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
## Detailed Examples
See [references/detailed-examples.md](references/detailed-examples.md) for complete code samples and advanced patterns.