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From: Jens Axboe <axboe@fb.com>
To: Jan Kara <jack@suse.cz>
Cc: <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 08:23:27 -0600
Message-ID: <5728B45F.6050200@fb.com> (raw)
In-Reply-To: <20160503093410.GD12748@quack2.suse.cz>

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

The latter part can still be handled independently, and the former as 
well of course, wbt just deals with throttling back #1 for buffered writes.

>> But yes, fairness is something that we have to pay attention to. Right now
>> the wait queue has no priority associated with it, that should probably be
>> improved to be able to wakeup in a more appropriate order.
>> Needs testing, but hopefully it works out since if you do run into
>> starvation, then you'll go to the back of the queue for the next attempt.
>
> Yeah, once I'll hunt down that regression with old disk, I can have a look
> into how writeback throttling plays together with blkio-controller.

Thanks!

-- 
Jens Axboe

  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 [this message]
2016-05-03 15:22           ` Jan Kara
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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