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%%EndProlog
%%Page: 0 1
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0 SI
14 /Times-Bold AF
18157 8138 MT
(A Unix Interface for Shared Memory and)SH
19828 9816 MT
(Memory Mapped Files Under Mach)SH
11 /Times-Roman AF
17143 14090 MT
(Avadis Tevanian, Jr., Richard F. Rashid, Michael W. Young,)SH
14284 15286 MT
(David B. Golub, Mary R. Thompson, William Bolosky and Richard Sanzi)SH
23223 17678 MT
(Department of Computer Science)SH
24383 18874 MT
(Carnegie-Mellon University)SH
25847 20070 MT
(Pittsburgh, PA 15213)SH
14 /Times-Bold AF
27995 25389 MT
(Abstract)SH
11 /Times-Roman AF
9000 27946 MT
(This paper describes)
54 W( an approach to Unix shared memory and memory mapped files currently in)53 W
9000 29142 MT
(use at CMU under)
122 W( the Mach Operating System.  It describes the rationale for Mach's memory)123 W
9000 30338 MT
(sharing and file mapping primitives as well as their impact on)
88 W( other system components and on)87 W
9000 31534 MT
(overall performance.)SH
13 /Times-Bold AF
9000 35285 MT
(1. Introduction)SH
11 /Times-Roman AF
9500 36933 MT
(The 4.2)
262 W( BSD mapped file interface \050)263 W
/Times-Italic SF
(mmap)SH
/Times-Roman SF
(\051 was designed to address two shortcomings of)263 W
9000 38581 MT
(previous Unix systems: a lack of shared memory between processes and)
171 W( the need to simplify)170 W
9000 40229 MT
(processing of file data.  Early Unix systems had provided no shared)
6 W( memory access and a stylized)7 W
9000 41877 MT
(way of accessing sequential file data through)195 W
/Times-Italic SF
30432 XM
(read)SH
/Times-Roman SF
32917 XM
(and)SH
/Times-Italic SF
34974 XM
(write)SH
/Times-Roman SF
37705 XM
(system calls.  Applications that)194 W
9000 43525 MT
(desired random access to data would use Unix's)103 W
/Times-Italic SF
31267 XM
(seek)SH
/Times-Roman SF
33538 XM
(operation or buffer their data themselves,)104 W
9000 45173 MT
(often incurring)
34 W( unwanted system overhead.  A mapped file facility could allow a user to treat file)33 W
9000 46821 MT
(data as normal memory without regard to buffering or concerns about sequential versus random)83 W
9000 48469 MT
(access. It)
358 W( would also provide an obvious mechanism for sharing memory by allowing more than)41 W
9000 50117 MT
(one process to map a file read/write simultaneously.)SH
9500 52589 MT
(The BSD file mapping facility was proposed as early as 1982.  Since then, similar)
72 W( mapped file)73 W
9000 54237 MT
(interfaces have)
270 W( been implemented by several vendors, both as part of 4.2 BSD Unix \050e.g.,)269 W
9000 55885 MT
(Sequent Dynix)
105 W( [Dynix]\051)
SH( and as part of a System V modified to contain 4.2 BSD enhancements)105 W
9000 57533 MT
(\050e.g., IBM's AIX\051.  A shared memory facility not based on mapped files)
58 W( is available in AT&T's)57 W
9000 59181 MT
(System V and has also)
13 W( been adapted to a variety of 4.2 BSD based systems such as DEC's Ultrix.)14 W
9000 60829 MT
(There is currently a lively debate going on within the Unix community about)
175 W( the appropriate)174 W
9000 62477 MT
(Unix interface to virtual memory and the relationship between)
19 W( mapped files, memory sharing and)20 W
9000 64125 MT
(other virtual memory concerns such as copy-on-write memory mapping.)SH
9500 66597 MT
(This paper describes the somewhat atypical approach to shared memory and)
275 W( file mapping)274 W
9000 68245 MT
(currently in use at CMU under the Mach Operating System.)
25 W( It)
327 W( describes the rationale for Mach's)26 W
9000 69893 MT
(memory sharing and file mapping primitives, their impact on other system components and)
127 W( on)126 W
9000 71541 MT
(overall performance and the experiences of the Mach group in implementing and using them.)SH
ES
%%Page: 1 2
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0 SI
13 /Times-Bold AF
9000 8071 MT
(2. The Problems of a Mapped File Interface)SH
11 /Times-Roman AF
9500 9719 MT
(Despite its obvious convenience, the notion that all memory sharing should be)
292 W( channeled)293 W
9000 11367 MT
(through a mapped file interface presents a number of problems:)SH
11200 12820 MT
(1.)SH
12300 XM
(A Unix file is \050in principle\051 a permanent on-disk)
224 W( data structure which must be)223 W
12300 14016 MT
(maintained consistent)
48 W( against crashes.  The use of disk files to exchange temporary)49 W
12300 15212 MT
(data can put an unnecessary I/O load on the system and impact performance.)SH
11200 17106 MT
(2.)SH
12300 XM
(A mapped file facility must take into account the sharing of)
43 W( remote \050network\051 files.)42 W
12300 18302 MT
(In order to handle remote file systems \050e.g. SUN NFS\051, the operating system must)80 W
12300 19498 MT
(be intimately involved in maintaining network data consistency.  This can)
67 W( increase)66 W
12300 20694 MT
(its complexity considerably by introducing within the OS kernel many of)
66 W( the same)67 W
12300 21890 MT
(concerns that complicate transaction processing systems.)SH
11200 23784 MT
(3.)SH
12300 XM
(Sharing semantics are limited to)
196 W( those supplied by the kernel.  In particular, an)195 W
12300 24980 MT
(application program cannot use domain)
108 W( specific knowledge to allow less then full)109 W
12300 26176 MT
(consistency in sharing access to file data.  This can result in inefficiency in)
186 W( the)185 W
12300 27372 MT
(handling of data sharing across node boundaries.)SH
9500 29844 MT
(These problems)
8 W( typically have led to compromises in the actual mapped file semantics provided.)9 W
9000 31492 MT
(Most have either assumed that modifications to read/write mapped files are)
62 W( not guaranteed to be)61 W
9000 33140 MT
(consistent in the face of multiple)
72 W( writers, or they guarantee consistency only for those processes)73 W
9000 34788 MT
(which share files on a single network node.)SH
13 /Times-Bold AF
9000 38539 MT
(3. The Uses of Shared Memory)SH
11 /Times-Roman AF
9500 40187 MT
(Many of the potential uses)
96 W( of shared memory do not require a file mapping interface.  In fact,)95 W
9000 41835 MT
(such an interface may present)
281 W( problems.  Memory sharing is often suggested as a way of)282 W
9000 43483 MT
(overcoming traditional Unix deficiencies by providing for:)SH
/Symbol SF
10969 45061 MT
(\267)SH
/Times-Roman SF
11750 XM
(fine granularity multiprocessing,)SH
/Symbol SF
10969 46955 MT
(\267)SH
/Times-Roman SF
11750 XM
(ultra-fast IPC,)SH
/Symbol SF
10969 48849 MT
(\267)SH
/Times-Roman SF
11750 XM
(database management support and/or)SH
/Symbol SF
10969 50743 MT
(\267)SH
/Times-Roman SF
11750 XM
(reduced overhead file management.)SH
9000 52391 MT
(But of these potential uses of shared memory, only two)
37 W( require some kind of mapped file facility)36 W
9000 54039 MT
(and of these only one fits the traditional)SH
/Times-Italic SF
26780 XM
(mmap)SH
/Times-Roman SF
29743 XM
(model of file access and shared data consistency.)SH
12 /Times-Bold AF
9000 57723 MT
(3.1. Fine grain multiprocessing)SH
11 /Times-Roman AF
9500 59371 MT
(The need to support fine grain multiprocessing has forced several)
47 W( multiprocessor manufacturers)48 W
9000 61019 MT
(to adopt some form of memory sharing)
17 W( in their multiprocessor versions of Unix, e.g. in Sequent's)16 W
9000 62667 MT
(Dynix [Dynix])
SH( and Encore's UMax)
20 W( [Umax].)
SH( This)
315 W( kind of shared memory)
20 W( often takes the form of)21 W
9000 64315 MT
(an)SH
/Times-Italic SF
10497 XM
(mmap)SH
/Times-Roman SF
(-like primitive which acts on a special device or)
184 W( file.  Fine grain multiprocessing is)183 W
9000 65963 MT
(accomplished by creating as many)
78 W( processes as there are available processors which then)79 W
/Times-Italic SF
49512 XM
(mmap)SH
/Times-Roman SF
9000 67611 MT
(the shared memory object and synchronize)
160 W( through it.  The only reason such a facility would)159 W
9000 69259 MT
(want a mapped file interface is the benefit provided)
100 W( by using Unix's filesystem name space for)101 W
9000 70907 MT
(referring to shared data.  There is no need for the shared data to be disk resident or permanent.)SH
10 SS 
30350 75600 MT
(1)SH
ES
%%Page: 2 3
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0 SI
12 /Times-Bold AF
9000 8004 MT
(3.2. Fast IPC)SH
11 /Times-Roman AF
9500 9652 MT
(Shared memory can be used as a kind)
188 W( of ultra-fast IPC facility, especially where large data)187 W
9000 11300 MT
(structures are built in shared memory by one process and then managed or manipulated by)245 W
9000 12948 MT
(another. An)
571 W( example of a potential use of this kind can be found)
148 W( in the relationship between)147 W
9000 14596 MT
(multiphase program components such as the typical C language)
267 W( preprocessor and compiler.)268 W
9000 16244 MT
(Already such programs use files or pipes to accomplish their goals.  The advantages)
8 W( of such a fast)7 W
9000 17892 MT
(IPC facility are)
156 W( actually diminished by tying it to a similar shared file construct which would)157 W
9000 19540 MT
(require some form of file system creation/destruction cost as well as disk I/O.)SH
12 /Times-Bold AF
9000 23224 MT
(3.3. Database management)SH
11 /Times-Roman AF
9500 24872 MT
(Designers of database management systems have argued against Unix at least partly)
59 W( because of)58 W
9000 26520 MT
(its inability to share data between potential database client programs)
115 W( and transaction managers,)116 W
9000 28168 MT
(data managers and recovery logs.  Systems of this sort need both)
62 W( sharing between processes and)61 W
9000 29816 MT
(sharing of data pages in files to accomplish their ends.)
