linux-kernel - Re: [PATCH] cfq-iosched: reduce write depth only if sync was delayed

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Message-ID: <x49k4wwnhbh.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 09 Dec 2009 13:16:50 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] cfq-iosched: reduce write depth only if sync was delayed

Corrado Zoccolo <czoccolo@...il.com> writes:

> Hi Jeff,
> On Tue, Dec 8, 2009 at 7:00 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
>> Corrado Zoccolo <czoccolo@...il.com> writes:
>>
>>> The introduction of ramp-up formula for async queue depths has
>>> slowed down dirty page reclaim, by reducing async write performance.
>>> This patch makes sure the formula kicks in only when sync request
>>> was recently delayed.
>>> @@ -3706,7 +3707,7 @@ static void *cfq_init_queue(struct request_queue *q)
>>>       cfqd->cfq_latency = 1;
>>>       cfqd->cfq_group_isolation = 0;
>>>       cfqd->hw_tag = -1;
>>> -     cfqd->last_end_sync_rq = jiffies;
>>> +     cfqd->last_delayed_sync = jiffies - HZ;
>>
>> So, umm, what's that about?
>
> Previously, when cfq started, it started in a state where writes were
> completely throttled. Now, we optimistically prefer to start with a
> reasonable max depth (10)

OK.  Can we put a comment in there and change the initialization to
cfq_slice_sync * 10?

Thanks,
Jeff
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