Reduce allocations in TextDocumentStates.AddRange #75640
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This method was calling ImmutableList.AddRange giving it the result of a Select linq expression. The underlying ImmutableList code doesn't end up special casing that collection type, and thus creates a FallbackWrapper around the enumeration. The FallbackWrapper.Count method uses Enumerable.ToArray to cache a collection object, which ends up 1) doing the double resize allocation dance pass until done 2) resizing that final array This essentially means we've allocated around 3x the amount of memory needed to hold our documentids. Instead, if we use a pooled list, we can generally avoid the allocations completely (the ImmutableList code instead creates a ListOfTWrapper around our List which doesn't need to allocate) This method is showing up as around 0.8% of allocations in the typing section of the csharp editing speedometer test.
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CyrusNajmabadi
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Oct 26, 2024
| filePathToDocumentIds: null); | ||
| { | ||
| using var pooledIds = SharedPools.Default<List<DocumentId>>().GetPooledObject(); | ||
| var ids = pooledIds.Object; |
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Worth setting the capacity of this list?
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I think I'd rather not unless it shows up. The reasons:
- Looks like it would need to ensure the desired size exceeds the existing capacity before setting the capacity, otherwise it may throw
- Setting the capacity explicitly avoids the double resizing code in EnsureCapacity. So, without doing that ourselves here, we'd potentially be allocating quite a bit more often.
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Going to go ahead and merge, let me know if you would like followup on this
This was referenced Nov 4, 2024
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This method was calling ImmutableList.AddRange giving it the result of a Select linq expression. The underlying ImmutableList code doesn't end up special casing that collection type, and thus creates a FallbackWrapper around the enumeration. The FallbackWrapper.Count method uses Enumerable.ToArray to cache a collection object, which ends up
This essentially means we've allocated around 3x the amount of memory needed to hold our documentids. Instead, if we use a pooled list, we can generally avoid the allocations completely (the ImmutableList code instead creates a ListOfTWrapper around our List which doesn't need to allocate)
This method is showing up as around 0.8% of allocations in the typing section of the csharp editing speedometer test.
*** allocations that should be removed by this change ***
