---
name: dotnet-gc-memory
description: >-
  Tunes GC and memory. GC modes, LOH/POH, Gen0/1/2, Span/Memory deep patterns,
  ArrayPool.
metadata:
  short-description: .NET skill guidance for foundation tasks
---
# dotnet-gc-memory

Garbage collection and memory management for .NET applications. Covers GC modes (workstation vs server, concurrent vs
non-concurrent), Large Object Heap (LOH) and Pinned Object Heap (POH), generational tuning (Gen0/1/2), memory pressure
notifications, deep Span<T>/Memory<T> ownership patterns beyond basics, buffer pooling with ArrayPool<T> and
MemoryPool<T>, weak references, finalizers vs IDisposable, and memory profiling with dotMemory and PerfView.

## Scope

- GC modes (workstation vs server, concurrent vs non-concurrent)
- Large Object Heap (LOH) and Pinned Object Heap (POH)
- Generational tuning (Gen0/1/2) and memory pressure
- Deep Span<T>/Memory<T> ownership patterns
- Buffer pooling with ArrayPool<T> and MemoryPool<T>
- Memory profiling with dotMemory and PerfView

## Out of scope

- Span<T>/Memory<T> syntax introduction and basic usage -- see [skill:dotnet-performance-patterns]
- Microbenchmarking setup -- see [skill:dotnet-benchmarkdotnet]
- CLI diagnostic tools (dotnet-counters, dotnet-trace, dotnet-dump) -- see [skill:dotnet-profiling]
- Channel<T> producer/consumer patterns -- see [skill:dotnet-channels]

Cross-references: [skill:dotnet-performance-patterns] for Span<T>/Memory<T> basics and sealed devirtualization,
[skill:dotnet-profiling] for runtime diagnostic tools (dotnet-counters, dotnet-trace, dotnet-dump),
[skill:dotnet-channels] for backpressure patterns that interact with memory management, [skill:dotnet-file-io] for
MemoryMappedFile usage and POH buffer patterns in file I/O.

---

## GC Modes and Configuration

### Workstation vs Server GC

| Aspect            | Workstation           | Server                       |
| ----------------- | --------------------- | ---------------------------- |
| **GC threads**    | Single thread         | One thread per logical core  |
| **Heap segments** | Single heap           | One heap per core            |
| **Pause latency** | Lower                 | Higher (more memory scanned) |
| **Throughput**    | Lower                 | Higher                       |
| **Default for**   | Console apps, desktop | ASP.NET Core web apps        |

````xml

<!-- In the .csproj file -->
<PropertyGroup>
  <ServerGarbageCollection>true</ServerGarbageCollection>
</PropertyGroup>

```csharp

```json

// Or in runtimeconfig.json
{
  "runtimeOptions": {
    "configProperties": {
      "System.GC.Server": true
    }
  }
}

```text

### Concurrent vs Non-Concurrent GC

| Mode                     | Behavior                                           | Use when                             |
| ------------------------ | -------------------------------------------------- | ------------------------------------ |
| **Concurrent** (default) | Gen2 collection runs alongside application threads | Latency-sensitive (web APIs, UI)     |
| **Non-concurrent**       | Application threads pause during Gen2 collection   | Maximum throughput, batch processing |

```json

{
  "runtimeOptions": {
    "configProperties": {
      "System.GC.Concurrent": true
    }
  }
}

```text

### DATAS (Dynamic Adaptation to Application Sizes) -- .NET 8+

DATAS dynamically adjusts GC heap size based on application memory usage patterns. It is enabled by default in .NET 8+
Server GC mode. DATAS reduces memory footprint for applications with variable load by shrinking the heap during
low-activity periods.

```json

{
  "runtimeOptions": {
    "configProperties": {
      "System.GC.DynamicAdaptationMode": 1
    }
  }
}

```text

Set to `0` to disable DATAS if you observe excessive GC frequency in steady-state workloads.

