Re: [PATCH 0/4 v3] ext3/4: enhance fsync performance when using CFQ

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Apr 15 2010, Jeff Moyer wrote:
> Jens Axboe <jens.axboe@oracle.com> writes:
> 
> > On Wed, Apr 14 2010, Jeff Moyer wrote:
> >> Hi,
> >> 
> >> The previous two postings can be found here:
> >>   http://lkml.org/lkml/2010/4/1/344
> >> and here:
> >>   http://lkml.org/lkml/2010/4/7/325
> >> 
> >> The basic problem is that, when running iozone on smallish files (up to
> >> 8MB in size) and including fsync in the timings, deadline outperforms
> >> CFQ by a factor of about 5 for 64KB files, and by about 10% for 8MB
> >> files.  From examining the blktrace data, it appears that iozone will
> >> issue an fsync() call, and subsequently wait until its CFQ timeslice
> >> has expired before the journal thread can run to actually commit data to
> >> disk.
> >> 
> >> The approach taken to solve this problem is to implement a blk_yield call,
> >> which tells the I/O scheduler not to idle on this process' queue.  The call
> >> is made from the jbd[2] log_wait_commit function.
> >> 
> >> This patch set addresses previous concerns that the sync-noidle workload
> >> would be starved by keeping track of the average think time for that
> >> workload and using that to decide whether or not to yield the queue.
> >> 
> >> My testing showed nothing but improvements for mixed workloads, though I
> >> wouldn't call the testing exhaustive.  I'd still very much like feedback
> >> on the approach from jbd/jbd2 developers.  Finally, I will continue to do
> >> performance analysis of the patches.
> >
> > This is starting to look better. Can you share what tests you did? I
> > tried reproducing with fs_mark last time and could not.
> 
> Did you use the fs_mark command line I (think I) had posted?  What
> storage were you using?

No, I didn't see any references to example command lines. I tested on a
few single disks, rotating and SSD. I expected the single spinning disk
to show the problem to some extent at least, but there was no difference
observed with 64kb blocks.

> I took Vivek's iostest and modified the mixed workload to do buffered
> random reader, buffered sequential reader, and buffered writer for all
> of 1, 2, 4, 8 and 16 threads each.
> 
> The initial problem was reported against iozone, which can show the
> problem quite easily when run like so:
>   iozone -s 64 -e -f /mnt/test/iozone.0 -i 0 -+n
> 
> You can also just run iozone in auto mode, but that can take quite a
> while to complete.
> 
> All of my tests for this round have been against a NetApp hardware
> RAID.  I wanted to test against a simple sata disk as well, but have
> become swamped with other issues.
> 
> I'll include all of this information in the next patch posting.  Sorry
> about that.

No problem, I'll try the above.

-- 
Jens Axboe