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Make "Built in Metrics for Runtime" doc accurate for memory metrics #44616

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merged 3 commits into from
Feb 18, 2025
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Make "Built in Metrics for Runtime" doc accurate for memory metrics #44616

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d509f11
The attribute for both `dotnet.gc.collections` and `dotnet.gc.last_co…
jwreford99 Jan 30, 2025
6c1c47d
The metric `dotnet.gc.last_collection.heap.fragmentation.size` has a …
jwreford99 Jan 30, 2025
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gewarren Feb 18, 2025
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8 changes: 6 additions & 2 deletions docs/core/diagnostics/built-in-metrics-runtime.md
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Expand Up @@ -49,7 +49,7 @@ Available starting in: .NET 9.0.

| Attribute | Type | Description | Examples | Presence |
|---|---|---|---|---|
| `dotnet.gc.heap.generation` | string | Name of the maximum managed heap generation being collected. | `gen0`; `gen1`; `gen2` | Always |
| `gc.heap.generation` | string | Name of the maximum managed heap generation being collected. | `gen0`; `gen1`; `gen2` | Always |

The .NET GC is a generational garbage collector. Each time the garbage collector runs, it uses heuristics to select a maximum generation and then collects objects in all generations up to the selected maximum. For example, a `gen1` collection collects all objects in generations 0 and 1. A `gen2` collection collects all objects in generations 0, 1, and 2. For more information about the .NET GC and generational garbage collection, see the [.NET garbage collection guide](../../standard/garbage-collection/fundamentals.md#generations).

Expand Down Expand Up @@ -83,7 +83,7 @@ Available starting in: .NET 9.0.

| Attribute | Type | Description | Examples | Presence |
|---|---|---|---|---|
| `dotnet.gc.heap.generation` | string | Name of the garbage collector managed heap generation. | `gen0`; `gen1`; `gen2`;`loh`;`poh` | Always |
| `gc.heap.generation` | string | Name of the garbage collector managed heap generation. | `gen0`; `gen1`; `gen2`; `loh`; `poh` | Always |

The .NET GC divides the heap into generations. In addition to the standard numbered generations, the GC also puts some objects into two special generations:

Expand All @@ -100,6 +100,10 @@ Available starting in: .NET 9.0.
| -------- | --------------- | ----------- | -------------- |
| `dotnet.gc.last_collection.heap.fragmentation.size` | UpDownCounter | `By` | The heap fragmentation, as observed during the latest garbage collection. |

| Attribute | Type | Description | Examples | Presence |
|---|---|---|---|---|
| `gc.heap.generation` | string | Name of the garbage collector managed heap generation. | `gen0`; `gen1`; `gen2`; `loh`; `poh` | Always |

This metric reports the same values as calling <xref:System.GCGenerationInfo.FragmentationAfterBytes?displayProperty=nameWithType>.

When .NET objects are allocated, initially they tend to be laid out contiguously in memory. However, if some of those objects are later collected by the GC, this creates gaps of unused memory between the live objects that remain. These gaps represent the portion of the GC heap that's not currently being used to store objects, often called "fragmentation." The GC can reuse the fragmentation bytes in the future for new object allocations if the object size is small enough to fit in one of the gaps. The GC can also perform a special compacting garbage collection that moves remaining live objects next to each other as long as the objects haven't been pinned in place.
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