linux-kernel - Re: [PATCH 7/8] wbt: add general throttling mechanism

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Message-ID: <20160503152249.GF25436@quack2.suse.cz>
Date:	Tue, 3 May 2016 17:22:49 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jens Axboe <axboe@...com>
Cc:	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
	dchinner@...hat.com, sedat.dilek@...il.com
Subject: Re: [PATCH 7/8] wbt: add general throttling mechanism

On Tue 03-05-16 08:23:27, Jens Axboe wrote:
> On 05/03/2016 03:34 AM, Jan Kara wrote:
> >On Thu 28-04-16 12:53:50, Jens Axboe wrote:
> >>>2) As far as I can see in patch 8/8, you have plugged the throttling above
> >>>    the IO scheduler. When there are e.g. multiple cgroups with different IO
> >>>    limits operating, this throttling can lead to strange results (like a
> >>>    cgroup with low limit using up all available background "slots" and thus
> >>>    effectively stopping background writeback for other cgroups)? So won't
> >>>    it make more sense to plug this below the IO scheduler? Now I understand
> >>>    there may be other problems with this but I think we should put more
> >>>    though to that and provide some justification in changelogs.
> >>
> >>One complexity is that we have to do this early for blk-mq, since once you
> >>get a request, you're already sitting on the hw tag. CoDel should actually
> >>work fine at each hop, so hopefully this will as well.
> >
> >OK, I see. But then this suggests that any IO scheduling and / or
> >cgroup-related throttling should happen before we get a request for blk-mq
> >as well? And then we can still do writeback throttling below that layer?
> 
> Not necessarily. For IO scheduling, basically we care about two parts:
> 
> 1) Are you allowed to allocate the resources to queue some IO
> 2) Are you allowed to dispatch

But then it seems suboptimal to waste a relatively scarce resource (which
HW tag is AFAIU) just because you happen to run from a cgroup that is
bandwidth limited and thus are not allowed to dispatch?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR