rudironsoni 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/Memory ownership patterns beyond basics, buffer pooling with ArrayPool and MemoryPool, 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/Memory ownership patterns
- Buffer pooling with ArrayPool and MemoryPool
- Memory profiling with dotMemory and PerfView
Out of scope
- Span/Memory 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 producer/consumer patterns -- see [skill:dotnet-channels]
Cross-references: [skill:dotnet-performance-patterns] for Span/Memory 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 |
<!-- 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)