linux-kernel - Re: cfq-iosched preempt issues

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Message-ID: <x49wrkh33ar.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 02 Mar 2011 11:17:16 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	jaxboe@...ionio.com, vgoyal@...hat.com, czoccolo@...il.com,
	guijianfeng@...fujitsu.com, linux-kernel@...r.kernel.org
Subject: Re: cfq-iosched preempt issues

Shaohua Li <shaohua.li@...el.com> writes:

> queue preemption is good for some workloads and not for others. With commit
> f8ae6e3eb825, the impact is amplified. I currently have two issues with it:
> 1. In a multi-threaded workload, each thread runs a random read/write (for
> example, mmap write) with iodepth 1. I found the queue depth gets smaller
> with commit f8ae6e3eb825. The reason is write gets preempted, so more threads
> are waitting for write, and on the other hand, there are less threads doing
> read. This will make the queue depth small, so performance drops a little.
> So in this case, speed up write can speed up read too, but we can't detect
> it.

I don't fully understand your workload.  What is the aio-stress or fio
command line/config file?

> 2. cfq_may_dispatch doesn't limit queue depth if the queue is the sole queue.
> What about if there are two queues, one sync and one async? If the sync queue's
> think time is small, we can treat it as the sole queue, because the sync queue
> will preempt async queue, so we don't need care about the async queue's latency.
> The issue exists before, but f8ae6e3eb825 amplifies it. Below is a patch for it.

I'm not sure I entirely agree with that reasoning.  Do you have real
workloads that are regressing due to this commit, or is it just these
cooked up benchmarks?

Cheers,
Jeff
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