Re: [Bugme-new] [Bug 41552] New: Performance of writing and reading from multiple drives decreases by 40% when going from Linux Kernel 2.6.36.4 to 2.6.37 (and beyond)

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Aug 22, 2011 at 12:24:43PM -0700, Andrew Morton wrote:
> 
> (switched to email.  Please respond via emailed reply-to-all, not via the
> bugzilla web interface).
> 
> On Mon, 22 Aug 2011 15:20:41 GMT
> bugzilla-daemon@bugzilla.kernel.org wrote:
> 
> > https://bugzilla.kernel.org/show_bug.cgi?id=41552
> > 
> >            Summary: Performance of writing and reading from multiple
> >                     drives decreases by 40% when going from Linux Kernel
> >                     2.6.36.4 to 2.6.37 (and beyond)
> >            Product: IO/Storage
> >            Version: 2.5
> >     Kernel Version: 2.6.37
> >           Platform: All
> >         OS/Version: Linux
> >               Tree: Mainline
> >             Status: NEW
> >           Severity: normal
> >           Priority: P1
> >          Component: SCSI
> >         AssignedTo: linux-scsi@vger.kernel.org
> >         ReportedBy: mpete_06@hotmail.com
> >         Regression: No
> > 
> > 
> > We have an application that will write and read from every sector on a drive. 
> > The application can perform these tasks on multiple drives at the same time. 
> > It is designed to run on top of the Linux Kernel, which we periodically update
> > so that we can get the latest device drivers.  When performing the last update
> > from 2.6.33.2 to 2.6.37, we found that the performance of a set of drives
> > decreased by some 40% (took 3 hours and 11 minutes to write and read from 5
> > drives on 2.6.37 versus 2 hours and 12 minutes on 2.6.33.3).  I was able to
> > determine that the issue was in the 2.6.37 Kernel as I was able to run it with
> > the 2.6.36.4 kernel, and it had the better performance.   After seeing that I/O
> > throttling was introduced in the 2.6.37 Kernel, I naturally suspected that. 
> > However, by default, all the throttling was turned off (I attached the actual
> > .config that was used to build the kernel).  I then tried to turn on the
> > throttling and set it to a high number to see what would happen.  When I did
> > that, I was able to reduce the time from 3 hours and 11 minutes to 2 hours and
> > 50 minutes.  There seems to be something there that changed that is impacting
> > performance on multiple drives.  When we do this same test with only one drive,
> > the performance is identical between the systems.  This issue still occurs on
> > Kernel 3.0.2.
> > 
> 
> Are you able to determine whether this regression is due to slower
> reading, to slower writing or to both?

Mark,

As your initial comment says that you see 40% regression even when block
throttling infrastructure is not enabled, I think it is not related to
throttling as blk_throtl_bio() is null when BLK_DEV_THROTTLING=n.

What IO scheduler are you using? Can you try switching IO scheduler to
deadline and see if regression is still there. Trying to figure out if
it has anything to do with IO scheduler.

What file system are you using with what options? Are you using device
mapper to create some special configuration on multiple disks?

Also can you take a trace (blktrace) of any of the disks for 30 seconds
both without regression and after regression and upload it somewhere.
Staring at it might give some clues. 

Thanks
Vivek

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