https://bugzilla.kernel.org/show_bug.cgi?id=41552

--- Comment #11 from Mark Petersen <mpete_06@hotmail.com>  2011-08-24 20:46:38 ---

I was finally able to run it with the deadline scheduler, and got the same
performance.  Unfortunately, I am not able to use the blktrace tool as it
requires a version of libc that we do not have on the system (we have 2.5 and
it requires at least 2.7).  Is there anything else I can use to trace it?

Thanks,
Mark

Date: Mon, 22 Aug 2011 15:48:54 -0400
From: vgoyal@redhat.com
To: mpete_06@hotmail.com
CC: bugme-daemon@bugzilla.kernel.org; axboe@kernel.dk; linux-mm@kvack.org; linux-scsi@vger.kernel.org; akpm@linux-foundation.org
Subject: 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)

On Mon, Aug 22, 2011 at 12:24:43PM -0700, Andrew Morton wrote:

quoted

(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:

quoted

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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