linux-kernel - Re: [patch]cfq-iosched: delete deep seeky queue idle logic

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Date:	Mon, 26 Sep 2011 08:51:39 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Corrado Zoccolo <czoccolo@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Jens Axboe <jaxboe@...ionio.com>,
	Maxim Patlasov <maxim.patlasov@...il.com>
Subject: Re: [patch]cfq-iosched: delete deep seeky queue idle logic

On Fri, 2011-09-23 at 21:24 +0800, Vivek Goyal wrote:
> On Wed, Sep 21, 2011 at 07:16:20PM +0800, Shaohua Li wrote:
> 
> [..]
> > > Try a workload with one shallow seeky queue and one deep (16) one, on
> > > a single spindle NCQ disk.
> > > I think the behaviour when I submitted my patch was that both were
> > > getting 100ms slice (if this is not happening, probably some
> > > subsequent patch broke it).
> > > If you remove idling, they will get disk time roughly in proportion
> > > 16:1, i.e. pretty unfair.
> > I thought you are talking about a workload with one thread depth 4, and
> > the other thread depth 16. I did some tests here. In an old kernel,
> > without the deep seeky idle logic, the threads have disk time in
> > proportion 1:5. With it, they get almost equal disk time. SO this
> > reaches your goal. In a latest kernel, w/wo the logic, there is no big
> > difference (the 16 depth thread get about 5x more disk time). With the
> > logic, the depth 4 thread gets equal disk time in first several slices.
> > But after an idle expiration(mostly because current block plug hold
> > requests in task list and didn't add them to elevator), the queue never
> > gets detected as deep, because the queue dispatch request one by one.
> 
> When the plugged requests are flushed, then they will be added to elevator
> and at that point of time queue should be marked as deep?
The problem is there are just 2 or 3 requests are hold to the per-task
list and then get flushed into elevator later, so the queue isn't marked
as deep.

> Anyway, what's wrong with the idea I suggested in other mail of expiring
> a sync-noidle queue afer few reuqest dispatches so that it does not
> starve other sync-noidle queues.
The problem is how many requests a queue should dispatch.
cfq_prio_to_maxrq() == 16, which is too many. Maybe use 4, but it has
its risk. seeky requests from one task might be still much far way with
requests from other tasks.

Thanks,
Shaohua

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