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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/os/freebsd/zfs/zfs_znode.c
+8
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@@ -153,6 +153,9 @@ zfs_znode_cache_constructor(void *buf, void *arg, int kmflags)
zp->z_xattr_cached = NULL;
zp->z_xattr_parent = 0;
zp->z_vnode = NULL;
zp->z_sync_writes_cnt = 0;
zp->z_async_writes_cnt = 0;
return (0);
}
@@ -172,6 +175,9 @@ zfs_znode_cache_destructor(void *buf, void *arg)
ASSERT3P(zp->z_acl_cached, ==, NULL);
ASSERT3P(zp->z_xattr_cached, ==, NULL);
ASSERT0(atomic_load_32(&zp->z_sync_writes_cnt));
ASSERT0(atomic_load_32(&zp->z_async_writes_cnt));
}
@@ -453,6 +459,8 @@ zfs_znode_alloc(zfsvfs_t *zfsvfs, dmu_buf_t *db, int blksz,
zp->z_blksz = blksz;
zp->z_seq = 0x7A4653;
zp->z_sync_cnt = 0;
zp->z_sync_writes_cnt = 0;
zp->z_async_writes_cnt = 0;
#if __FreeBSD_version >= 1300139
atomic_store_ptr(&zp->z_cached_symlink, NULL);
#endif