CacheLocality.cpp

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/*
 * Copyright 2013-present Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <folly/concurrency/CacheLocality.h>

#ifndef _MSC_VER
#define _GNU_SOURCE 1 // for RTLD_NOLOAD
#include <dlfcn.h>
#endif
#include <fstream>

#include <folly/Conv.h>
#include <folly/Exception.h>
#include <folly/FileUtil.h>
#include <folly/Format.h>
#include <folly/ScopeGuard.h>

namespace folly {


static CacheLocality getSystemLocalityInfo() {
  if (kIsLinux) {
    try {
      return CacheLocality::readFromSysfs();
    } catch (...) {
      // keep trying
    }
  }

  long numCpus = sysconf(_SC_NPROCESSORS_CONF);
  if (numCpus <= 0) {
    // This shouldn't happen, but if it does we should try to keep
    // going.  We are probably not going to be able to parse /sys on
    // this box either (although we will try), which means we are going
    // to fall back to the SequentialThreadId splitter.  On my 16 core
    // (x hyperthreading) dev box 16 stripes is enough to get pretty good
    // contention avoidance with SequentialThreadId, and there is little
    // improvement from going from 32 to 64.  This default gives us some
    // wiggle room
    numCpus = 32;
  }
  return CacheLocality::uniform(size_t(numCpus));
}

template <>
const CacheLocality& CacheLocality::system<std::atomic>() {
  static auto* cache = new CacheLocality(getSystemLocalityInfo());
  return *cache;
}

// Each level of cache has sharing sets, which are the set of cpus
// that share a common cache at that level.  These are available in a
// hex bitset form (/sys/devices/system/cpu/cpu0/index0/shared_cpu_map,
// for example).  They are also available in a human-readable list form,
// as in /sys/devices/system/cpu/cpu0/index0/shared_cpu_list.  The list
// is a comma-separated list of numbers and ranges, where the ranges are
// a pair of decimal numbers separated by a '-'.
//
// To sort the cpus for optimum locality we don't really need to parse
// the sharing sets, we just need a unique representative from the
// equivalence class.  The smallest value works fine, and happens to be
// the first decimal number in the file.  We load all of the equivalence
// class information from all of the cpu*/index* directories, order the
// cpus first by increasing last-level cache equivalence class, then by
// the smaller caches.  Finally, we break ties with the cpu number itself.

static size_t parseLeadingNumber(const std::string& line) {
  auto raw = line.c_str();
  char* end;
  unsigned long val = strtoul(raw, &end, 10);
  if (end == raw || (*end != ',' && *end != '-' && *end != '\n' && *end != 0)) {
    throw std::runtime_error(
        to<std::string>("error parsing list '", line, "'").c_str());
  }
  return val;
}

CacheLocality CacheLocality::readFromSysfsTree(
    const std::function<std::string(std::string)>& mapping) {
  // number of equivalence classes per level
  std::vector<size_t> numCachesByLevel;

  // the list of cache equivalence classes, where equivalance classes
  // are named by the smallest cpu in the class
  std::vector<std::vector<size_t>> equivClassesByCpu;

  std::vector<size_t> cpus;

  while (true) {
    auto cpu = cpus.size();
    std::vector<size_t> levels;
    for (size_t index = 0;; ++index) {
      auto dir =
          sformat("/sys/devices/system/cpu/cpu{}/cache/index{}/", cpu, index);
      auto cacheType = mapping(dir + "type");
      auto equivStr = mapping(dir + "shared_cpu_list");
      if (cacheType.size() == 0 || equivStr.size() == 0) {
        // no more caches
        break;
      }
      if (cacheType[0] == 'I') {
        // cacheType in { "Data", "Instruction", "Unified" }. skip icache
        continue;
      }
      auto equiv = parseLeadingNumber(equivStr);
      auto level = levels.size();
      levels.push_back(equiv);

      if (equiv == cpu) {
        // we only want to count the equiv classes once, so we do it when
        // we first encounter them
        while (numCachesByLevel.size() <= level) {
          numCachesByLevel.push_back(0);
        }
        numCachesByLevel[level]++;
      }
    }

