On Fri, Oct 08, 2010 at 05:56:12PM -0400, Ric Wheeler wrote:
quoted text
>  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@tunkums /]# 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.

Yeah, elm3c65 and elm3c75 in that spreadsheet are a new pokey SATA disk and a
really old IDE disk, which ought to represent the low end case.  elm3c44-sas is
a midrange storage server... which doesn't like the patch so much.

--D
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