### GC Regions -- .NET 7+

Regions replace the older segment-based heap management. Each region is a small, fixed-size block of memory that the GC
can allocate and free independently. Regions are enabled by default in .NET 7+ and improve:

- Memory return to the OS after usage spikes
- Heap compaction efficiency
- Server GC scalability on high-core-count machines

No configuration is needed -- regions are the default. To revert to segments (rarely needed):

```json

{
  "runtimeOptions": {
    "configProperties": {
      "System.GC.Regions": false
    }
  }
}

```text

---

## Generational GC (Gen0/1/2)

### How Generations Work

| Generation | Contains                | Collection frequency         | Collection cost            |
| ---------- | ----------------------- | ---------------------------- | -------------------------- |
| **Gen0**   | Newly allocated objects | Very frequent (milliseconds) | Very cheap (small heap)    |
| **Gen1**   | Objects surviving Gen0  | Frequent                     | Cheap                      |
| **Gen2**   | Long-lived objects      | Infrequent                   | Expensive (full heap scan) |

Objects promote from Gen0 to Gen1 to Gen2 as they survive collections. The GC budget for Gen0 is tuned dynamically --
when Gen0 fills, a Gen0 collection triggers.

### Tuning Principles

1. **Minimize Gen0 allocation rate** -- reduce temporary object creation on hot paths. Every allocation contributes to
   Gen0 pressure.
2. **Avoid mid-life crisis** -- objects that live just long enough to promote to Gen1/Gen2 but then become garbage are
   the most expensive. They survive cheap Gen0 collections and require expensive Gen2 collections to reclaim.
3. **Reduce Gen2 collection frequency** -- Gen2 collections cause the longest pauses. Use object pooling, Span<T>, and
   value types to keep long-lived heap allocations low.

### Monitoring Generations

```bash

# Real-time GC metrics
dotnet-counters monitor --process-id <PID> \
  --counters System.Runtime[gen-0-gc-count,gen-1-gc-count,gen-2-gc-count,gc-heap-size]

```bash

```csharp

// Programmatic GC observation
var gen0 = GC.CollectionCount(0);
var gen1 = GC.CollectionCount(1);
var gen2 = GC.CollectionCount(2);
var totalMemory = GC.GetTotalMemory(forceFullCollection: false);
var memoryInfo = GC.GetGCMemoryInfo();

logger.LogInformation(
    "GC: Gen0={Gen0} Gen1={Gen1} Gen2={Gen2} Heap={HeapMB:F1}MB",
    gen0, gen1, gen2, totalMemory / (1024.0 * 1024));

```text

---

## Large Object Heap (LOH) and Pinned Object Heap (POH)

### LOH

Objects >= 85,000 bytes are allocated on the LOH. LOH collections only happen during Gen2 collections, and by default
the LOH is not compacted (causing fragmentation).

```csharp

// Force LOH compaction (use sparingly -- expensive)
GCSettings.LargeObjectHeapCompactionMode =
    GCLargeObjectHeapCompactionMode.CompactOnce;
GC.Collect();

```text

### LOH Fragmentation Prevention

| Strategy                                   | Implementation                          |
| ------------------------------------------ | --------------------------------------- |
| **ArrayPool<T>** for large arrays          | `ArrayPool<byte>.Shared.Rent(100_000)`  |
| **MemoryPool<T>** for IMemoryOwner pattern | `MemoryPool<byte>.Shared.Rent(100_000)` |
| **Pre-allocate and reuse**                 | Create large buffers once at startup    |
| **Avoid frequent large string concat**     | Use `StringBuilder` or `string.Create`  |

