ddidderr/zstd-rs
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

first implementation of delayed update for btlazy2

Browse Source 
This is a pretty nice speed win.

The new strategy consists in stacking new candidates as if it was a hash chain.
Then, only if there is a need to actually consult the chain, they are batch-updated,
before starting the match search itself.
This is supposed to be beneficial when skipping positions,
which happens a lot when using lazy strategy.

The baseline performance for btlazy2 on my laptop is :
15#calgary.tar       :   3265536 ->    955985 (3.416),  7.06 MB/s , 618.0 MB/s
15#enwik7            :  10000000 ->   3067341 (3.260),  4.65 MB/s , 521.2 MB/s
15#silesia.tar       : 211984896 ->  58095131 (3.649),  6.20 MB/s , 682.4 MB/s
(only level 15 remains for btlazy2, as this strategy is squeezed between lazy2 and btopt)

After this patch, and keeping all parameters identical,
speed is increased by a pretty good margin (+30-50%),
but compression ratio suffers a bit :
15#calgary.tar       :   3265536 ->    958060 (3.408),  9.12 MB/s , 621.1 MB/s
15#enwik7            :  10000000 ->   3078318 (3.249),  6.37 MB/s , 525.1 MB/s
15#silesia.tar       : 211984896 ->  58444111 (3.627),  9.89 MB/s , 680.4 MB/s

That's because I kept `1<<searchLog` as a maximum number of candidates to update.
But for a hash chain, this represents the total number of candidates in the chain,
while for the binary, it represents the maximum depth of searches.
Keep in mind that a lot of candidates won't even be visited in the btree,
since they are filtered out by the binary sort.

As a consequence, in the new implementation,
the effective depth of the binary tree is substantially shorter.

To compensate, it's enough to increase `searchLog` value.
Here is the result after adding just +1 to searchLog (level 15 setting in this patch):
15#calgary.tar       :   3265536 ->    956311 (3.415),  8.32 MB/s , 611.4 MB/s
15#enwik7            :  10000000 ->   3067655 (3.260),  5.43 MB/s , 535.5 MB/s
15#silesia.tar       : 211984896 ->  58113144 (3.648),  8.35 MB/s , 679.3 MB/s

aka, almost the same compression ratio as before,
but with a noticeable speed increase (+20-30%).

This modification makes btlazy2 more competitive.
A new round of paramgrill will be necessary to determine which levels are impacted and could adopt the new strategy.
This commit is contained in:
Yann Collet
2017-12-28 16:58:57 +01:00
parent d55aea3c3b
commit 5235d8d6ba
5 changed files with 322 additions and 150 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/compress/zstd_compress.c
+5 -1
View File
@@ -1922,6 +1922,10 @@ static size_t ZSTD_loadDictionaryContent(ZSTD_CCtx* zc, const void* src, size_t
break;
case ZSTD_btlazy2:
if (srcSize >= HASH_READ_SIZE)
ZSTD_updateDUBT(zc, iend-HASH_READ_SIZE, iend, zc->appliedParams.cParams.searchLength);
break;
case ZSTD_btopt:
case ZSTD_btultra:
if (srcSize >= HASH_READ_SIZE)
@@ -2974,7 +2978,7 @@ static const ZSTD_compressionParameters ZSTD_defaultCParameters[4][ZSTD_MAX_CLEV
{ 22, 20, 22, 5, 5, 16, ZSTD_lazy2 }, /* level 12 */
{ 22, 21, 22, 5, 5, 16, ZSTD_lazy2 }, /* level 13 */
{ 22, 21, 22, 6, 5, 16, ZSTD_lazy2 }, /* level 14 */
{ 22, 21, 22, 4, 5, 16, ZSTD_btlazy2 }, /* level 15 */
{ 22, 21, 22, 5, 5, 16, ZSTD_btlazy2 }, /* level 15 */
{ 22, 21, 22, 4, 5, 48, ZSTD_btopt }, /* level 16 */
{ 23, 22, 22, 4, 4, 48, ZSTD_btopt }, /* level 17 */
{ 23, 22, 22, 5, 3, 64, ZSTD_btopt }, /* level 18 */