The kernel uses internal mounts created by kern_mount() and populated
with files with no lookup path by alloc_file_pseudo() for a variety of
reasons. An relevant example is anonymous pipes because every vfs_write
also checks if atime needs to be updated even though it is unnecessary.
Most of the relevant users for alloc_file_pseudo() either have no statfs
helper or use simple_statfs which does not return st_atime. The closest
proxy measure is the proc fd representations of such inodes which do not
appear to change once they are created. This patch sets the S_NOATIME
on inode->i_flags for inodes created by new_inode_pseudo() so that atime
will not be updated.
The test motivating this was "perf bench sched messaging --pipe" where
atime-related functions were noticeable in the profiles. On a single-socket
machine using threads the difference of the patch was as follows. The
difference in performance was
5.8.0-rc1 5.8.0-rc1
vanilla pseudoatime-v1r1
Amean 1 0.4807 ( 0.00%) 0.4623 * 3.81%*
Amean 3 1.5543 ( 0.00%) 1.4610 ( 6.00%)
Amean 5 2.5647 ( 0.00%) 2.5183 ( 1.81%)
Amean 7 3.7407 ( 0.00%) 3.7120 ( 0.77%)
Amean 12 5.9900 ( 0.00%) 5.5233 ( 7.79%)
Amean 18 8.8727 ( 0.00%) 6.8353 * 22.96%*
Amean 24 11.1510 ( 0.00%) 8.9123 * 20.08%*
Amean 30 13.9330 ( 0.00%) 10.8743 * 21.95%*
Amean 32 14.2177 ( 0.00%) 10.9923 * 22.69%*
Note that I consider the impact to be disproportionate and so it may not
be universal. On a profiled run for just *one* group, the difference in
perf profiles for atime-related functions were
0.23% -0.18% [kernel.vmlinux] [k] atime_needs_update
0.13% -0.02% [kernel.vmlinux] [k] touch_atime
So there is a large reduction in atime overhead which on this particular
machine must have gotten incrementally worse as the group count
increased. I could measure it specifically but I think it's reasonable
to reduce atime overhead for pseudo files unconditionally.
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
fs/inode.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/fs/inode.c b/fs/inode.c
index 72c4c347afb7..6d4ea0c9fe3e 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -930,6 +930,7 @@ struct inode *new_inode_pseudo(struct super_block *sb)
if (inode) {
spin_lock(&inode->i_lock);
inode->i_state = 0;
+ inode->i_flags |= S_NOATIME;
spin_unlock(&inode->i_lock);
INIT_LIST_HEAD(&inode->i_sb_list);
}