### POH (Pinned Object Heap) -- .NET 5+

The POH is a dedicated heap for objects that must remain at a fixed memory address (pinned). Before .NET 5, pinning
objects on the regular heap prevented compaction. The POH isolates pinned objects so they do not block compaction of
Gen0/1/2 heaps.

```csharp

// Allocate on POH -- useful for I/O buffers passed to native code
byte[] buffer = GC.AllocateArray<byte>(4096, pinned: true);

// The buffer's address will not change, safe for native interop
// and overlapped I/O without explicit GCHandle pinning

```text

Use POH for:

- I/O buffers passed to native/unmanaged code
- Memory-mapped file backing arrays
- Buffers used with `Socket.ReceiveAsync` (overlapped I/O)

---

## Span<T>/Memory<T> Deep Ownership Patterns

See [skill:dotnet-performance-patterns] for Span<T>/Memory<T> introduction and basic slicing. This section covers
ownership semantics and lifetime management for shared buffers.

### IMemoryOwner<T> for Pooled Buffers

```csharp

// Rent from MemoryPool and manage lifetime with IDisposable
using IMemoryOwner<byte> owner = MemoryPool<byte>.Shared.Rent(4096);
Memory<byte> buffer = owner.Memory[..4096]; // Slice to exact size needed

// Pass the Memory<T> to async I/O
int bytesRead = await stream.ReadAsync(buffer, cancellationToken);
Memory<byte> data = buffer[..bytesRead];

// Process the data
await ProcessDataAsync(data, cancellationToken);
// owner.Dispose() returns the buffer to the pool

```text

### Ownership Transfer Pattern

When transferring buffer ownership between components, use `IMemoryOwner<T>` to make lifetime responsibility explicit:

```csharp

public sealed class MessageParser
{
    // Caller transfers ownership -- this method is responsible for disposal
    public async Task ProcessAsync(
        IMemoryOwner<byte> messageOwner,
        CancellationToken ct)
    {
        using (messageOwner)
        {
            Memory<byte> data = messageOwner.Memory;
            // Parse and process...
            await HandleMessageAsync(data, ct);
        }
        // Buffer returned to pool on dispose
    }
}

```text

### Span<T> Stack Discipline

```csharp

// Span<T> enforces stack-only usage (ref struct)
// These are compile-time errors:
// Span<byte> field;              // Cannot store in class/struct field
// async Task Foo(Span<byte> s);  // Cannot use in async method
// var list = new List<Span<byte>>(); // Cannot use as generic type argument

// When you need heap storage or async, use Memory<T> instead
public async Task ProcessAsync(Memory<byte> buffer, CancellationToken ct)
{
    // Can use Memory<T> in async methods
    int bytesRead = await stream.ReadAsync(buffer, ct);

    // Convert to Span<T> for synchronous processing within a method
    Span<byte> span = buffer.Span;
    ParseHeader(span[..bytesRead]);
}

```text

---

## ArrayPool<T> and MemoryPool<T>

### ArrayPool<T>

`ArrayPool<T>` reduces GC pressure by reusing array allocations. Always return rented arrays, and never assume the
returned array is exactly the requested size.

```csharp

// Rent and return pattern
byte[] buffer = ArrayPool<byte>.Shared.Rent(minimumLength: 4096);
try
{
    // IMPORTANT: Rented array may be larger than requested
    int bytesRead = await stream.ReadAsync(
        buffer.AsMemory(0, 4096), cancellationToken);
    ProcessData(buffer.AsSpan(0, bytesRead));
}
finally
{
    // clearArray: true when buffer contained sensitive data
    ArrayPool<byte>.Shared.Return(buffer, clearArray: false);
}

```text

### Custom Pool Sizing

```csharp

// Create a custom pool for specific allocation patterns
var pool = ArrayPool<byte>.Create(
    maxArrayLength: 1_048_576,  // 1 MB max array
    maxArraysPerBucket: 50);    // Keep up to 50 arrays per size bucket

// Use for workloads with predictable buffer sizes
byte[] buffer = pool.Rent(65_536);
try
{
    // Process...
}
finally
{
    pool.Return(buffer);
}