118 W( Unfortunately,)
512 W( an mmap-like construct)119 W
9000 31464 MT
(does not, by itself, resolve the problems)
165 W( posed by database systems.  For example, it may be)164 W
9000 33112 MT
(important for a database system)
30 W( to know when data is going to be moved from volatile storage to)31 W
9000 34760 MT
(disk so that a database recovery manager can update crucial portions of the recovery)
219 W( log in)218 W
9000 36408 MT
(advance [TABSSOSP])
SH( \050i.e., write-ahead logging\051.  In addition, the consistency of shared)
20 W( memory)21 W
9000 38056 MT
(must either be absolute, or the consistency)
5 W( model must be well understood and manageable by the)4 W
9000 39704 MT
(database transaction manager -- a fact often)
31 W( remarked by database builders on other systems with)32 W
9000 41352 MT
(shared file constructs such as Apollo's Aegis [Leach83].)SH
12 /Times-Bold AF
9000 45036 MT
(3.4. Efficient file access)SH
11 /Times-Roman AF
9500 46684 MT
(By far the most)
59 W( compelling general argument for linking shared memory with memory mapped)58 W
9000 48332 MT
(files is the need in Unix for reducing)
36 W( the overhead of file management.  Partly because Unix was)37 W
9000 49980 MT
(originally designed at a time when primary memory was a scarce commodity, traditional Unix)132 W
9000 51628 MT
(programs are)
158 W( I/O intensive.  Even the Unix pipe facility was once implemented as file I/O to)159 W
9000 53276 MT
(conserve memory.  As the relationship between the costs of)
47 W( memory and secondary storage have)46 W
9000 54924 MT
(changed, large memory Unix systems are limited)
38 W( more by their I/O capacity than by memory.  A)39 W
9000 56572 MT
(mapped file facility)
36 W( could reduce the cost of I/O operations by eliminating a copy operation from)35 W
9000 58220 MT
(the Unix buffer)
218 W( cache to process memory and also provide for better memory utilization by)219 W
9000 59868 MT
(allowing more than one process to share the same physical memory when accessing the same file.)SH
13 /Times-Bold AF
9000 63619 MT
(4. Mach Memory Primitives)SH
11 /Times-Roman AF
9500 65267 MT
(Rather than support sharing only through)
167 W( an)166 W
/Times-Italic SF
30372 XM
(mmap)SH
/Times-Roman SF
33501 XM
(model of shared memory through shared)166 W
9000 66915 MT
(files, Mach provides a number of non-file based mechanisms for sharing data)
492 W( among)493 W
9000 68563 MT
(computational entities:)SH
/Symbol SF
10969 70141 MT
(\267)SH
/Times-Roman SF
11750 XM
(Unrestricted, fine grain sharing between processors in a tightly coupled)700 W
11750 71337 MT
(multiprocessor can be achieved)
16 W( by using the Mach notion of)17 W
/Times-Italic SF
38737 XM
(thread)SH
/Times-Roman SF
(. A)
309 W( thread can be)17 W
10 SS 
30350 75600 MT
(2)SH
ES
%%Page: 3 4
BS
0 SI
11 /Times-Roman AF
11750 7955 MT
(thought of as a lightweight process which)
332 W( shares an address space with other)331 W
11750 9151 MT
(lightweight processes.  The Unix)
239 W( notion of)240 W
/Times-Italic SF
32490 XM
(process)SH
/Times-Roman SF
36365 XM
(has been split into)240 W
/Times-Italic SF
45575 XM
(task)SH
/Times-Roman SF
47862 XM
(and)SH
/Times-Italic SF
11750 10347 MT
(thread)SH
/Times-Roman SF
(. A)
407 W( task defines an address space and resource domain in which a)
66 W( number of)65 W
11750 11543 MT
(program control)
175 W( flows \050threads\051 may coexist.  Using this multiple thread per task)176 W
11750 12739 MT
(mechanism, an application may easily share a single address space among separate)112 W
11750 13935 MT
(executing entities.)SH
/Symbol SF
10969 15829 MT
(\267)SH
/Times-Roman SF
11750 XM
(In addition to unrestricted)
194 W( sharing using threads, Mach allows tasks to read/write)195 W
11750 17025 MT
(share protected ranges of virtual addresses)
162 W( through inheritance.  A Mach task can)161 W
11750 18221 MT
(specify any portion of its address space to be shared read/write)
105 W( with its children as)106 W
11750 19417 MT
(the result of a)108 W
/Times-Italic SF
18474 XM
(task_create)SH
/Times-Roman SF
23927 XM
(operation. The)
489 W( fact that memory is shared only through)107 W
11750 20613 MT
(inheritance guarantees that the shared memory is always located)
46 W( within a single host)47 W
11750 21809 MT
(\050or cluster within a host\051.  This allows the kernel to guarantee cache consistency)
64 W( for)63 W
11750 23005 MT
(such memory.  Another advantage of)
124 W( this method of data sharing is that it ensures)125 W
11750 24201 MT
(that shared memory is always located at the same virtual address in)
97 W( each inheriting)96 W
11750 25397 MT
(task. This)
885 W( avoids the)
305 W( often difficult programming problems caused by pointer)306 W
11750 26593 MT
(address aliasing in shared data structures.)SH
/Symbol SF
10969 28487 MT
(\267)SH
/Times-Roman SF
11750 XM
(Physical memory can be shared copy-on-write by taking advantage of Mach's)337 W
11750 29683 MT
(integration of IPC and virtual memory)
247 W( management.  Applications not requiring)248 W
11750 30879 MT
(read/write memory sharing can use this feature to transfer)
210 W( large amounts of data)209 W
11750 32075 MT
(between tasks without actually copying data.)
