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Speed up WB_SYNC_NONE when a WB_SYNC_ALL occurs simultaneously

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Page writebacks with WB_SYNC_NONE can take several seconds to complete 
since they wait for the transaction group to close before being 
committed. This is usually not a problem since the caller does not 
need to wait. However, if we're simultaneously doing a writeback 
with WB_SYNC_ALL (e.g via msync), the latter can block for several 
seconds (up to zfs_txg_timeout) due to the active WB_SYNC_NONE 
writeback since it needs to wait for the transaction to complete 
and the PG_writeback bit to be cleared.

This commit deals with 2 cases:

- No page writeback is active. A WB_SYNC_ALL page writeback starts 
  and even completes. But when it's about to check if the PG_writeback 
  bit has been cleared, another writeback with WB_SYNC_NONE starts. 
  The sync page writeback ends up waiting for the non-sync page 
  writeback to complete.

- A page writeback with WB_SYNC_NONE is already active when a 
  WB_SYNC_ALL writeback starts. The WB_SYNC_ALL writeback ends up 
  waiting for the WB_SYNC_NONE writeback.

The fix works by carefully keeping track of active sync/non-sync 
writebacks and committing when beneficial.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shaan Nobee <sniper111@gmail.com>
Closes #12662
Closes #12790
This commit is contained in:
Shaan Nobee
2022-05-04 00:23:26 +04:00
committed by GitHub
parent a64d757aa4
commit 411f4a018d
17 changed files with 351 additions and 20 deletions
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module/zfs/zfs_vnops.c
+2
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@@ -68,7 +68,9 @@ zfs_fsync(znode_t *zp, int syncflag, cred_t *cr)
if (zfsvfs->z_os->os_sync != ZFS_SYNC_DISABLED) {
ZFS_ENTER(zfsvfs);
ZFS_VERIFY_ZP(zp);
atomic_inc_32(&zp->z_sync_writes_cnt);
zil_commit(zfsvfs->z_log, zp->z_id);
atomic_dec_32(&zp->z_sync_writes_cnt);
ZFS_EXIT(zfsvfs);
}
tsd_set(zfs_fsyncer_key, NULL);