From 4fbc52495552b7e8b3337c94d0b7080db65657b8 Mon Sep 17 00:00:00 2001 From: Alexander Motin Date: Fri, 20 Oct 2023 15:37:16 -0400 Subject: [PATCH] Remove lock from dsl_pool_need_dirty_delay() 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 Signed-off-by: Alexander Motin Sponsored by: iXsystems, Inc. Closes #15390 --- module/zfs/dsl_pool.c | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-) diff --git a/module/zfs/dsl_pool.c b/module/zfs/dsl_pool.c index 9120fef93c..17b9712482 100644 --- a/module/zfs/dsl_pool.c +++ b/module/zfs/dsl_pool.c @@ -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];