From e4e3ff61f9d0bc4419c52bb9d8b6162fe1503e33 Mon Sep 17 00:00:00 2001 From: Rob Norris Date: Wed, 24 Apr 2024 15:31:36 +1000 Subject: [PATCH] zio: try to execute TYPE_NULL ZIOs on the current task Many TYPE_NULL ZIOs are used to provide a sync point for child ZIOs, and do not do any actual work themselves. However, they are still dispatched to a dedicated, single-thread taskq, which leads to their execution being entirely task switch and dequeue overhead for no actual reason. This commit changes it so that when selecting a parent ZIO to execute, if the parent is TYPE_NULL and has no done function (that is, no additional work), it is executed on the same thread. This reduces task switches and frees up CPU cores for other work. Sponsored-by: Klara, Inc. Sponsored-by: Wasabi Technology, Inc. Signed-off-by: Rob Norris --- module/zfs/zio.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/module/zfs/zio.c b/module/zfs/zio.c index 1ba99f4d46..9b2ae4adc8 100644 --- a/module/zfs/zio.c +++ b/module/zfs/zio.c @@ -803,9 +803,10 @@ zio_notify_parent(zio_t *pio, zio_t *zio, enum zio_wait_type wait, /* * If we can tell the caller to execute this parent next, do - * so. We only do this if the parent's zio type matches the - * child's type. Otherwise dispatch the parent zio in its - * own taskq. + * so. We do this if the parent's zio type matches the child's + * type, or if it's a zio_null() with no done callback, and so + * has no actual work to do. Otherwise dispatch the parent zio + * in its own taskq. * * Having the caller execute the parent when possible reduces * locking on the zio taskq's, reduces context switch @@ -825,7 +826,8 @@ zio_notify_parent(zio_t *pio, zio_t *zio, enum zio_wait_type wait, * of writes for spa_sync(), and the chain of ZIL blocks. */ if (next_to_executep != NULL && *next_to_executep == NULL && - pio->io_type == zio->io_type) { + (pio->io_type == zio->io_type || + (pio->io_type == ZIO_TYPE_NULL && !pio->io_done))) { *next_to_executep = pio; } else { zio_taskq_dispatch(pio, type, B_FALSE);