linux-kernel - Re: IOPS based scheduler (Was: Re: [PATCH 18/21] blkcg: move blkio_group

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Date:	Thu, 05 Apr 2012 01:18:05 +0800
From:	Tao Ma <tm@....ma>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	Shaohua Li <shli@...nel.org>, Tejun Heo <tj@...nel.org>,
	axboe@...nel.dk, ctalbott@...gle.com, rni@...gle.com,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	containers@...ts.linux-foundation.org
Subject: Re: IOPS based scheduler (Was: Re: [PATCH 18/21] blkcg: move blkio_group_conf->weight
 to cfq)

On 04/05/2012 12:50 AM, Vivek Goyal wrote:
> On Thu, Apr 05, 2012 at 12:45:05AM +0800, Tao Ma wrote:
> 
> [..]
>>> In iops_mode(), expire each cfqq after dispatch of 1 or bunch of requests
>>> and you should get the same behavior (with slice_idle=0 and group_idle=0).
>>> So why write a new scheduler.
>> really? How could we config cfq to work like this? Or you mean we can
>> change the code for it?
> 
> You can just put a few lines of code to expire queue after 1-2 requests
> dispatched from the queue. Than run your workload with slice_idle=0
> and group_idle=0 and see what happens.
oh, yes I can do this to see whether the latency helps, but it is
hacking and doesn't work with the cgroup proportion...
> 
> I don't even know what your workload is. 
Sorry for not allowing to say more about it.
> 
>>>
>>> Only thing is that with above, current code will provide iops fairness only
>>> for groups. We should be able to tweak queue scheduling to support iops
>>> fairness also.
>> OK, as I have said in another e-mail another my concern is the
>> complexity. It will make cfq too much complicated. I just checked the
>> source code of shaohua's original patch, fiops scheduler is only ~700
>> lines, so with cgroup support added it would be ~1000 lines I guess.
>> Currently cfq-iosched.c is around ~4000 lines even after Tejun's cleanup
>> of io context...
> 
> I think a large chunk of that iops scheduler code will be borrowed from
> CFQ code. All the cgroup logic, queue creation logic, group scheduling
> logic etc. And that's the reason I was still exploring the possibility 
> of having common code base.
Yeah, actually I was thinking of abstracting a generic logic, but it
seems a lot bit hard. Maybe we can try to unify the code later?

Thanks
Tao
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