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....
> 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>
--
Dave Chinner
david@xxxxxxxxxxxxx
