Improve compression ratio of the --patch-from mode #4288
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terrelln
approved these changes
Feb 10, 2025
lib/compress/zstd_ldm.c
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| } | ||
| } else { | ||
| assert(1 <= (int)cParams->strategy && (int)cParams->strategy <= 9); | ||
| /* mapping: strat1 -> rate8 ... strat9 -> rate4*/ |
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nit: Is this comment out of date?
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Ah yes, indeed,
it should be from 7 to 4 now
to be updated
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--patch-frommode essentially combines--longwith dictionary, in order to create a very small patch when compressing a new document based on an older revision of the same document.The
--longmode features advanced parameters, that can be tuned by the user, but are rarely employed. As a consequence, the "default" values are almost always used, resulting in non-optimal performance.This patch dynamically updates the advanced parameters of the
--longmode, spending more cpu time to favor compression ratio at higher levels.For illustration, here is an extract of the updated performance, using the following scenario:
zstd -# -T6 ~/dev/bench/linux-v6.13.tar --patch-from ~/dev/bench/linux-v6.12.tarrunning on a ubuntu desktop with a i7-9700k cpu:
devtimePRtimedevcSizePRcSize