51 W( In)
378 W( effect, a multiphase application can)52 W
11750 33271 MT
(effectively forward between components the actual physical memory containing)299 W
11750 34467 MT
(important data.  The sender in such an exchange is always protected because data)
64 W( is)65 W
11750 35663 MT
(logically sent by value. The kernel uses memory management tricks)
156 W( to make sure)155 W
11750 36859 MT
(that the same physical page is available to)
59 W( both sender and receiver unless or until a)60 W
11750 38055 MT
(write operation occurs.)SH
/Symbol SF
10969 39949 MT
(\267)SH
/Times-Roman SF
11750 XM
(Finally, applications may define their own sharing semantics)
211 W( within a distributed)210 W
11750 41145 MT
(system of Mach hosts using the Mach)127 W
/Times-Italic SF
29534 XM
(external pager)127 W
/Times-Roman SF
36508 XM
(facility. This)
530 W( external pager)128 W
11750 42341 MT
(mechanism allows an application to control many)
410 W( aspects of virtual memory)409 W
11750 43537 MT
(management for regions of virtual memory.  An)28 W
/Times-Italic SF
33387 XM
(external pager)28 W
/Times-Roman SF
40164 XM
(may implement fully)29 W
11750 44733 MT
(coherent network shared memory, or a shared memory paradigm)
29 W( that requires clients)28 W
11750 45929 MT
(to maintain their own cache consistency \050if consistency is even desired\051.  It allows)
55 W( a)56 W
11750 47125 MT
(database recovery manager to be advised of)
40 W( the kernel's need to flush data to disk in)39 W
11750 48321 MT
(advance and thus permit efficient write-ahead logging.)SH
12 /Times-Bold AF
9000 52005 MT
(4.1. Mach virtual memory operations)SH
11 /Times-Roman AF
9500 53653 MT
(Table VMOPS lists the set of operations that)
37 W( can be performed on the virtual address space of a)38 W
9000 55301 MT
(task. Mach)
1011 W( calls)
368 W( are specified to act on object handles called)367 W
/Times-Italic SF
40498 XM
(ports)SH
/Times-Roman SF
43402 XM
(which are simplex)367 W
9000 56949 MT
(communication channels on)
169 W( which messages are sent.  A more complete description of Mach)170 W
9000 58597 MT
(ports and calling conventions can be found in [USENIX86].)SH
9500 61069 MT
(A task address space consists of an ordered collection)
227 W( of mappings to memory objects; all)226 W
9000 62717 MT
(threads within a task share access)
108 W( to that address space.  A Mach)109 W
/Times-Italic SF
39182 XM
(memory object)109 W
/Times-Roman SF
46180 XM
(\050also called a)109 W
/Times-Italic SF
9000 64365 MT
(paging object)114 W
/Times-Roman SF
(\051 is a data repository, provided and managed by a server.  The size of an)
114 W( address)113 W
9000 66013 MT
(space is limited only by the addressing restrictions of the)
55 W( underlying hardware.  For example, an)56 W
9000 67661 MT
(IBM RT PC task can address a full 4 gigabytes of memory under Mach,)
285 W( while the VAX)284 W
9000 69309 MT
(architecture allows at most 2 gigabytes of user address space.)SH
9500 71781 MT
(The basic memory operations permit both copy-on-write and read/write)
230 W( sharing of memory)231 W
10 SS 
30350 75600 MT
(3)SH
ES
%%Page: 4 5
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0 SI
11 /Times-Bold AF
23971 7937 MT
(Virtual Memory Operations)SH
39600 50 10800 9111 UL
9 SS 
12600 9690 MT
(vm_allocate)SH
/Times-Roman SF
17425 XM
(\050task, address, size, anywhere\051)SH
/Times-Italic SF
37080 XM
(Allocate and fill with zeros new)SH
37080 10541 MT
(virtual memory either anywhere or)SH
37080 11392 MT
(at a specified address on demand.)SH
/Times-Bold SF
12600 13094 MT
(vm_copy)SH
/Times-Roman SF
16275 XM
(\050task, src_addr, count, dst_addr\051)SH
/Times-Italic SF
37080 XM
(Virtually copy a range of memory)SH
37080 13945 MT
(from one address to another.)SH
/Times-Bold SF
12600 15647 MT
(vm_deallocate)SH
/Times-Roman SF
18325 XM
(\050task, address, size\051)SH
/Times-Italic SF
37080 XM
(Deallocate a range of addresses,)SH
37080 16498 MT
(i.e. make them no longer valid.)SH
/Times-Bold SF
12600 18200 MT
(vm_inherit)SH
/Times-Roman SF
17075 XM
(\050task, address, size, inheritance\051)SH
/Times-Italic SF
37080 XM
(Set the inheritance attribute)SH
37080 19051 MT
(of an address range.)SH
/Times-Bold SF
12600 20753 MT
(vm_protect)SH
/Times-Roman SF
17225 XM
(\050task, address, size, set_max, protection\051)SH
/Times-Italic SF
37080 XM
(Set the protection attribute)SH
37080 21604 MT
(of an address range.)SH
/Times-Bold SF
12600 23306 MT
(vm_read)SH
/Times-Roman SF
16225 XM
(\050task, address, size, data, data_count\051)SH
/Times-Italic SF
37080 XM
(Read the contents of a region)SH
37080 24157 MT
(of a task's address space.)SH
/Times-Bold SF
12600 25859 MT
(vm_regions)SH
/Times-Roman SF
17275 XM
(\050task, address, size, elements, elements_count\051)SH
/Times-Italic SF
37080 XM
(Return description of specified)SH
37080 26710 MT
(region of task's address space.)SH
/Times-Bold SF
12600 28412 MT
(vm_statistics)SH
/Times-Roman SF
17775 XM
(\050task, vm_stats\051)SH
/Times-Italic SF
37080 XM
(Return statistics about the use)SH
37080 29263 MT
(of memory by task.)SH
/Times-Bold SF
12600 30965 MT
(vm_write)SH
/Times-Roman SF
16475 XM
(\050task, address, count, data, data_count\051)SH
/Times-Italic SF
37080 XM
(Write the contents of a region)SH
37080 31816 MT
(of a task's address space.)SH
39600 50 10800 32867 UL
11 /Times-Bold AF
27926 34329 MT
(Table 4-1:)SH
9 /Times-Roman AF
18950 35943 MT
(All VM operations apply to a)SH
/Times-Italic SF
29750 XM
(task)SH
/Times-Roman SF
31425 XM
(\050represented by a port\051 and all)SH
20912 36794 MT
(but vm_statistics specify an)SH
/Times-Italic SF
31112 XM
(address)SH
/Times-Roman SF
34137 XM
(and)SH
/Times-Italic SF
35662 XM
(size)SH
/Times-Roman SF
37237 XM
(in bytes.)SH
/Times-Italic SF
18300 37645 MT
(anywhere)SH
/Times-Roman SF
22025 XM
(is a boolean which indicates whether or not a vm_allocate)SH
18875 38496 MT
(allocates memory anywhere or at a location specified by address.)SH
39600 50 10800 40344 UL
11 SS 
9000 42095 MT
(regions between tasks.  Copy-on-write sharing between unrelated tasks is usually the result)
151 W( of)150 W
9000 43743 MT
(large message transfers. An entire address space may be sent)
71 W( in a single message with no actual)72 W
9000 45391 MT
(data copy operations performed.  Read/write)
126 W( shared memory within a task creation tree can be)125 W
9000 47039 MT
(created by)
42 W( allocating a memory region and setting its inheritance attribute.  Subsequently created)43 W
9000 48687 MT
(child tasks share the memory of their parent according to)
282 W( its inheritance value.  The only)281 W
9000 50335 MT
(restriction imposed by Mach on the nature of the regions that may be specified)
259 W( for virtual)260 W
9000 51983 MT
(memory operations is that they must be aligned on)
131 W( system page boundaries.  The system page)130 W
9000 53631 MT
(size is a boot time parameter and can be any power of two that is a multiple of the hardware page)22 W
9000 55279 MT
(size.)SH
12 /Times-Bold AF
9000 58963 MT
(4.2. Managing external pagers)SH
11 /Times-Roman AF
9500 60611 MT
(The basic task virtual memory operations allow memory sharing)
136 W( through inheritance between)135 W
9000 62259 MT
(tasks in the same task creation subtree.  Read/write shared memory between)
77 W( unrelated tasks can)78 W
9000 63907 MT
(be implemented through the use of external pagers -- tasks which allocate and)
36 W( manage secondary)35 W
9000 65555 MT
(storage objects.)SH
9500 68027 MT
(The Mach)
73 W( interface for external pagers can best be thought of as a message protocol used by a)74 W
9000 69675 MT
(pager and the kernel to communicate with each)
12 W( other about the contents of a memory object.  The)11 W
9000 71323 MT
(external pager interface to the kernel can be)
137 W( described in terms of operations requested by the)138 W
10 SS 
30350 75600 MT
(4)SH
ES
%%Page: 5 6
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0 SI
11 /Times-Roman AF
9000 7955 MT
(kernel \050messages sent to a)24 W
/Times-Italic SF
20879 XM
(paging_object)SH
/Times-Roman SF
27472 XM
(port\051 and calls made by)
24 W( the external pager on the kernel)23 W
9000 9603 MT
(\050messages sent to the kernel's)186 W
/Times-Italic SF
23339 XM
(pager_request_port)SH
/Times-Roman SF
32538 XM
(associated with)
186 W( a memory object\051.  Tables)187 W
9000 11251 MT
(EXPOPS and POPS describe these two interfaces.)SH
/Times-Bold SF
22415 13184 MT
(Kernel to External Pager Interface)SH
36000 50 12600 14358 UL
9 SS 
14400 14937 MT
(pager_init)SH
/Times-Roman SF
18575 XM
(\050paging_object,)SH
/Times-Italic SF
34200 XM
(Initialize a memory object.)SH
/Times-Roman SF
18000 15788 MT
(pager_request_port, pager_name\051)SH
/Times-Bold SF
14400 17490 MT
(pager_data_request)SH
/Times-Roman SF
22275 XM
(\050paging_object,)SH
/Times-Italic SF
34200 XM
(Requests data from an external pager.)SH
/Times-Roman SF
18000 18341 MT
(pager_request_port, offset,)SH
18000 19192 MT
(length, desired_access\051)SH
/Times-Bold SF
14400 20894 MT
(pager_data_write)SH
/Times-Roman SF
21425 XM
(\050paging_object, offset)SH
/Times-Italic SF
34200 XM
(Writes data back to a memory object.)SH
/Times-Roman SF
18000 21745 MT
(data, data_count\051)SH
/Times-Bold SF
14400 23447 MT
(pager_data_unlock)SH
/Times-Roman SF
22025 XM
(\050paging_object,)SH
/Times-Italic SF
34200 XM
(Requests that data be unlocked.)SH
/Times-Roman SF
18000 24298 MT
(pager_request_port, offset,)SH
18000 25149 MT
(length, desired_access\051)SH
/Times-Bold SF
14400 26851 MT
(pager_create)SH
/Times-Roman SF
19625 XM
(\050old_paging_object,)SH
/Times-Italic SF
34200 XM
(Accept ownership of a memory object.)SH
/Times-Roman SF
18000 27702 MT
(new_paging_object, new_request_port,)SH
18000 28553 MT
(new_name\051)SH
36000 50 12600 29604 UL
11 /Times-Bold AF
27926 31066 MT
(Table 4-2:)SH
9 /Times-Roman AF
14000 32680 MT
(Calls made by Mach kernel to a task providing external paging service for a memory object.)SH
36000 50 12600 34528 UL
11 SS 
9500 36800 MT
(A memory object)
406 W( may be mapped into the address space of a task by exercising the)405 W
/Times-Italic SF
9000 38448 MT
(vm_allocate_with_pager)SH
/Times-Roman SF
20304 XM
(primitive, specifying a paging object)
91 W( port.  This port will then be used)92 W
9000 40096 MT
(by the kernel to refer to that object.)
