Re: [PATCH] xfs: Reduce unnecessary searches when searching for the best extents

From: Chi Zhiling
Date: Sun Oct 27 2024 - 00:05:46 EST



On 2024/10/25 15:09, Dave Chinner wrote:
On Fri, Oct 25, 2024 at 10:33:20AM +0800, Chi Zhiling wrote:
From: Chi Zhiling <chizhiling@xxxxxxxxxx>

Recently, we found that the CPU spent a lot of time in
xfs_alloc_ag_vextent_size when the filesystem has millions of fragmented
spaces.

The reason is that we conducted much extra searching for extents that
could not yield a better result, and these searches would cost a lot of
time when there were millions of extents to search through. Even if we
get the same result length, we don't switch our choice to the new one,
so we can definitely terminate the search early.

Since the result length cannot exceed the found length, when the found
length equals the best result length we already have, we can conclude
the search.

We did a test in that filesystem:
[root@localhost ~]# xfs_db -c freesp /dev/vdb
from to extents blocks pct
1 1 215 215 0.01
2 3 994476 1988952 99.99
Ok, so you have *badly* fragmented free space. That going to cause
lots more problems than only "allocation searches take a long
time". e.g. you can't allocate inodes in a AG that is fragmented
this badly - not even sparse inode clusters....

Yes, this usually happens in some systems that use Mysql table compression, which continuously punches holes in file, eventually causing most fragment lengths to converge to the hole size.


Before this patch:
0) | xfs_alloc_ag_vextent_size [xfs]() {
0) * 15597.94 us | }

After this patch:
0) | xfs_alloc_ag_vextent_size [xfs]() {
0) 19.176 us | }
Yup, that's a good improvement.


Signed-off-by: Chi Zhiling <chizhiling@xxxxxxxxxx>
---
fs/xfs/libxfs/xfs_alloc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index 04f64cf9777e..22bdbb3e9980 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -1923,7 +1923,7 @@ xfs_alloc_ag_vextent_size(
error = -EFSCORRUPTED;
goto error0;
}
- if (flen < bestrlen)
+ if (flen <= bestrlen)
break;
busy = xfs_alloc_compute_aligned(args, fbno, flen,
&rbno, &rlen, &busy_gen);
Yup, I think that works fine. We aren't caring about using locality
as a secondary search key so as soon as we have a candidate extent
of a length that that the remaining extents in the free space btree
can't improve on, we are done.

Nice work!

Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
Thanks!