fix speed of --patch-from mode at high compression levels #4276
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Cyan4973
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improves compression ratio at low levels
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by avoiding to duplicate in memory a dictionary that was passed by reference.
thus saving a bit of memory and a little bit of cpu time
--patch-from no longer blocked on first job dictionary loading
terrelln
approved these changes
Feb 3, 2025
| } else { | ||
| /* note : a loadPrefix becomes an internal CDict */ | ||
| mtctx->cdictLocal = ZSTD_createCDict_advanced(dict, dictSize, | ||
| dictLoadMethod, dictContentType, |
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We don't actually care about the dictLoadMethod here, right? If it was ZSTD_dlm_byCopy, the outer ZSTD_CCtx must have already token ownership of the buffer. So we can use byRef here.
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| dictLoadMethod, dictContentType, | |
| ZSTD_dlm_byRef, dictContentType, |
| @@ -1232,7 +1248,7 @@ static size_t ZSTDMT_computeOverlapSize(const ZSTD_CCtx_params* params) | |||
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| size_t ZSTDMT_initCStream_internal( | |||
| ZSTDMT_CCtx* mtctx, | |||
| const void* dict, size_t dictSize, ZSTD_dictContentType_e dictContentType, | |||
| const void* dict, size_t dictSize, ZSTD_dictContentType_e dictContentType, ZSTD_dictLoadMethod_e dictLoadMethod, | |||
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Since we can always use byRef, we don't need to ass a dictLoadMethod in here.
| const void* dict; | ||
| size_t dictSize; | ||
| ZSTD_dictContentType_e dictContentType; | ||
| } ZSTD_prefixDict; | ||
| ZSTD_dictLoadMethod_e loadMethod; |
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Since we can always use byRef in ZSTDMT, we don't need to store it here.
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When employing
--patch-fromin combination with very high compression levels,compression speed plummets significantly, becoming extremely slow.
This issue is mitigated by this patch,
which sensibly reduces compression time for this scenario.
Benchmark: patching
linux-6.7.tar(~1.4GB) fromlinux-6.6.taron a M1 Pro laptop (employing 2 threads by default)
devThe impact is even more pronounced with a larger amount of threads.
Benchmark on a core i7-9700k with 6 threads :
devAlso: improved memory usage of
--patch-from, by skipping an internalCDictcreation that was duplicating reference content in memory.