125 W( A)
523 W( single memory object may be mapped more than once)124 W
9000 41744 MT
(\050possibly in different tasks\051.)
98 W( The)
473 W( Mach kernel provides consistent shared memory access to all)99 W
9000 43392 MT
(mappings of the same memory)
103 W( object on the same uniprocessor or multiprocessor.  The role of)102 W
9000 45040 MT
(the kernel in paging is primarily that of a physical page cache manager for objects.)SH
9500 47512 MT
(When asked to map a memory)
253 W( object for the first time, the kernel responds by making a)254 W
/Times-Italic SF
9000 49160 MT
(pager_init)SH
/Times-Roman SF
13859 XM
(call on the paging object port.  Included in this message are:)SH
/Symbol SF
10969 50738 MT
(\267)SH
/Times-Roman SF
11750 XM
(a)SH
/Times-Italic SF
12589 XM
(pager request)76 W
/Times-Roman SF
19095 XM
(port, which the pager may use to make cache management requests)76 W
11750 51934 MT
(of the Mach kernel,)SH
/Symbol SF
10969 53828 MT
(\267)SH
/Times-Roman SF
11750 XM
(a)SH
/Times-Italic SF
12566 XM
(pager name)53 W
/Times-Roman SF
18170 XM
(port, which the kernel will use to identify this memory object to)
53 W( other)54 W
9 SS 
23054 54686 MT
(1)SH
11 SS 
11750 55049 MT
(tasks in)SH
/Times-Italic SF
15356 XM
(vm_regions)SH
/Times-Roman SF
20763 XM
(calls.)SH
9000 56697 MT
(The Mach kernel holds send rights)
118 W( to the paging object port, and send, receive, and ownership)119 W
9000 58345 MT
(rights on the paging request and paging name ports.)SH
9500 60817 MT
(In order to fulfill a cache)
192 W( miss \050i.e. page fault\051, the kernel issues a)191 W
/Times-Italic SF
41286 XM
(pager_data_request)SH
/Times-Roman SF
50612 XM
(call)SH
9000 62465 MT
(specifying the range \050usually a)
281 W( single page\051 desired.  The pager is expected to supply the)282 W
9000 64113 MT
(requested data using the)29 W
/Times-Italic SF
19990 XM
(pager_data_provided)SH
/Times-Roman SF
29826 XM
(call on the specified paging request port.)
29 W( To)
331 W( flush)28 W
9000 65761 MT
(modified cached data, the kernel performs a)228 W
/Times-Italic SF
30203 XM
(pager_data_write)SH
/Times-Roman SF
38591 XM
(call, including the data to be)229 W
10800 50 9000 69158 UL
7 SS 
9900 70659 MT
(1)SH
9 SS 
10250 70986 MT
(The paging object and request ports cannot be used for this)
11 W( purpose, as access to those ports allows complete access)10 W
9000 72000 MT
(to the data and management functions.)SH
10 SS 
30350 75600 MT
(5)SH
ES
%%Page: 6 7
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0 SI
11 /Times-Roman AF
9000 7955 MT
(written and its location in the memory object.  When the pager no longer needs the data \050e.g.)
87 W( it)86 W
9000 9603 MT
(has been successfully written)
8 W( to secondary storage\051, it is expected to use the)9 W
/Times-Italic SF
42774 XM
(vm_deallocate)SH
/Times-Roman SF
49472 XM
(call to)9 W
9000 11251 MT
(release the cache resources.)SH
9500 13723 MT
(Since the pager may have external constraints on the consistency of its memory object,)
4 W( the Mach)3 W
9000 15371 MT
(interface provides some functions to control caching; these calls are made)
18 W( using the pager request)19 W
9000 17019 MT
(port provided at initialization time.)SH
/Times-Bold SF
22415 18952 MT
(External Pager to Kernel Interface)SH
36000 50 12600 20126 UL
9 SS 
14400 20705 MT
(vm_allocate_with_pager)SH
/Times-Roman SF
24025 XM
(\050task, address,)SH
/Times-Italic SF
32400 XM
(Allocate a region of memory at specified)SH
/Times-Roman SF
18000 21556 MT
(size, anywhere, paging_object, offset\051)SH
/Times-Italic SF
32400 XM
(address backed by a memory object.)SH
/Times-Bold SF
14400 23258 MT
(pager_data_provided)SH
/Times-Roman SF
22875 XM
(\050paging_object_request,)SH
/Times-Italic SF
32400 XM
(Supplies the kernel with the data contents)SH
/Times-Roman SF
18000 24109 MT
(offset, data, data_count, lock_value\051)SH
/Times-Italic SF
32400 XM
(of a region of a memory object.)SH
/Times-Bold SF
14400 25811 MT
(pager_data_lock)SH
/Times-Roman SF
21025 XM
(\050paging_object_request,)SH
/Times-Italic SF
32400 XM
(Prevents further access to the specified)SH
/Times-Roman SF
18000 26662 MT
(offset, length, lock_value\051)SH
/Times-Italic SF
32400 XM
(data until an unlock.)SH
/Times-Bold SF
14400 28364 MT
(pager_flush_request)SH
/Times-Roman SF
22475 XM
(\050paging_object_request,)SH
/Times-Italic SF
32400 XM
(Forces physically cached data)SH
/Times-Roman SF
18000 29215 MT
(offset, length\051)SH
/Times-Italic SF
32400 XM
(to be destroyed.)SH
/Times-Bold SF
14400 30917 MT
(pager_clean_request)SH
/Times-Roman SF
22575 XM
(\050paging_object_request,)SH
/Times-Italic SF
32400 XM
(Forces modified physically cached data)SH
/Times-Roman SF
18000 31768 MT
(offset, length\051)SH
/Times-Italic SF
32400 XM
(to be written back to a memory object.)SH
/Times-Bold SF
14400 33470 MT
(pager_cache)SH
/Times-Roman SF
19425 XM
(\050paging_object_request,)SH
/Times-Italic SF
32400 XM
(Notifies the kernel that it should retain)SH
/Times-Roman SF
18000 34321 MT
(should_cache_object\051)SH
/Times-Italic SF
32400 XM
(knowledge about the memory object even)SH
32400 35172 MT
(after all references to it have been removed.)SH
/Times-Bold SF
14400 36874 MT
(pager_data_unavailable)SH
/Times-Roman SF
18000 37725 MT
(\050paging_object_request,)SH
/Times-Italic SF
32400 XM
(Notifies kernel that no data is)SH
/Times-Roman SF
18000 38576 MT
(offset, size\051)SH
/Times-Italic SF
32400 XM
(available for that region of a memory object.)SH
36000 50 12600 39627 UL
11 /Times-Bold AF
27926 41089 MT
(Table 4-3:)SH
9 /Times-Roman AF
16150 42703 MT
(Calls made by a task on the kernel to allocate and and manage a memory object.)SH
36000 50 12600 44551 UL
11 SS 
9500 46823 MT
(A)SH
/Times-Italic SF
10862 XM
(pager_flush_request)SH
/Times-Roman SF
20474 XM
(call causes the kernel to invalidate its cached copy of the data in)292 W
9000 48471 MT
(question, writing back modifications if necessary.  A)45 W
/Times-Italic SF
32882 XM
(pager_clean_request)SH
/Times-Roman SF
42488 XM
(call asks the)
45 W( kernel to)46 W
9000 50119 MT
(write back modifications, but allows the kernel to continue to use the)
53 W( cached data.  A pager may)52 W
9000 51767 MT
(restrict the)
207 W( use of cached data by issuing a)208 W
/Times-Italic SF
29716 XM
(pager_data_lock)SH
/Times-Roman SF
37653 XM
(request, specifying the types of)208 W
9000 53415 MT
(access \050of read, write, execute\051 which may)
181 W( be permitted.  For example, a pager may wish to)180 W
9000 55063 MT
(temporarily allow read-only access to cached data.  The locking on a page may)
60 W( later be changed)61 W
9000 56711 MT
(as deemed necessary by the pager.)SH
9500 59183 MT
(When a user task requires greater access to cached data \050e.g.)