```text

### MemoryPool<T>

`MemoryPool<T>` wraps `ArrayPool<T>` and returns `IMemoryOwner<T>` for RAII-style lifetime management:

```csharp

// MemoryPool returns IMemoryOwner<T> -- dispose to return
using IMemoryOwner<byte> owner = MemoryPool<byte>.Shared.Rent(8192);
Memory<byte> buffer = owner.Memory;

// Slice to exact size (owner.Memory may be larger)
int bytesRead = await stream.ReadAsync(buffer[..8192], ct);
await ProcessAsync(buffer[..bytesRead], ct);
// Dispose returns the underlying array to the pool

```text

### Pool Usage Guidelines

| Guideline                                            | Rationale                                              |
| ---------------------------------------------------- | ------------------------------------------------------ |
| Always return rented buffers in `finally` or `using` | Leaked buffers defeat the purpose of pooling           |
| Slice to exact size before processing                | Rented arrays may be larger than requested             |
| Use `clearArray: true` for sensitive data            | Pool reuse could expose secrets to other consumers     |
| Do not cache rented arrays in long-lived fields      | Holds pool buffers indefinitely, reducing availability |
| Prefer `MemoryPool<T>` over raw `ArrayPool<T>`       | Disposal-based lifetime is harder to misuse            |

---

## Weak References and Caching

### WeakReference<T>

Weak references allow the GC to collect the target object when no strong references remain. Use for caches where
reclamation under memory pressure is acceptable.

```csharp

public sealed class ImageCache
{
    private readonly ConcurrentDictionary<string, WeakReference<byte[]>> _cache = new();

    public byte[]? TryGet(string key)
    {
        if (_cache.TryGetValue(key, out var weakRef)
            && weakRef.TryGetTarget(out var data))
        {
            return data;
        }
        return null;
    }

    public void Set(string key, byte[] data)
    {
        _cache[key] = new WeakReference<byte[]>(data);
    }

    // Periodically clean up dead references
    public void Purge()
    {
        foreach (var key in _cache.Keys)
        {
            if (_cache.TryGetValue(key, out var weakRef)
                && !weakRef.TryGetTarget(out _))
            {
                _cache.TryRemove(key, out _);
            }
        }
    }
}

```text

### When to Use Weak References

- Large object caches where memory pressure should trigger eviction
- Caches for expensive-to-compute but recreatable data (image thumbnails, rendered templates)
- Do NOT use for small objects -- the `WeakReference<T>` overhead outweighs the benefit

For most caching scenarios, prefer `MemoryCache` with size limits and expiration policies. Weak references are a last
resort when you need GC-driven eviction.

---

## Finalizers vs IDisposable

### IDisposable (Preferred)

Implement `IDisposable` to release unmanaged resources deterministically:

```csharp

public sealed class NativeBufferWrapper : IDisposable
{
    private IntPtr _handle;
    private bool _disposed;

    public NativeBufferWrapper(int size)
    {
        _handle = Marshal.AllocHGlobal(size);
    }

    public void Dispose()
    {
        if (_disposed) return;
        _disposed = true;

        Marshal.FreeHGlobal(_handle);
        _handle = IntPtr.Zero;
        // No GC.SuppressFinalize needed -- no finalizer
    }
}

```text

### Finalizer (Safety Net Only)

Finalizers run on the GC finalizer thread when an object is collected. They are a safety net for unmanaged resources
that were not disposed explicitly.

```csharp

public class UnmanagedResourceHolder : IDisposable
{
    private IntPtr _handle;
    private bool _disposed;

    public UnmanagedResourceHolder(int size)
    {
        _handle = Marshal.AllocHGlobal(size);
    }

    ~UnmanagedResourceHolder()
    {
        Dispose(disposing: false);
    }

    public void Dispose()
    {
        Dispose(disposing: true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (_disposed) return;
        _disposed = true;

        if (disposing)
        {
            // Free managed resources
        }

        // Free unmanaged resources
        if (_handle != IntPtr.Zero)
        {
            Marshal.FreeHGlobal(_handle);
            _handle = IntPtr.Zero;
        }
    }
}

