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extentwalker: don't fetch absurd numbers of extents just to throw them away

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ExtentWalker doesn't gain significant benefits from caching, and the
extra SEARCH_V2 ioctls were blamed for a 33% kernel CPU overhead by perf.

Reduce the number of extents to 16 in lieu of fixing the caching.

This gives a significant speed boost on CPU-bound workloads compared
to the original 1024--almost 40% faster on a single SSD with a filesystem
consisting of raw VM images mounted with compress=zstd.

This also seems to reduce LOGICAL_INO overhead.  Perhaps SEARCH_V2 and
LOGICAL_INO were trying to lock the same extents, and interfering with
each other?

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell 2018-09-26 22:44:59 -04:00
parent f053e0e1a7
commit d4b3836493
1 changed files with 1 additions and 1 deletions
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2
include/crucible/extentwalker.h
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@ -58,7 +58,7 @@ namespace crucible {
virtual Vec get_extent_map(off_t pos);
static const unsigned sc_extent_fetch_max = 1024;
static const unsigned sc_extent_fetch_max = 16;
static const unsigned sc_extent_fetch_min = 4;
static const off_t sc_step_size = 0x1000 * (sc_extent_fetch_max / 2);