52 W( a)
377 W( write fault on a read-only page\051)51 W
9000 60831 MT
(than the)
67 W( pager has permitted, the kernel issues a)68 W
/Times-Italic SF
30931 XM
(pager_data_unlock)SH
/Times-Roman SF
39828 XM
(call. The)
411 W( pager is expected)68 W
9000 62479 MT
(to respond by changing the locking on that data when it is able to do so.)SH
9500 64951 MT
(When no references to a memory object remain, and)
25 W( all modifications have been written back to)24 W
9000 66599 MT
(the paging object port, the kernel deallocates)
183 W( its rights to the three ports associated with that)184 W
9000 68247 MT
(memory object.  The pager receives notification of the death of the request)
127 W( and name ports, at)126 W
9000 69895 MT
(which time it can perform appropriate shutdown.)SH
10 SS 
30350 75600 MT
(6)SH
ES
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0 SI
11 /Times-Roman AF
9500 7955 MT
(In order to attain better cache performance, a pager may permit the data for a memory object)
47 W( to)48 W
9000 9603 MT
(be cached even after)
157 W( all address map references are gone by calling)156 W
/Times-Italic SF
40737 XM
(pager_cache)SH
/Times-Roman SF
(. Permitting)587 W
9000 11251 MT
(such caching is in no way binding; the kernel may choose to relinquish)
52 W( its access to the memory)53 W
9000 12899 MT
(object ports as it deems necessary for its cache management.)SH
9500 15371 MT
(The Mach kernel may itself need to)
17 W( create memory objects, either to provide backing storage for)16 W
9000 17019 MT
(zero-filled memory \050)264 W
/Times-Italic SF
(vm_allocate)SH
/Times-Roman SF
(\051, or to implement virtual)
264 W( copy operations.  These memory)265 W
9000 18667 MT
(objects are managed by)
22 W( a)21 W
/Times-Italic SF
20499 XM
(default pager)21 W
/Times-Roman SF
26713 XM
(task, which is known to the kernel at system initialization)21 W
9000 20315 MT
(time. When)
565 W( the kernel creates such a memory object, it performs a)146 W
/Times-Italic SF
40675 XM
(pager_create)SH
/Times-Roman SF
46960 XM
(call \050on the)146 W
9000 21963 MT
(default pager port\051; this call is similar in form to)90 W
/Times-Italic SF
31408 XM
(pager_init)SH
/Times-Roman SF
(. Since)
455 W( these kernel-created objects)90 W
9000 23611 MT
(have no initial memory, the default pager may not have data to)
63 W( provide in response to a request.)64 W
9000 25259 MT
(In this case, it should perform a)SH
/Times-Italic SF
23174 XM
(pager_data_unavailable)SH
/Times-Roman SF
34265 XM
(call.)SH
9500 27731 MT
(Since interaction with pagers is conducted only through ports, it is possible to map)
146 W( the same)145 W
9000 29379 MT
(memory object into tasks on different hosts in a distributed system.  While each)
63 W( kernel keeps its)64 W
9000 31027 MT
(own uses of the cached data consistent, the pager is responsible)
180 W( for any further coordination.)179 W
9000 32675 MT
(Since each Mach kernel will perform a)166 W
/Times-Italic SF
27539 XM
(pager_init)SH
/Times-Roman SF
32565 XM
(call upon its first use of a memory object,)167 W
9000 34323 MT
(including its own request and name ports, a pager can easily distinguish the various)
113 W( uses of its)112 W
9000 35971 MT
(data.)SH
13 /Times-Bold AF
9000 39722 MT
(5. A Unix Interface for File Mapping)SH
11 /Times-Roman AF
9500 41370 MT
(Shared memory can be obtained in Mach)
235 W( either through the use of memory inheritance or)236 W
9000 43018 MT
(external pagers.  Given these mechanisms for sharing data,)
47 W( there is no need to overload the Unix)46 W
9000 44666 MT
(filesystem in order to provide shared memory.)
431 W( Nevertheless,)
1138 W( the potential performance)432 W
9000 46314 MT
(advantages of mapped files make them desirable for Unix emulation)
3 W( under Mach.  In addition, the)2 W
9000 47962 MT
(ease of programming)
244 W( associated with mapped files is attractive in both the Unix and Mach)245 W
9000 49610 MT
(environments.)SH
9500 52082 MT
(At present, Mach provides a single new Unix domain system call for file mapping:)SH
/Courier SF
11640 53608 MT
(map_fd\050fd, offset, addr, find_space, numbytes\051)SH
16920 54722 MT
(int fd;)7920 W
16920 55836 MT
(vm_offset_t offset;)2640 W
16920 56950 MT
(vm_offset_t *addr;)2640 W
16920 58064 MT
(boolean_t find_space;)3960 W
16920 59178 MT
(vm_size_t numbytes;)3960 W
/Times-Italic SF
9500 61650 MT
(Map_fd)SH
/Times-Roman SF
13224 XM
(is called with an open Unix file descriptor \050)27 W
/Times-Italic SF
(fd)SH
/Times-Roman SF
(\051 and if)
27 W( successful results in a virtual copy)26 W
9000 63298 MT
(of the file mapped into the address space of the)
117 W( calling Unix process.)118 W
/Times-Italic SF
41524 XM
(Offset)SH
/Times-Roman SF
44545 XM
(is the byte offset)118 W
9000 64946 MT
(within the file at which mapping is to begin.)
65 W( The)
403 W( offset may be any byte offset in the file, page)64 W
9000 66594 MT
(alignment is not required.)141 W
/Times-Italic SF
21559 XM
(Addr)SH
/Times-Roman SF
24175 XM
(is a pointer to the address in the address space)
141 W( of the calling)142 W
9000 68242 MT
(process at which the)
152 W( mapped file should start.  This address, unlike the offset, must be paged)151 W
9000 69890 MT
(aligned. If)479 W
/Times-Italic SF
14377 XM
(find_space)SH
/Times-Roman SF
19521 XM
(is TRUE, the kernel will select an unused address range and return it in)103 W
/Times-Italic SF
9000 71538 MT
(*addr)SH
/Times-Roman SF
(. The)
275 W( number of bytes to be mapped is specified by)SH
/Times-Italic SF
34691 XM
(numbytes)SH
/Times-Roman SF
(.)SH
10 SS 
30350 75600 MT
(7)SH
ES
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0 SI
11 /Times-Roman AF
9500 7955 MT
(The implementation of)15 W
/Times-Italic SF
19902 XM
(map_fd)SH
/Times-Roman SF
23492 XM
(was a straightforward application of internal Mach)
15 W( primitives for)14 W
9000 9603 MT
(virtual copying)
449 W( regions of memory and external pagers)
450 W( [MACH-ASPLOS, MACH-SOSP].)SH
9000 11251 MT
(When a request is made for a file to be mapped into a user address space, the kernel)
120 W( creates a)119 W
9000 12899 MT
(temporary internal)
102 W( address space into which the file is mapped.  This mapping is accomplished)103 W
9000 14547 MT
(with the)231 W
/Times-Italic SF
13312 XM
(vm_allocate_with_pager)SH
/Times-Roman SF
24756 XM
(primitive. The)
737 W( kernel)
231 W( specifies that new memory is to be)230 W
9000 16195 MT
(allocated and that the)
68 W( new memory will be backed by the internal kernel)69 W
/Times-Italic SF
41935 XM
(inode pager)69 W
/Times-Roman SF
(. Then)
413 W( the)69 W
9000 17843 MT
(file data is moved to the process address space by a call to)4 W
/Times-Italic SF
34922 XM
(vm_copy)SH
/Times-Roman SF
(. Once)
281 W( this is done, the kernel)3 W
9000 19491 MT
(can deallocate the temporary map.)SH
13 /Times-Bold AF
9000 23242 MT
(6. Uses of Mapped Files in Mach)SH
11 /Times-Roman AF
9500 24890 MT
(Files mapped using)139 W
/Times-Italic SF
18748 XM
(map_fd)SH
/Times-Roman SF
22462 XM
(can be used in a variety of ways.  Mach itself uses file mapping)140 W
9000 26538 MT
(internally to implement program loading.  File mapping can also be used as)
135 W( a replacement for)134 W
9000 28186 MT
(buffer management in the standard I/O library.)SH
12 /Times-Bold AF
9000 31870 MT
(6.1. File Mapping and Shared Libraries)SH
11 /Times-Roman AF
9500 33518 MT
(Mach uses the mapped file interface to implement both program loading and)
75 W( a general form of)76 W
9000 35166 MT
(shared libraries.)
