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new block splitting variant _fromBorders

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less precise but still suitable for `fast` strategy.
This commit is contained in:
Yann Collet
2024-10-25 16:13:55 -07:00
parent 7fb5347e88
commit e557abc8a0
4 changed files with 71 additions and 21 deletions
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lib/compress/zstd_compress.c
+6 -16
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@@ -4493,6 +4493,8 @@ static void ZSTD_overflowCorrectIfNeeded(ZSTD_matchState_t* ms,
static size_t ZSTD_optimalBlockSize(ZSTD_CCtx* cctx, const void* src, size_t srcSize, size_t blockSizeMax, ZSTD_strategy strat, S64 savings)
{
/* split level based on compression strategy, from `fast` to `btultra2` */
static const int splitLevels[] = { 0, 0, 1, 2, 2, 3, 3, 4, 4, 4 };
/* note: conservatively only split full blocks (128 KB) currently.
* While it's possible to go lower, let's keep it simple for a first implementation.
* Besides, benefits of splitting are reduced when blocks are already small.
@@ -4500,25 +4502,13 @@ static size_t ZSTD_optimalBlockSize(ZSTD_CCtx* cctx, const void* src, size_t src
if (srcSize < 128 KB || blockSizeMax < 128 KB)
return MIN(srcSize, blockSizeMax);
/* do not split incompressible data though:
* ensure a 3 bytes per full block overhead limit.
* Note: as a consequence, the first full block skips the splitting detector.
* require verified savings to allow pre-splitting.
* Note: as a consequence, the first full block is not split.
*/
if (savings < 3) return 128 KB;
/* dynamic splitting has a cpu cost for analysis,
* due to that cost it's only used for higher levels */
if (strat >= ZSTD_btopt)
return ZSTD_splitBlock(src, blockSizeMax, 3, cctx->tmpWorkspace, cctx->tmpWkspSize);
if (strat >= ZSTD_lazy2)
return ZSTD_splitBlock(src, blockSizeMax, 2, cctx->tmpWorkspace, cctx->tmpWkspSize);
if (strat >= ZSTD_greedy)
return ZSTD_splitBlock(src, blockSizeMax, 1, cctx->tmpWorkspace, cctx->tmpWkspSize);
if (strat >= ZSTD_dfast)
return ZSTD_splitBlock(src, blockSizeMax, 0, cctx->tmpWorkspace, cctx->tmpWkspSize);
/* blind split strategy
* heuristic value, tested as being "generally better".
* no cpu cost, but can over-split homegeneous data.
*/
return 92 KB;
* select a variant among multiple gradual speed/accuracy tradeoffs */
return ZSTD_splitBlock(src, blockSizeMax, splitLevels[strat], cctx->tmpWorkspace, cctx->tmpWkspSize);
}
/*! ZSTD_compress_frameChunk() :