Release the GVL while decompressing in Zstd.decompress - #156
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Zstd.decompress runs its decode loop by calling ZSTD_decompressStream directly, so it holds the GVL for the whole operation. Zstd.compress and both streaming paths already go through the rb_thread_call_without_gvl wrappers in common.h; decompress is the one hot path that does not, so a large decompress blocks every other thread in the process until it finishes. Route the decode loop through the existing zstd_stream_decompress wrapper with gvl=false, matching the streaming decompress path. The GVL is released and reacquired once per output chunk (ZSTD_DStreamOutSize), so other threads run between chunks. This is safe here: the scratch buffer is malloc'd (ALLOC_N), not a movable Ruby String, and the input pointer comes from a stack-pinned VALUE, so nothing the call touches can move while the GVL is released.
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@SpringMT Can you approve the pending workflow for the CI to run on this fix. |
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Summary
Fixes #155.
The one-shot
Zstd.decompresspath holds Ruby's GVL throughout the libzstd call, blocking every other thread in the process. This routes it through the existingzstd_stream_decompresswrapper (already used by the streaming classes), which releases the GVL once per output chunk.Change
In
ext/zstdruby/zstdruby.c, functiondecode_one_frame:The
falseargument selects the GVL-releasing branch of the wrapper. The GVL is released and reacquired once per output chunk (ZSTD_DStreamOutSize, 128 KB) — the same granularity the streaming decompress path already uses, so other threads get a turn between chunks.Why it's safe to release the GVL here
decode_one_frameismalloc'd (ALLOC_N), not a Ruby String, so GC compaction cannot move it during the released window.VALUE, which is conservatively pinned by the GC.So nothing the libzstd call touches can be relocated while the GVL is released. On very old Rubies without
ruby/thread.h, the wrapper falls back to a direct call, so behaviour there is unchanged.Verification