88 W( In)
449 W( the current Mach system, there are two types of program loaders.  The first)87 W
9000 36814 MT
(program loader executes in the kernel and implements the Unix)135 W
/Times-Italic SF
38574 XM
(exec)SH
/Times-Roman SF
40936 XM
(system call.  This)
135 W( loader)136 W
9000 38462 MT
(handles both)64 W
/Times-Italic SF
14994 XM
(a.out)SH
/Times-Roman SF
17564 XM
(and)SH
/Times-Italic SF
19491 XM
(COFF)SH
/Times-Roman SF
22701 XM
(format binary files for binary compatibility with existing systems.)63 W
9000 40110 MT
(The second loader executes in a user task and handles)6 W
/Times-Italic SF
33005 XM
(MACH-O)SH
/Times-Roman SF
37563 XM
(format binary files.  Both loaders)7 W
9000 41758 MT
(use mapped files.)SH
9500 44230 MT
(The MACH-O format was devised to)
103 W( be flexible enough to be used as a single file format for)102 W
9000 45878 MT
(fully resolved binaries, unresolved object files, shared libraries and "core" files.  It)
223 W( provides)224 W
9000 47526 MT
(enough backward compatibility)
60 W( with older formats \050e.g., a.out\051 to salvage most existing code for)59 W
9000 49174 MT
(debuggers and related applications.)SH
9500 51646 MT
(The MACH-O format can roughly be though of as a sequence of commands to be executed by a)27 W
9000 53294 MT
(program loader.  The layout of a MACH-O file is summarized as:)SH
/Courier SF
11640 54820 MT
(start)SH
16920 55934 MT
(header)SH
16920 57048 MT
(command_id, command_info)SH
16920 58162 MT
(command_id, command_info)SH
22200 59276 MT
(.)SH
22200 60390 MT
(.)SH
22200 61504 MT
(.)SH
16920 62618 MT
(command_id, command_info)SH
11640 63732 MT
(ENDMARKER)SH
/Times-Roman SF
9500 66204 MT
(Each command consists of a command identifier followed by)
51 W( a command-dependent number of)50 W
9000 67852 MT
(arguments. Some)
275 W( of the commands supported are:)SH
14600 69803 MT
(READ_ONLY)SH
23400 XM
(Map in data read-only \050e.g. a text segment\051.)SH
14600 71498 MT
(WRITEABLE)SH
23400 XM
(Map in data read/write \050e.g. a data segment\051.)SH
10 SS 
30350 75600 MT
(8)SH
ES
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0 SI
11 /Times-Roman AF
14600 7955 MT
(ZEROFILL)SH
23400 XM
(Allocate zero-fill memory \050e.g. a bss segment\051.)SH
14600 9650 MT
(REGISTER)SH
23400 XM
(Create a thread in the task and set its register state.)SH
14600 11345 MT
(LOADFILE)SH
23400 XM
(Map in data from another file \050e.g. a shared library\051.)SH
14600 13040 MT
(RELOCATE)SH
23400 XM
(Relocate a specified address.)SH
14600 14735 MT
(END_LOAD)SH
23400 XM
(Loading complete.)SH
9500 17207 MT
(The)SH
/Times-Italic SF
11539 XM
(header)SH
/Times-Roman SF
14922 XM
(contains a magic number indicating MACH-O format.  It also)
54 W( contains other useful)55 W
9000 18855 MT
(information such)
274 W( as version information, and a machine-type specifier.  Finally, the header)273 W
9000 20503 MT
(specifies the type of file represented, e.g. executable, object file or shared library.)SH
9500 22975 MT
(The MACH-O program loader operates by)
185 W( scanning a load file and executing commands as)186 W
9000 24623 MT
(necessary. In)
535 W( the typical)
130 W( case, it uses the)129 W
/Times-Italic SF
28460 XM
(map_fd)SH
/Times-Roman SF
32164 XM
(call to map portions of files into its address)129 W
9000 26271 MT
(space. It)
459 W( then places the data in the image to be executed using the)92 W
/Times-Italic SF
40130 XM
(vm_write)SH
/Times-Roman SF
44591 XM
(operation. Since)461 W
9000 27919 MT
(copy-on-write is used at the base of the virtual memory primitives it is possible to share both)
2 W( code)1 W
9000 29567 MT
(and writable data.  Each task that writes data within a shared library will get a new copy)
52 W( as each)53 W
9000 31215 MT
(page is written for the first time.  Pages that are not written will be physically shared by all tasks.)SH
12 /Times-Bold AF
9000 34899 MT
(6.2. File Mapping and Standard I/O)SH
11 /Times-Roman AF
9500 36547 MT
(The Mach mapped file mechanism has been used to build a new version of the C library)248 W
9000 38195 MT
(buffered I/O package.  When a file is)39 W
/Times-Italic SF
25894 XM
(fopen)SH
/Times-Roman SF
(ed it is mapped in its entirety into the caller's address)40 W
9000 39843 MT
(space. The)
343 W( semantics)
34 W( of the buffered i/o package are not changed.  The existing stdio buffer has,)33 W
9000 41491 MT
(in effect, been enlarged to the size of the file.  When a)
89 W( write takes place only the data buffer is)90 W
9000 43139 MT
(changed. The)
331 W( file is not guaranteed to change on disk until a)28 W
/Times-Italic SF
36427 XM
(fflush)SH
/Times-Roman SF
39175 XM
(or)SH
/Times-Italic SF
40393 XM
(fclose)SH
/Times-Roman SF
43261 XM
(takes place. As with)27 W
9000 44787 MT
(normal buffered I/O, if two processes)
27 W( have the same file open for reading and writing, there is no)28 W
9000 46435 MT
(guarantee how the reads and writes will intermix.  A read may get new information off the disk)76 W
9000 48083 MT
(copy of the file, or it may use information that was already buffered.)SH
9500 50555 MT
(The primary rationale for this change is)
65 W( improved performance.  Table STDIOPERF shows the)66 W
9000 52203 MT
(time for simple buffered)
278 W( I/O operations both with and without the change.  In addition to)277 W
9000 53851 MT
(improved performance, the use of file mapping also has the effect of)
82 W( reducing the memory load)83 W
9000 55499 MT
(on the system.  In a traditional)
131 W( Unix implementation)130 W
/Times-Italic SF
33458 XM
(fopen)SH
/Times-Roman SF
36307 XM
(would allocate new memory to the)130 W
9000 57147 MT
(calling process and copy the data from the Unix)
17 W( buffer cache into that new memory at the time of)18 W
9000 58795 MT
(a)SH
/Times-Italic SF
9862 XM
(read)SH
/Times-Roman SF
(. Using)
473 W( this new package and Mach file mapping, each new call to)99 W
/Times-Italic SF
43014 XM
(fopen)SH
/Times-Roman SF
45832 XM
(will reuse any)99 W
9000 60443 MT
(physical memory containing file data pages, reducing the number of I/O operations.  \050See table)101 W
9000 62091 MT
(STDIOPERF2.\051)SH
9500 64563 MT
(In addition to traditional buffered I/O calls, the mapped file version of buffered I/O has)
167 W( had)166 W
9000 66211 MT
(added to it a new call which allows)
9 W( an application program to directly access the mapped file data)10 W
9000 67859 MT
(and thus further improve performance by eliminating the copying of data by)151 W
/Times-Italic SF
44596 XM
(fread)SH
/Times-Roman SF
47344 XM
(and)SH
/Times-Italic SF
49357 XM
(fwrite)SH
/Times-Roman SF
(.)SH
9000 69507 MT
(The new routine is called)SH
/Times-Italic SF
20393 XM
(fmap)SH
/Times-Roman SF
22868 XM
(and is a buffered I/O compatible version of)SH
/Times-Italic SF
42110 XM
(map_fd)SH
/Times-Roman SF
(.)SH
9500 71979 MT
(To read map a file with fmap the user calls:)SH
10 SS 
30350 75600 MT
(9)SH
ES
%%Page: 10 11
BS
36000 50 12600 7400 UL
0 SI
11 /Times-Bold AF
18807 8839 MT
(Unmapped vs. Mapped Buffered I/O Performance)SH
35500 50 13100 10013 UL
9 /Times-Roman AF
16200 10592 MT
(Test program)SH
27000 XM
(First time)SH
37800 XM
(Second time)SH
27000 11443 MT
(user system elapsed)SH
37800 XM
(user system elapsed)SH
36000 50 12600 12494 UL
16200 13073 MT
(old_read)SH
27000 XM
(6.1u 0.62s 0:08)450 W
37800 XM
(6.1u 0.62s 0:08)450 W
16200 14775 MT
(new_read)SH
27000 XM
(6.0u 0.71s 0:08)450 W
37800 XM
(6.0u 0.21s 0:06)450 W
16200 16477 MT
(map_read)SH
27000 XM
(2.8u 0.76s 0:04)450 W
37800 XM
(2.7u 0.17s 0:03)450 W
36000 50 12600 17528 UL
11 /Times-Bold AF
27926 18990 MT
(Table 6-1:)SH
9 /Times-Roman AF
21375 20604 MT
(Time to read a 492544 byte file using standard I/O.)SH
23712 21455 MT
(\050Mach, 4K file system, MicroVAX II\051)SH
20275 23157 MT
(old_read performs)SH
/Times-Italic SF
27125 XM
(fopen)SH
/Times-Roman SF
29350 XM
(followed by a loop of getc calls.)SH
18900 24008 MT
(new_read is identical to old_read with new mapped file package.)SH
19050 24859 MT
(map_read uses fmap and reads data by array reference, not getc.)SH
36000 50 12600 26707 UL
36000 50 12600 27903 UL
11 /Times-Bold AF
21742 29342 MT
(Multiple Access File I/O Performance)SH
35500 50 13100 30516 UL
9 /Times-Roman AF
18000 31095 MT
(Test program)SH
