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Remove lock from dsl_pool_need_dirty_delay()

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Torn reads/writes of dp_dirty_total are unlikely: on 64-bit systems
due to register size, while on 32-bit due to memory constraints.
And even if we hit some race, the code implementing the delay takes
the lock any way.

Removal of the poll-wide lock acquisition saves ~1% of CPU time on
8-thread 8KB write workload.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #15390
This commit is contained in:
Alexander Motin 2023-10-20 15:37:16 -04:00 committed by GitHub
parent de7b1ae30a
commit 4fbc524955
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1 changed files with 7 additions and 7 deletions
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14
module/zfs/dsl_pool.c
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@ -965,18 +965,18 @@ dsl_pool_need_dirty_delay(dsl_pool_t *dp)
uint64_t delay_min_bytes =
zfs_dirty_data_max * zfs_delay_min_dirty_percent / 100;
mutex_enter(&dp->dp_lock);
uint64_t dirty = dp->dp_dirty_total;
mutex_exit(&dp->dp_lock);
return (dirty > delay_min_bytes);
/*
* We are not taking the dp_lock here and few other places, since torn
* reads are unlikely: on 64-bit systems due to register size and on
* 32-bit due to memory constraints. Pool-wide locks in hot path may
* be too expensive, while we do not need a precise result here.
*/
return (dp->dp_dirty_total > delay_min_bytes);
}
static boolean_t
dsl_pool_need_dirty_sync(dsl_pool_t *dp, uint64_t txg)
{
ASSERT(MUTEX_HELD(&dp->dp_lock));
uint64_t dirty_min_bytes =
zfs_dirty_data_max * zfs_dirty_data_sync_percent / 100;
uint64_t dirty = dp->dp_dirty_pertxg[txg & TXG_MASK];