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

[Chi Zhiling <chizhiling@163.com>]

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!
>