32400 XM
(user system)
900 W( elapsed I/O)225 W
36000 50 12600 32146 UL
18000 32725 MT
(old_read[1])SH
32400 XM
(25.4u 1.8s 1:23)
900 W( 217io)450 W
18000 33576 MT
(old_read[2])SH
32400 XM
(25.3u 2.0s 1:26)
900 W( 326io)450 W
18000 34427 MT
(old_read[3])SH
32400 XM
(25.1u 2.2s 1:26)
900 W( 439io)450 W
18000 36129 MT
(new_read[1])SH
32400 XM
(24.0u 1.6s 1:17)
900 W( 89io)675 W
18000 36980 MT
(new_read[2])SH
32400 XM
(24.0u 1.8s 1:18)
900 W( 194io)450 W
18000 37831 MT
(new_read[3])SH
32400 XM
(24.2u 1.6s 1:18)
900 W( 197io)450 W
36000 50 12600 38882 UL
11 /Times-Bold AF
27926 40344 MT
(Table 6-2:)SH
9 /Times-Roman AF
21150 41958 MT
(Time to read a 1970176 byte file using standard I/O.)SH
23712 42809 MT
(\050Mach, 4K file system, MicroVAX II\051)SH
19875 44511 MT
(Each program is run 3 times in parallel and times are listed.)SH
21000 45362 MT
(Instance numbers for each invocation are in brackets.)SH
21062 46213 MT
(Each program accesses the same file simultaneously.)SH
20275 47915 MT
(old_read performs)SH
/Times-Italic SF
27125 XM
(fopen)SH
/Times-Roman SF
29350 XM
(followed by a loop of getc calls.)SH
18900 48766 MT
(new_read is identical to old_read with new mapped file package.)SH
36000 50 12600 50614 UL
11 /Courier AF
11640 52296 MT
(stream = fopen\050"filename", "r"\051;  /* existing call */)SH
11640 53410 MT
(data = fmap\050stream, size\051;)
SH( /*)
4620 W( new call */)SH
/Times-Roman SF
9000 55058 MT
(where data is)
115 W( a pointer to a region of virtual memory where the file contents are buffered, and)116 W
9000 56706 MT
(size is the)
145 W( suggested size for the data buffer; if that size is zero, then the implementation will)144 W
9000 58354 MT
(choose a suitable size. As before,)SH
/Courier SF
11640 59880 MT
(bufsize = fbufsize\050stream\051)SH
/Times-Roman SF
9000 61528 MT
(returns the actual size of the buffer.  Once)15 W
/Times-Italic SF
27885 XM
(fmap)SH
/Times-Roman SF
30375 XM
(is called,)
15 W( the user can reference file data by using)16 W
9000 63176 MT
(the data pointer)
110 W( and any offset less than bufsize.  The user may also mix)109 W
/Times-Italic SF
42649 XM
(fseek)SH
/Times-Roman SF
(, and)109 W
/Times-Italic SF
47478 XM
(fread)SH
/Times-Roman SF
50184 XM
(calls)SH
9000 64824 MT
(with direct data references.  Once the user is finished with the file the call)SH
/Courier SF
11640 66350 MT
(fclose\050stream\051; /*)
10560 W( existing call */)SH
/Times-Roman SF
9000 67998 MT
(should be used to deallocate the virtual address space used by the mapped file.)SH
9500 70470 MT
(To write map a file the user would:)SH
10 SS 
30100 75600 MT
(10)SH
ES
%%Page: 11 12
BS
0 SI
11 /Courier AF
11640 7886 MT
(stream = fopen\050"filename", "w"\051;/* existing call */)SH
11640 9000 MT
(data = fmap\050stream, size\051;)
SH( /*)
3300 W( new call */)SH
/Times-Roman SF
9000 10648 MT
(where size is used as an initial buffer size; if that size is zero, the implementation)
80 W( will choose a)81 W
9000 12296 MT
(suitable size.  Initially, the buffer will be zero-filled.  Once)14 W
/Times-Italic SF
35300 XM
(fmap)SH
/Times-Roman SF
37789 XM
(is called, the user may write into)14 W
9000 13944 MT
(any part)
50 W( of the file with an offset less than bufsize.  An)51 W
/Times-Italic SF
34098 XM
(fwrite)SH
/Times-Roman SF
36992 XM
(or)SH
/Times-Italic SF
38234 XM
(fseek)SH
/Times-Roman SF
40758 XM
(call with an offset greater)51 W
9000 15592 MT
(than bufsize will cause an)
83 W( error. To expand the buffer size, the user may call)82 W
/Times-Italic SF
44103 XM
(fmap)SH
/Times-Roman SF
46660 XM
(again with a)82 W
9000 17240 MT
(larger size parameter.  The calls)SH
/Courier SF
11640 18766 MT
(fflush\050stream\051; /*)
10560 W( existing call */)SH
11640 19880 MT
(fclose\050stream\051; /*)
10560 W( existing call */)SH
/Times-Roman SF
9000 21528 MT
(continue to work as before. Similarly, files opened for append and read/write may be)SH
/Times-Italic SF
46630 XM
(fmap)SH
/Times-Roman SF
(ed.)SH
9500 24000 MT
(Table STDIOPERF3 shows)
73 W( the time advantage which can be gained by using fmap rather than)74 W
9000 25648 MT
(conventional I/O.)SH
36000 50 12600 27044 UL
/Times-Bold SF
18807 28483 MT
(Unmapped vs. Mapped Buffered I/O Performance)SH
35500 50 13100 29657 UL
9 /Times-Roman AF
16200 30236 MT
(Test program)SH
27000 XM
(First time)SH
37800 XM
(Second time)SH
27000 31087 MT
(user system elapsed  I/O)SH
37800 XM
(user system elapsed I/O)SH
36000 50 12600 32138 UL
16200 32717 MT
(old_read)SH
27000 XM
(11.5u 3.1s  0:21 481io)225 W
37800 XM
(11.5u 3.0s  0:21 482io)225 W
16200 34419 MT
(map_read)SH
27000 XM
(11.2u 2.9s  0:15 480io)225 W
37800 XM
(11.0u 0.9s  0:12  0io)225 W
36000 50 12600 35470 UL
11 /Times-Bold AF
27926 36932 MT
(Table 6-3:)SH
9 /Times-Roman AF
24587 38546 MT
(Time to read a 1970176 byte file.)SH
23712 39397 MT
(\050Mach, 4K file system, MicroVAX II\051)SH
19625 41099 MT
(old_read performs)SH
/Times-Italic SF
26475 XM
(open)SH
/Times-Roman SF
(,)SH
/Times-Italic SF
28675 XM
(malloc)SH
/Times-Roman SF
(s buffer, calls)SH
/Times-Italic SF
36225 XM
(read)SH
/Times-Roman SF
38100 XM
(for whole)SH
22925 41950 MT
(file and then reads data by array reference.)SH
18862 43652 MT
(map_read uses)SH
/Times-Italic SF
24412 XM
(fopen)SH
/Times-Roman SF
26637 XM
(and)SH
/Times-Italic SF
28162 XM
(fmap)SH
/Times-Roman SF
30187 XM
(and reads data by array reference.)SH
36000 50 12600 45500 UL
13 /Times-Bold AF
9000 49051 MT
(7. The Effect of Mach Memory Mapping on Performance)SH
9 /Times-Roman AF
21446 50336 MT
(2)SH
11 SS 
9500 50699 MT
(File mapping is hardly free)8 W
21896 XM
(. Even when a page)
8 W( is already in physical memory, a page fault must)7 W
9000 52347 MT
(be taken)
362 W( on the first process access to validate the corresponding hardware map entries.)363 W
9000 53995 MT
(Currently such a)
154 W( fault takes approximately 1.0-1.4 milliseconds on a MicroVAX II with a 4K)153 W
9000 55643 MT
(page size.  There are also several ways in which mapped files can adversely affect performance:)SH
/Symbol SF
10969 57221 MT
(\267)SH
/Times-Roman SF
11750 XM
(If the file to be mapped)
32 W( is smaller than a single page, file mapping will always result)33 W
11750 58417 MT
(in a full page being allocated in physical memory with excess data filled with zeroes.)SH
/Symbol SF
10969 60311 MT
(\267)SH
/Times-Roman SF
11750 XM
(Mapped files compete with program text and data for)
297 W( physical memory.  In a)296 W
11750 61507 MT
(traditional Unix system, user programs maintain a fixed-size buffer, so)
181 W( the buffer)182 W
11750 62703 MT
(cache limits the amount of memory which can be consumed in accessing a file.)SH
9500 65175 MT
(Nevertheless, as the performance)
41 W( of the new standard I/O library points out, useful performance)40 W
9000 66823 MT
(gains can be achieved using Mach memory mapping.  In fact, because the)
186 W( Mach kernel uses)187 W
10800 50 9000 70172 UL
7 SS 
9900 71673 MT
(2)SH
9 SS 
10250 72000 MT
(Unless the output is going to /dev/null!)SH
10 SS 
30100 75600 MT
(11)SH
ES
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0 SI
11 /Times-Roman AF
9000 7955 MT
(mapped files internally to implement)102 W
/Times-Italic SF
26039 XM
(exec)SH
/Times-Roman SF
(, overall performance of)
102 W( vanilla 4.3 BSD programs is)101 W
9000 9603 MT
(often improved when run on Mach. Particularly dramatic performance gains are seen on)
1 W( machines)2 W
9000 11251 MT
(where the processor speed is high, memory is plentiful and)
121 W( disk is a bottleneck.  For example,)120 W
9000 12899 MT
(performance gains of over 20% have been achieved on)
12 W( a VAX 8650.  Improvement can be found,)13 W
9000 14547 MT
(however, even on small memory systems with moderately heavy loads.  The multiuser)428 W
9000 16195 MT
(benchmark load used to study)
504 W( the performance of the CMU ITC VICE/VIRTUE file)505 W
9000 17843 MT
(system [SATYA85])
SH( ran 10-15% faster under Mach than a comparable BSD derived kernel on an)48 W
9000 19491 MT
(IBM RT PC with 4 megabytes of memory.)SH
9500 21963 MT
(These performance improvements are especially surprising)
363 W( because many basic operating)364 W
9000 23611 MT
(system overheads are actually larger in Mach than in 4.3 BSD.)
