From: Jens Axboe <axboe@kernel.dk>
To: Edoardo Comar <ECOMAR@uk.ibm.com>
Cc: fio@vger.kernel.org
Subject: Re: latency measurements when adding thinktime
Date: Fri, 17 May 2013 12:48:59 +0200 [thread overview]
Message-ID: <20130517104859.GX697@kernel.dk> (raw)
In-Reply-To: <20130517104305.GW697@kernel.dk>
On Fri, May 17 2013, Jens Axboe wrote:
> On Fri, May 17 2013, Jens Axboe wrote:
> > On Fri, May 17 2013, Edoardo Comar wrote:
> > > Hi Jens,
> > >
> > > thanks a lot, I tried your suggestion straight away:
> > > The cpu line in the output shows that the job has not been idle at all.
> > > but even with thinktime_spin, the latency stays high at at 150ms.
> > >
> > > Note also that if I increase thinktime & thinktime_spin by another 10000
> > > us
> > > to a total of 20000, then the reported latency goes up another 150ms to a
> > > total of 300ms.
> > > Could this be a bug?
> >
> > It certainly could. Let me take a look here and see if I can reproduce
> > it.
>
> And it was... Basically the same issue that was fixed for rate limiting.
> When going to sleep or spinning, ensure that we have all IOs flushed.
> Otherwise we could unfairly attribute the sleep towards the completion
> latencies.
>
> Fixed here:
>
> http://git.kernel.dk/?p=fio.git;a=commit;h=002e7183cb86d6100efef690b6fa90bf0988b084
>
> or just git pull and you'll get the fix.
Also note this:
http://git.kernel.dk/?p=fio.git;a=commit;h=4d01ece69f7b4d7bd56210e0f839944a91c5679f
which explains how thinktime_blocks will effectively reduce iodepth=, if
the former is smaller. So if you want a larger queue depth with
thinktime being set, consider setting thinktime_blocks= to only wait
every X blocks instead. Or keep it at 1, but do realize that this then
caps your effective device queue depth to 1.
--
Jens Axboe
next prev parent reply other threads:[~2013-05-17 10:49 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-17 0:00 latency measurements when adding thinktime Edoardo Comar
2013-05-17 7:47 ` Jens Axboe
2013-05-17 9:24 ` Edoardo Comar
2013-05-17 10:22 ` Jens Axboe
2013-05-17 10:38 ` Edoardo Comar
2013-05-17 10:43 ` Jens Axboe
2013-05-17 10:48 ` Jens Axboe [this message]
2013-05-17 10:48 ` Edoardo Comar
2013-05-17 10:50 ` Jens Axboe
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