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From: Kevin Wolf <kwolf@redhat.com>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: "Zhi Yong Wu" <wuzhy@cn.ibm.com>,
	qemu-devel@nongnu.org, "Benoît Canet" <benoit@irqsave.net>
Subject: Re: [Qemu-devel] [PATCH v2 0/4] block: fix I/O throttling oscillations
Date: Fri, 5 Apr 2013 14:32:36 +0200	[thread overview]
Message-ID: <20130405123236.GB2351@dhcp-200-207.str.redhat.com> (raw)
In-Reply-To: <1365154342-10513-1-git-send-email-stefanha@redhat.com>

Am 05.04.2013 um 11:32 hat Stefan Hajnoczi geschrieben:
> Benoît Canet <benoit@irqsave.net> reported that QEMU I/O throttling can
> oscillate under continuous I/O.  The test case runs 50 threads performing
> random writes and a -drive iops=150 limit is used.
> 
> Since QEMU I/O throttling is implemented using 100 millisecond time slices,
> we'd expect 150 +/- 15 IOPS.  Anything outside that range indicates a problem
> with the I/O throttling algorithm.
> 
> It turned out that even a single thread performing sequential I/O continuously
> is throttled outside the 150 +/- 15 IOPS range.  The continous stream of I/O
> slows down as time goes on but resets to 150 IOPS again when interrupted.  This
> can be tested with:
> 
>   $ iostat -d 1 -x /dev/vdb &
>   $ dd if=/dev/vdb of=/dev/null bs=4096 iflag=direct
> 
> This patches addresses these problems as follows:
> 
> 1. Account for I/O requests when they are submitted instead of completed.  This
>    ensures that we do not exceed the budget for this slice.  Exceeding the
>    budget leads to fluctuations since we have to make up for this later.
> 
> 2. Use constant 100 millisecond slice time.  Adjusting the slice time at
>    run-time led to oscillations.  Since the reason for adjusting slice time is
>    not clear, drop this behavior.
> 
> I have also included two code clean-up patches.
> 
> Tested-By: Benoit Canet <benoit@irqsave.net>
> 
> v2:
>  * Account slice_submitted after both bps and iops checks pass [kwolf]

Thanks, applied all to the block branch.

Kevin

      parent reply	other threads:[~2013-04-05 12:33 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-04-05  9:32 [Qemu-devel] [PATCH v2 0/4] block: fix I/O throttling oscillations Stefan Hajnoczi
2013-04-05  9:32 ` [Qemu-devel] [PATCH v2 1/4] block: fix I/O throttling accounting blind spot Stefan Hajnoczi
2013-04-05  9:32 ` [Qemu-devel] [PATCH v2 2/4] block: keep I/O throttling slice time constant Stefan Hajnoczi
2013-04-05  9:32 ` [Qemu-devel] [PATCH v2 3/4] block: drop duplicated slice extension code Stefan Hajnoczi
2013-04-05  9:32 ` [Qemu-devel] [PATCH v2 4/4] block: clean up I/O throttling wait_time code Stefan Hajnoczi
2013-04-05 12:32 ` Kevin Wolf [this message]

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