xfs: Reduce unnecessary searches when searching for the best extents

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

Posted by Chi Zhiling 1 month ago

From: Chi Zhiling <chizhiling@kylinos.cn>

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

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    |  }

Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
---
 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);
-- 
2.43.0

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

Posted by Carlos Maiolino 3 weeks, 2 days ago

On Fri, 25 Oct 2024 10:33:20 +0800, Chi Zhiling wrote:
> 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.
> 
> [...]

Applied to for-next, thanks!

[1/1] xfs: Reduce unnecessary searches when searching for the best extents
      commit: 3ef22684038aa577c10972ee9c6a2455f5fac941

Best regards,
-- 
Carlos Maiolino <cem@kernel.org>

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

Posted by Dave Chinner 1 month ago

On Fri, Oct 25, 2024 at 10:33:20AM +0800, Chi Zhiling wrote:
> From: Chi Zhiling <chizhiling@kylinos.cn>
> 
> 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@kylinos.cn>
> ---
>  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@redhat.com>

-- 
Dave Chinner
david@fromorbit.com

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

Posted by Chi Zhiling 4 weeks, 1 day ago

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@kylinos.cn>
>>
>> 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@kylinos.cn>
>> ---
>>   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@redhat.com>
Thanks!
>