Date:	Fri, 08 Oct 2010 17:56:12 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	djwong@...ibm.com
CC:	Andreas Dilger <adilger.kernel@...ger.ca>,
	"Ted Ts'o" <tytso@....edu>, Mingming Cao <cmm@...ibm.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Keith Mannthey <kmannth@...ibm.com>,
	Mingming Cao <mcao@...ibm.com>, Tejun Heo <tj@...nel.org>,
	hch@....de, Josef Bacik <josef@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>
Subject: Re: Performance testing of various barrier reduction patches [was:
 Re: [RFC v4] ext4: Coordinate fsync requests]

  On 10/08/2010 05:26 PM, Darrick J. Wong wrote:
> On Mon, Sep 27, 2010 at 04:01:11PM -0700, Darrick J. Wong wrote:
>> Other than those regressions, the jbd2 fsync coordination is about as fast as
>> sending the flush directly from ext4.  Unfortunately, where there _are_
>> regressions they seem rather large, which makes this approach (as implemented,
>> anyway) less attractive.  Perhaps there is a better way to do it?
> Hmm, not much chatter for two weeks.  Either I've confused everyone with the
> humongous spreadsheet, or ... something?
>
> I've performed some more extensive performance and safety testing with the
> fsync coordination patch.  The results have been merged into the spreadsheet
> that I linked to in the last email, though in general the results have not
> really changed much at all.
>
> I see two trends happening here with regards to comparing the use of jbd2 to
> coordinate the flushes vs. measuring and coodinating flushes directly in ext4.
> The first is that for loads that most benefit from having any kind of fsync
> coordination (i.e. storage with slow flushes), the jbd2 approach provides the
> same or slightly better performance than the direct approach.  However, for
> storage with fast flushes, the jbd2 approach seems to cause major slowdowns
> even compared to not changing any code at all.  To me this would suggest that
> ext4 needs to coordinate the fsyncs directly, even at a higher code maintenance
> cost, because a huge performance regression isn't good.
>
> Other people in my group have been running their own performance comparisons
> between no-coordination, jbd2-coordination, and direct-coordination, and what
> I'm hearing is tha the direct-coordination mode is slightly faster than jbd2
> coordination, though either are better than no coordination at all.  Happily, I
> haven't seen an increase in fsck complaints in my poweroff testing.
>
> Given the nearness of the merge window, perhaps we ought to discuss this on
> Monday's ext4 call?  In the meantime I'll clean up the fsync coordination patch
> so that it doesn't have so many debugging knobs and whistles.
>
> Thanks,
>
> --D

Hi Darrick,

We have been busily testing various combinations at Red Hat (we being not me 
:)), but here is one test that we used a long time back to validate the batching 
impact.

You need a slow, poky S-ATA drive - the slower it spins, the better.

A single fs_mark run against that drive should drive some modest number of 
files/sec with 1 thread:


[root@...kums /]# fs_mark -s 20480 -n 500 -L 5 -d /test/foo

On my disk, I see:

      5          500        20480         31.8             6213

Now run with 4 threads to give the code a chance to coalesce.

On my box, I see it jump up:

      5         2000        20480        113.0            25092

And at 8 threads it jumps again:

      5         4000        20480        179.0            49480

This work load is very device specific. On a very low latency device (arrays, 
high performance SSD), the coalescing "wait" time could be slower than just 
dispatching the command. Ext3/4 work done by Josef a few years back was meant to 
use high res timers to dynamically adjust that wait to avoid slowing down.

Have we tested the combined patchset with this?

Thanks!

Ric






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