Re: delalloc is crippling fs_mark performance

[Andreas Dilger <adilger@sun.com>]

On Jul 21, 2008  11:22 -0500, 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
> 
> I've updated this graph with another run where the group_prealloc
> tuneable was set to a perfect multiple of the allocation size, or 500
> blocks.  This way the leftover 2-block preallocations don't wind up
> causing the list to grow with unuseable tiny leftover preallocations.
> After tuning this way, it does clearly seem to be the problem here.

Looking at that graph it would seem that allowing 1000 PAs to accumulate
with Aneesh's patch adds a constant slowdown.  Compared with the "perfect"
case where the PA list is always empty it is noticably slower.

I'd guess that the right thing to do is have a few buckets for PAs of
different sizes, and keep them very short (e.g. <= 8) to avoid a lot of
list walking overhead on each access.

I think keeping a single PA of "each size" would likely run out if
different-sized allocations are being done, requiring a re-search.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.