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From: Jan Kara <jack@suse.cz>
To: Jens Axboe <axboe@fb.com>
Cc: Jan Kara <jack@suse.cz>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-block@vger.kernel.org, dchinner@redhat.com,
	sedat.dilek@gmail.com
Subject: Re: [PATCH 7/8] wbt: add general throttling mechanism
Date: Tue, 3 May 2016 17:22:49 +0200
Message-ID: <20160503152249.GF25436@quack2.suse.cz> (raw)
In-Reply-To: <5728B45F.6050200@fb.com>

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@suse.com>
SUSE Labs, CR

  reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-04-26 15:55 [PATCHSET v5] Make background writeback great again for the first time Jens Axboe
2016-04-26 15:55 ` [PATCH 1/8] block: add WRITE_BG Jens Axboe
2016-04-26 15:55 ` [PATCH 2/8] writeback: add wbc_to_write_cmd() Jens Axboe
2016-04-26 15:55 ` [PATCH 3/8] writeback: use WRITE_BG for kupdate and background writeback Jens Axboe
2016-04-26 15:55 ` [PATCH 4/8] writeback: track if we're sleeping on progress in balance_dirty_pages() Jens Axboe
2016-04-26 15:55 ` [PATCH 5/8] block: add code to track actual device queue depth Jens Axboe
2016-04-26 15:55 ` [PATCH 6/8] block: add scalable completion tracking of requests Jens Axboe
2016-05-05  7:52   ` Ming Lei
2016-04-26 15:55 ` [PATCH 7/8] wbt: add general throttling mechanism Jens Axboe
2016-04-27 12:06   ` xiakaixu
2016-04-27 15:21     ` Jens Axboe
2016-04-28  3:29       ` xiakaixu
2016-04-28 11:05   ` Jan Kara
2016-04-28 18:53     ` Jens Axboe
2016-04-28 19:03       ` Jens Axboe
2016-05-03  9:34       ` Jan Kara
2016-05-03 14:23         ` Jens Axboe
2016-05-03 15:22           ` Jan Kara [this message]
2016-05-03 15:32             ` Jens Axboe
2016-05-03 15:40         ` Jan Kara
2016-05-03 15:48           ` Jan Kara
2016-05-03 16:59             ` Jens Axboe
2016-05-03 18:14               ` Jens Axboe
2016-05-03 19:07                 ` Jens Axboe
2016-04-26 15:55 ` [PATCH 8/8] writeback: throttle buffered writeback Jens Axboe
2016-04-27 18:01 ` [PATCHSET v5] Make background writeback great again for the first time Jan Kara
2016-04-27 18:17   ` Jens Axboe
2016-04-27 20:37     ` Jens Axboe
2016-04-27 20:59       ` Jens Axboe
2016-04-28  4:06         ` xiakaixu
2016-04-28 18:36           ` Jens Axboe
2016-04-28 11:54         ` Jan Kara
2016-04-28 18:46           ` Jens Axboe
2016-05-03 12:17             ` Jan Kara
2016-05-03 12:40               ` Chris Mason
2016-05-03 13:06                 ` Jan Kara
2016-05-03 13:42                   ` Chris Mason
2016-05-03 13:57                     ` Jan Kara
2016-05-11 16:36               ` Jan Kara
2016-05-13 18:29                 ` Jens Axboe
2016-05-16  7:47                   ` Jan Kara
2016-08-31 17:05 [PATCHSET v6] Throttled background buffered writeback Jens Axboe
2016-08-31 17:05 ` [PATCH 7/8] wbt: add general throttling mechanism Jens Axboe
2016-09-01 18:05   ` Omar Sandoval
2016-09-01 18:51     ` Jens Axboe
2016-09-07 14:46 [PATCH 0/8] Throttled background buffered writeback v7 Jens Axboe
2016-09-07 14:46 ` [PATCH 7/8] wbt: add general throttling mechanism Jens Axboe

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