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Pool the dictionary buffer when training a Zstandard dictionary #129125

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Pool the dictionary buffer when training a Zstandard dictionary #129125

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35 changes: 21 additions & 14 deletions ...ibraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardDictionary.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -127,23 +127,30 @@ public static ZstandardDictionary Train(ReadOnlySpan<byte> samples, ReadOnlySpan

ArgumentOutOfRangeException.ThrowIfLessThan(maxDictionarySize, 256, nameof(maxDictionarySize));

byte[] dictionaryBuffer = new byte[maxDictionarySize];

nuint dictSize;

unsafe
byte[] dictionaryBuffer = ArrayPool<byte>.Shared.Rent(maxDictionarySize);
try
{
fixed (byte* samplesPtr = &MemoryMarshal.GetReference(samples))
fixed (byte* dictPtr = dictionaryBuffer)
fixed (nuint* lengthsAsNuintPtr = &MemoryMarshal.GetReference(lengthsAsNuint))
nuint dictSize;

unsafe
{
dictSize = Interop.Zstd.ZDICT_trainFromBuffer(
dictPtr, (nuint)maxDictionarySize,
samplesPtr, lengthsAsNuintPtr, (uint)sampleLengths.Length);
fixed (byte* samplesPtr = &MemoryMarshal.GetReference(samples))
fixed (byte* dictPtr = dictionaryBuffer)
fixed (nuint* lengthsAsNuintPtr = &MemoryMarshal.GetReference(lengthsAsNuint))
{
dictSize = Interop.Zstd.ZDICT_trainFromBuffer(
dictPtr, (nuint)maxDictionarySize,
samplesPtr, lengthsAsNuintPtr, (uint)sampleLengths.Length);
}

ZstandardUtils.ThrowIfError(dictSize);
return Create(dictionaryBuffer.AsSpan(0, (int)dictSize));
}

ZstandardUtils.ThrowIfError(dictSize);
return Create(dictionaryBuffer.AsSpan(0, (int)dictSize));
}
finally
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@MihaZupan MihaZupan Jun 8, 2026

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We already have a try/finally for the lengthsArray buffer, can we avoid even more nesting by reusing the existing blocks?

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@AustinWise AustinWise Jun 8, 2026

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Another option is to not return the array to the pool on the exceptional path. I can't find a reference for this in learn.microsoft.com or this repo's docs, but I've seen Stephen Toub and others mentioned in a PRs that generally returning arrays to the pool in exceptions paths is more trouble than its worth. For example:

#71249 (comment)

{
// Clear before returning: the trained dictionary is derived from caller-supplied samples.
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@jkotas jkotas Jun 8, 2026

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This clearing is expensive and unnecessary. We are not doing it anywhere else in similar situations

ArrayPool<byte>.Shared.Return(dictionaryBuffer, clearArray: true);
Comment on lines +152 to +153
Comment on lines +152 to +153
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@MihaZupan MihaZupan Jun 8, 2026

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Suggested change
// Clear before returning: the trained dictionary is derived from caller-supplied samples.
ArrayPool<byte>.Shared.Return(dictionaryBuffer, clearArray: true);
ArrayPool<byte>.Shared.Return(dictionaryBuffer);

We practically never clear buffers outside of crypto, I don't see a reason to do it here

}
}
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