```text

### Finalizer Costs

| Cost                                                  | Impact                                            |
| ----------------------------------------------------- | ------------------------------------------------- |
| Objects with finalizers survive at least one extra GC | Promotes to Gen1/Gen2, increasing memory pressure |
| Finalizer thread is single-threaded                   | Slow finalizers block all other finalization      |
| Execution order is non-deterministic                  | Cannot depend on other finalizable objects        |
| Not guaranteed to run on process exit                 | Critical cleanup may not execute                  |

**Rule:** Use `sealed` classes with `IDisposable` (no finalizer) unless you own unmanaged handles. Only add a finalizer
as a safety net for unmanaged resources.

---

## Memory Pressure Notifications

### GC.AddMemoryPressure / RemoveMemoryPressure

Inform the GC about unmanaged memory allocations so it accounts for them in collection decisions:

```csharp

public sealed class NativeImageBuffer : IDisposable
{
    private readonly IntPtr _buffer;
    private readonly long _size;
    private bool _disposed;

    public NativeImageBuffer(long sizeBytes)
    {
        _size = sizeBytes;
        _buffer = Marshal.AllocHGlobal((IntPtr)sizeBytes);
        GC.AddMemoryPressure(sizeBytes);
    }

    public void Dispose()
    {
        if (_disposed) return;
        _disposed = true;

        Marshal.FreeHGlobal(_buffer);
        GC.RemoveMemoryPressure(_size);
    }
}

```text

### GC.GetGCMemoryInfo for Adaptive Behavior

```csharp

// React to memory pressure in application logic
var memoryInfo = GC.GetGCMemoryInfo();
double loadPercent = (double)memoryInfo.MemoryLoadBytes
    / memoryInfo.TotalAvailableMemoryBytes * 100;

if (loadPercent > 85)
{
    logger.LogWarning("High memory pressure: {Load:F1}%", loadPercent);
    // Shed load: reduce cache sizes, reject non-critical requests
}

```text

---

## Memory Profiling

### dotMemory (JetBrains)

dotMemory provides heap snapshots and allocation tracking with a visual UI. Use it for investigating memory leaks and
high-allocation hot paths.

**Workflow:**

1. Attach dotMemory to the running process (or launch with profiling enabled)
2. Capture a baseline snapshot after application warm-up
3. Execute the scenario under investigation
4. Capture a second snapshot
5. Compare snapshots to identify retained objects and growth

**Key views:**

- **Sunburst** -- shows allocation tree by type hierarchy
- **Dominator tree** -- shows which objects prevent GC of retained memory
- **Survived objects** -- objects allocated between snapshots that survived GC

### PerfView

PerfView is a free Microsoft tool for detailed GC and allocation analysis. It uses ETW (Event Tracing for Windows)
events for low-overhead profiling.

```bash

# Collect GC and allocation events for 30 seconds
PerfView.exe /GCCollectOnly /MaxCollectSec:30 collect

# Collect allocation stacks (higher overhead)
PerfView.exe /ClrEvents:GC+Stack /MaxCollectSec:30 collect

```text

**Key PerfView views:**

- **GCStats** -- GC pause times, generation counts, promotion rates, fragmentation
- **GC Heap Alloc Stacks** -- call stacks responsible for allocations
- **Any Stacks** -- CPU sampling for identifying hot methods

### Profiling Workflow

1. **Identify the symptom** -- high memory usage, growing Gen2, frequent Gen2 collections, LOH fragmentation
2. **Monitor with dotnet-counters** (see [skill:dotnet-profiling]) to confirm GC metrics match the symptom
3. **Profile with dotMemory or PerfView** to identify the objects and allocation sites
4. **Apply fixes** -- pool buffers, use Span<T>, reduce allocations, fix leaks
5. **Validate with BenchmarkDotNet** (see [skill:dotnet-benchmarkdotnet]) `[MemoryDiagnoser]` to confirm improvement
6. **Monitor in production** via OpenTelemetry runtime metrics (see [skill:dotnet-observability])

