linux-ext4 - Re: delalloc is crippling fs

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Date:	Sat, 19 Jul 2008 10:44:34 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: delalloc is crippling fs_mark performance

Eric Sandeen wrote:
> Eric Sandeen wrote:
>> running fs_mark like this:
>>
>> fs_mark -d /mnt/test -D 256 -n 100000 -t 4 -s 20480 -F -S 0
>>
>> (256 subdirs, 100000 files/iteration, 4 threads, 20k files, no sync)
>>
>> on a 1T fs, with and without delalloc (mount option), is pretty interesting:
>>
>> http://people.redhat.com/esandeen/ext4/fs_mark.png
>>
>> somehow delalloc is crushing performance here.  I'm planning to wait
>> 'til the fs is full and see what the effect is on fsck, and look at the
>> directory layout for differences compared to w/o delalloc.
>>
>> But something seems to have gone awry here ...
>>
>> This is on 2.6.26 with the patch queue applied up to stable.
>>
>> -Eric
> 
> I oprofiled both with and without delalloc for the first 15% of the fs fill:
> 
> ==> delalloc.op <==
> CPU: AMD64 processors, speed 2000 MHz (estimated)
> Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a
> unit mask of 0x00 (No unit mask) count 100000
> samples  %        image name               app name
> symbol name
> 56094537 73.6320  ext4dev.ko               ext4dev
> ext4_mb_use_preallocated
> 642479    0.8433  vmlinux                  vmlinux
> __copy_user_nocache
> 523803    0.6876  vmlinux                  vmlinux                  memcmp
> 482874    0.6338  jbd2.ko                  jbd2
> do_get_write_access
> 480687    0.6310  vmlinux                  vmlinux
> kmem_cache_free
> 403604    0.5298  ext4dev.ko               ext4dev
> str2hashbuf
> 400471    0.5257  vmlinux                  vmlinux
> __find_get_block
> 
> ==> nodelalloc.op <==
> CPU: AMD64 processors, speed 2000 MHz (estimated)
> Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a
> unit mask of 0x00 (No unit mask) count 100000
> samples  %        image name               app name
> symbol name
> 56167198 56.8949  ext4dev.ko               ext4dev
> ext4_mb_use_preallocated

This was wrong, I forgot to clear stats before re-running.

With delalloc, the lg_prealloc list seems to just grow & grow in
ext4_mb_use_preallocated, searching up to 90,000 entries before finding
something, I think this is what's hurting - I need to look into how this
should work.

-Eric
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