linux-kernel - Re: cfq-iosched preempt issues

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Message-ID: <x49bp1t2pyd.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 02 Mar 2011 16:05:30 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Shaohua Li <shaohua.li@...el.com>, jaxboe@...ionio.com,
	czoccolo@...il.com, guijianfeng@...fujitsu.com,
	linux-kernel@...r.kernel.org
Subject: Re: cfq-iosched preempt issues

Vivek Goyal <vgoyal@...hat.com> writes:

> On Wed, Mar 02, 2011 at 08:43:41PM +0800, Shaohua Li wrote:
>> 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.
>> 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.
>> 
>> Any idea?
>
> CFQ is already very complicated, lets try to keep it simple. Because it
> is complicated, making it hierarchical for cgroup becomes even harder.
>
> IIUC, you are saying that cfqd->busy_queues check is not sufficient as
> it takes async queues also in account.
>
> So we can keep another count say, cfqd->busy_sync_queues and if there
> are no busy_sync_queues, allow unlimited depth and that should be
> a really simple few lines change.

That covers workload 2, but what about 1?  I'm really not sure what the
workload there is.

Cheers,
Jeff
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