---

## Agent Gotchas

1. **Do not default to workstation GC for ASP.NET Core applications** -- server GC is the default and correct choice for
   web workloads. Workstation GC has lower throughput on multi-core servers. Only override for specific
   latency-sensitive scenarios.
2. **Do not forget to return ArrayPool buffers** -- leaked pool buffers are worse than regular allocations because they
   hold pool capacity indefinitely. Always use `try/finally` or `IMemoryOwner<T>` with `using`.
3. **Do not assume rented arrays are the requested size** -- `ArrayPool<T>.Rent()` may return an array larger than
   requested. Always slice to the exact size needed before processing.
4. **Do not add finalizers to classes that only use managed resources** -- finalizers promote objects to Gen1/Gen2 and
   add overhead to GC. Use `sealed class` with `IDisposable` (no finalizer) for managed-only cleanup.
5. **Do not call GC.Collect() in production code** -- forcing full collections causes long pauses and disrupts the GC's
   dynamic tuning. Use `GC.AddMemoryPressure()` to hint at unmanaged memory instead.
6. **Do not ignore LOH fragmentation** -- large arrays (>= 85,000 bytes) allocated and freed repeatedly fragment the
   LOH. Use `ArrayPool<T>` to rent and return large buffers instead of allocating new arrays.
7. **Do not cache IMemoryOwner<T> in long-lived fields without disposal tracking** -- the underlying pooled buffer is
   held indefinitely, preventing pool reuse. Transfer ownership explicitly or limit cache lifetimes.

---

## References

- [Fundamentals of garbage collection](https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/fundamentals)
- [Workstation and server GC](https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/workstation-server-gc)
- [Large Object Heap](https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/large-object-heap)
- [Pinned Object Heap](https://devblogs.microsoft.com/dotnet/internals-of-the-poh/)
- [Memory<T> and Span<T> usage guidelines](https://learn.microsoft.com/en-us/dotnet/standard/memory-and-spans/memory-t-usage-guidelines)
- [ArrayPool<T> class](https://learn.microsoft.com/en-us/dotnet/api/system.buffers.arraypool-1)
- [GC.GetGCMemoryInfo](https://learn.microsoft.com/en-us/dotnet/api/system.gc.getgcmemoryinfo)
- [PerfView GC analysis tutorial](https://learn.microsoft.com/en-us/dotnet/core/diagnostics/debug-highcpu?tabs=windows#analyze-with-perfview)
- [Stephen Toub -- Performance Improvements in .NET series](https://devblogs.microsoft.com/dotnet/author/toub/)
  (published annually)
- [IDisposable pattern](https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-dispose)
````

## Code Navigation (Serena MCP)

**Primary approach:** Use Serena symbol operations for efficient code navigation:

1. **Find definitions**: `serena_find_symbol` instead of text search
2. **Understand structure**: `serena_get_symbols_overview` for file organization
3. **Track references**: `serena_find_referencing_symbols` for impact analysis
4. **Precise edits**: `serena_replace_symbol_body` for clean modifications

**When to use Serena vs traditional tools:**

- **Use Serena**: Navigation, refactoring, dependency analysis, precise edits
- **Use Read/Grep**: Reading full files, pattern matching, simple text operations
- **Fallback**: If Serena unavailable, traditional tools work fine

**Example workflow:**

```text
# Instead of:
Read: src/Services/OrderService.cs
Grep: "public void ProcessOrder"

# Use:
serena_find_symbol: "OrderService/ProcessOrder"
serena_get_symbols_overview: "src/Services/OrderService.cs"
```