    if (levels.size() == 0) {
      // no levels at all for this cpu, we must be done
      break;
    }
    equivClassesByCpu.emplace_back(std::move(levels));
    cpus.push_back(cpu);
  }

  if (cpus.size() == 0) {
    throw std::runtime_error("unable to load cache sharing info");
  }

  std::sort(cpus.begin(), cpus.end(), [&](size_t lhs, size_t rhs) -> bool {
    // sort first by equiv class of cache with highest index,
    // direction doesn't matter.  If different cpus have
    // different numbers of caches then this code might produce
    // a sub-optimal ordering, but it won't crash
    auto& lhsEquiv = equivClassesByCpu[lhs];
    auto& rhsEquiv = equivClassesByCpu[rhs];
    for (ssize_t i = ssize_t(std::min(lhsEquiv.size(), rhsEquiv.size())) - 1;
         i >= 0;
         --i) {
      auto idx = size_t(i);
      if (lhsEquiv[idx] != rhsEquiv[idx]) {
        return lhsEquiv[idx] < rhsEquiv[idx];
      }
    }

    // break ties deterministically by cpu
    return lhs < rhs;
  });

  // the cpus are now sorted by locality, with neighboring entries closer
  // to each other than entries that are far away.  For striping we want
  // the inverse map, since we are starting with the cpu
  std::vector<size_t> indexes(cpus.size());
  for (size_t i = 0; i < cpus.size(); ++i) {
    indexes[cpus[i]] = i;
  }

  return CacheLocality{
      cpus.size(), std::move(numCachesByLevel), std::move(indexes)};
}

CacheLocality CacheLocality::readFromSysfs() {
  return readFromSysfsTree([](std::string name) {
    std::ifstream xi(name.c_str());
    std::string rv;
    std::getline(xi, rv);
    return rv;
  });
}

CacheLocality CacheLocality::uniform(size_t numCpus) {
  CacheLocality rv;

  rv.numCpus = numCpus;

  // one cache shared by all cpus
  rv.numCachesByLevel.push_back(numCpus);

  // no permutations in locality index mapping
  for (size_t cpu = 0; cpu < numCpus; ++cpu) {
    rv.localityIndexByCpu.push_back(cpu);
  }

  return rv;
}


Getcpu::Func Getcpu::resolveVdsoFunc() {
#if !FOLLY_HAVE_LINUX_VDSO
  return nullptr;
#else
  void* h = dlopen("linux-vdso.so.1", RTLD_LAZY | RTLD_LOCAL | RTLD_NOLOAD);
  if (h == nullptr) {
    return nullptr;
  }

  auto func = Getcpu::Func(dlsym(h, "__vdso_getcpu"));
  if (func == nullptr) {
    // technically a null result could either be a failure or a successful
    // lookup of a symbol with the null value, but the second can't actually
    // happen for this symbol.  No point holding the handle forever if
    // we don't need the code
    dlclose(h);
  }

  return func;
#endif
}

#ifdef FOLLY_TLS
template struct SequentialThreadId<std::atomic>;
#endif

template struct AccessSpreader<std::atomic>;

SimpleAllocator::SimpleAllocator(size_t allocSize, size_t sz)
    : allocSize_{allocSize}, sz_(sz) {}

SimpleAllocator::~SimpleAllocator() {
  std::lock_guard<std::mutex> g(m_);
  for (auto& block : blocks_) {
    folly::aligned_free(block);
  }
}

void* SimpleAllocator::allocateHard() {
  // Allocate a new slab.
  mem_ = static_cast<uint8_t*>(folly::aligned_malloc(allocSize_, allocSize_));
  if (!mem_) {
    throw_exception<std::bad_alloc>();
  }
  end_ = mem_ + allocSize_;
  blocks_.push_back(mem_);

  // Install a pointer to ourselves as the allocator.
  *reinterpret_cast<SimpleAllocator**>(mem_) = this;
  static_assert(max_align_v >= sizeof(SimpleAllocator*), "alignment too small");
  mem_ += std::min(sz_, max_align_v);

  // New allocation.
  auto mem = mem_;
  mem_ += sz_;
  assert(intptr_t(mem) % 128 != 0);
  return mem;
}

} // namespace folly