linux-kernel - Re: RFC: default group_isolation to 1, remove option

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Date:	Mon, 7 Mar 2011 10:20:10 -0800
From:	Justin TerAvest <teravest@...gle.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	Chad Talbott <ctalbott@...gle.com>,
	Nauman Rafique <nauman@...gle.com>,
	Divyesh Shah <dpshah@...gle.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	Corrado Zoccolo <czoccolo@...il.com>
Subject: Re: RFC: default group_isolation to 1, remove option

On Wed, Mar 2, 2011 at 7:45 PM, Jens Axboe <axboe@...nel.dk> wrote:
> On 2011-03-01 09:20, Vivek Goyal wrote:
>> I think creating per group request pool will complicate the
>> implementation further. (we have done that once in the past). Jens
>> once mentioned that he liked number of requests per iocontext limit
>> better than overall queue limit. So if we implement per iocontext
>> limit, it will get rid of need of doing anything extra for group
>> infrastructure.
>>
>> Jens, do you think per iocontext per queue limit on request
>> descriptors make sense and we can get rid of per queue overall limit?
>
> Since we practically don't need a limit anymore to begin with (or so is
> the theory), then yes we can move to per-ioc limits instead and get rid
> of that queue state. We'd have to hold on to the ioc for the duration of
> the IO explicitly from the request then.
>
> I primarily like that implementation since it means we can make the IO
> completion lockless, at least on the block layer side. We still have
> state to complete in the schedulers that require that, but it's a good
> step at least.

So, the primary advantage of using per-ioc limits that we can make
IO completions lockless?

I'm concerned that looking up the correct iocontext for a page will be more
complicated, and require more storage (than a css_id, anyway). I think Vivek
mentioned this too.

I don't understand what the advantage is of offering isolation between
iocontexts within a cgroup; if the user wanted isolation, shouldn't
they just create multiple cgroups? It seems like per-cgroup limits would work
as well.

Thanks,
Justin

>
> --
> Jens Axboe
>
>
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