211 W( The)
695 W( use of special purpose)210 W
9000 25259 MT
(scheduling instructions has, for example, been eliminated in the VAX version of)
177 W( Mach.  The)178 W
9000 26907 MT
(Mach equivalent of the Unix u-area is not at)
113 W( a fixed address so as to allow multiple threads of)112 W
9000 28555 MT
(control. This increases the cost of task and thread data structure)
951 W( references)952 W
9000 30203 MT
(significantly [MACH-THREADS].)
SH( In)
599 W( addition,)
162 W( VAX Mach is run as a multiprocessor system)161 W
9000 31851 MT
(even on uniprocessors at CMU, so virtually all)
SH( kernel operations have had their costs increased by)1 W
9000 33499 MT
(locking concerns.)SH
13 /Times-Bold AF
9000 37250 MT
(8. Conclusion)SH
11 /Times-Roman AF
9500 38898 MT
(Mach's basic memory primitives provide applications with several mechanisms for sharing)256 W
9000 40546 MT
(memory. As)
319 W( such, a mapped file interface under Mach is not)
22 W( required for shared memory.  Mach)23 W
9000 42194 MT
(does provide a non-shared interface for mapped files.)
91 W( This)
456 W( interface is not only appropriate for)90 W
9000 43842 MT
(implementing various)
72 W( applications \050e.g. shared libraries and program loading\051, but has increased)73 W
9000 45490 MT
(both the performance and functionality of the system.)SH
9500 47962 MT
(The internal implementation of Mach VM)
176 W( does not preclude shared read/write file mapping.)175 W
9000 49610 MT
(Mach does, in fact, support the 4.2)2 W
/Times-Italic SF
24532 XM
(mmap)SH
/Times-Roman SF
27497 XM
(call for)
2 W( the purposes of mapping special device memory)3 W
9000 51258 MT
(\050typically used for frame buffers\051.  The)9 W
/Times-Italic SF
26531 XM
(mmap)SH
/Times-Roman SF
29503 XM
(call will also work on normal files but)
9 W( will not map)8 W
9000 52906 MT
(files shared between processes.  This restriction was not based on technical issues, but was an)133 W
9000 54554 MT
(intentional modification)
156 W( of the)155 W
/Times-Italic SF
23309 XM
(mmap)SH
/Times-Roman SF
26427 XM
(semantics. The)
585 W( Mach designers felt it was important to)155 W
9000 56202 MT
(discourage programmers from writing programs which depended)
46 W( on sharing data which might or)47 W
9000 57850 MT
(might not be consistently maintained in a loosely coupled environment.)SH
13 /Times-Bold AF
9000 61601 MT
(9. Acknowledgements)SH
11 /Times-Roman AF
9500 63249 MT
(The Mach VM implementation)
232 W( was done primarily by Avie Tevanian, Michael Young and)231 W
9000 64897 MT
(David Golub.  The implementation has been ported five different)
106 W( machine types and has yet to)107 W
9000 66545 MT
(need modification to accommodate new architectures.)SH
9500 69017 MT
(The authors would also like to acknowledge)
140 W( others who have contributed to the Mach kernel)139 W
9000 70665 MT
(including Mike Accetta, Robert Baron, David Black, Jonathan Chew,)
163 W( Eric Cooper, Dan Julin,)164 W
10 SS 
30100 75600 MT
(12)SH
ES
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0 SI
11 /Times-Roman AF
9000 7955 MT
(Glenn Marcy and Robert Sansom.  Bob Beck \050of Sequent\051 and Charlie Hill \050of)
78 W( North American)77 W
9000 9603 MT
(Philips\051 helped with the)
196 W( port to the Balance 21000.  Fred Olivera and Jim Van Sciver \050both)197 W
9000 11251 MT
(formerly of Encore\051 helped with the port to the MultiMax.)SH
10 SS 
30100 75600 MT
(13)SH
ES
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14 /Times-Bold AF
27334 8138 MT
(References)SH
11 /Times-Roman AF
9000 10695 MT
([1])SH
12300 XM
(Mike Accetta, Robert Baron, William Bolosky, David Golub, Richard Rashid, Avadis)SH
12300 11891 MT
(Tevanian, Michael Young.)SH
12300 13087 MT
(Mach: A New Kernel Foundation for UNIX Development.)SH
12300 14283 MT
(In)SH
/Times-Italic SF
13491 XM
(Proceedings of Summer Usenix)SH
/Times-Roman SF
(. July,)
275 W( 1986.)SH
9000 16234 MT
([2])SH
12300 XM
(Sequent Computer Systems, Inc.)SH
/Times-Italic SF
12300 17430 MT
(Dynix Programmer's Manual)SH
/Times-Roman SF
12300 18626 MT
(Sequent Computer Systems, Inc., 1986.)SH
9000 20577 MT
([3])SH
12300 XM
(Encore Computer Corporation.)SH
/Times-Italic SF
12300 21773 MT
(UMAX 4.2 Programmer's Reference Manual)SH
/Times-Roman SF
12300 22969 MT
(Encore Computer Corporation, 1986.)SH
9000 24920 MT
([4])SH
12300 XM
(Leach, P.L., P.H. Levine, B.P. Douros, J.A.  Hamilton, D.L. Nelson and B.L. Stumpf.)SH
12300 26116 MT
(The Architecture of an Integrated Local Network.)SH
/Times-Italic SF
12300 27312 MT
(IEEE Journal on Selected Areas in Communications)SH
/Times-Roman SF
35671 XM
(SAC-1\0505\051:842-857, November, 1983.)SH
9000 29263 MT
([5])SH
12300 XM
(Rashid, R., Tevanian, A., Young, M., Golub, D., Baron, R., Black, D., Bolosksy, W. and)SH
12300 30459 MT
(Chew, J.)SH
/Times-Italic SF
12300 31655 MT
(Machine-Independent Virtual Memory Management for Paged Uniprocessor and)SH
13950 32851 MT
(Multiprocessor Architectures)SH
/Times-Roman SF
(.)SH
12300 34047 MT
(Technical Report , Carnegie-Mellon University, February, 1987.)SH
9000 35998 MT
([6])SH
12300 XM
(Satyanarayanan, M., et.al.)SH
12300 37194 MT
(The ITC Distributed File System: Principles and Design.)SH
12300 38390 MT
(In)SH
/Times-Italic SF
13491 XM
(Proc. 10th Symposium on Operating Systems Principles)SH
/Times-Roman SF
(, pages 35-50.  ACM,)SH
13950 39586 MT
(December, 1985.)SH
9000 41537 MT
([7])SH
12300 XM
(Alfred Z. Spector, Dean S. Daniels, Daniel J. Duchamp, Jeffrey L. Eppinger, Randy)SH
12300 42733 MT
(Pausch.)SH
12300 43929 MT
(Distributed Transactions for Reliable Systems.)SH
12300 45125 MT
(In)SH
/Times-Italic SF
13491 XM
(Proceedings of the Tenth Symposium on Operating System Principles)SH
/Times-Roman SF
(, pages 127-146.)SH
13950 46321 MT
(ACM, December, 1985.)SH
12300 47517 MT
(Also available in)SH
/Times-Italic SF
20029 XM
(Concurrency Control and Reliability in Distributed Systems)SH
/Times-Roman SF
(, Van)SH
13950 48713 MT
(Nostrand Reinhold Company, New York, and as Technical Report CMU-CS-85-117,)SH
13950 49909 MT
(Carnegie-Mellon University, September 1985.)SH
9000 51860 MT
([8])SH
12300 XM
(Young, M., Tevanian, A., Rashid, R., Golub, D., Eppinger, J., Chew, J., Bolosky, W.,)SH
12300 53056 MT
(Black, D. and Baron, R.)SH
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(The Duality of Memory and Communication in the Implementation of a Multiprocessor)SH
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(Operating System)SH
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(Technical Report , Carnegie-Mellon University, February, 1987.)SH
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(Tevanian, A., Rashid, R., Golub, D., Black, D., Cooper, E., and Young, M.)SH
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(Mach Threads and the UNIX kernel:  The Battle for Control)SH
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(Technical Report , Carnegie-Mellon University, April, 1987.)SH
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(Table of Contents)SH
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(1. Introduction)SH
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(2. The Problems of a Mapped File Interface)SH
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(3. The Uses of Shared Memory)SH
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(3.1. Fine grain multiprocessing)SH
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(3.2. Fast IPC)SH
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(3.3. Database management)SH
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(3.4. Efficient file access)SH
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(4. Mach Memory Primitives)SH
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(4.1. Mach virtual memory operations)SH
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(4.2. Managing external pagers)SH
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(5. A Unix Interface for File Mapping)SH
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(6. Uses of Mapped Files in Mach)SH
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(6.1. File Mapping and Shared Libraries)SH
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(6.2. File Mapping and Standard I/O)SH
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(7. The Effect of Mach Memory Mapping on Performance)SH
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(8. Conclusion)SH
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(9. Acknowledgements)SH
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(List of Tables)SH
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(Table 4-1:)SH
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(Table 4-2:)SH
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(Table 4-3:)SH
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(Table 6-1:)SH
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(Table 6-2:)SH
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(Table 6-3:)SH
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