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From: "Michal Koutný" <mkoutny@suse.com>
To: Tejun Heo <tj@kernel.org>
Cc: hannes@cmpxchg.org, clm@fb.com, dennisz@fb.com,
	Josef Bacik <jbacik@fb.com>,
	kernel-team@fb.com, newella@fb.com, lizefan@huawei.com,
	axboe@kernel.dk, Paolo Valente <paolo.valente@linaro.org>,
	Rik van Riel <riel@surriel.com>,
	josef@toxicpanda.com, cgroups@vger.kernel.org,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 08/10] blkcg: implement blk-iocost
Date: Wed, 9 Oct 2019 17:36:29 +0200	[thread overview]
Message-ID: <20191009153629.GA5400@blackbody.suse.cz> (raw)
In-Reply-To: <20191003164552.GA3247445@devbig004.ftw2.facebook.com>

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On Thu, Oct 03, 2019 at 09:45:52AM -0700, Tejun Heo <tj@kernel.org> wrote:
> [...] but the qos file gets weird because the content of the file is
> more resource control policies than device properties.
I see two facets on this -- the semantics of the QoS controls and
storing controller parameters generally.

Because I'm not fully convinced using the root cgroup for the latter is
a good idea and I don't have a better one (what about
/sys/kernel/cgroup/?), I'd like to question the former to potentially
postpone finding the place for its parameters :-)


On Wed, Aug 28, 2019 at 03:05:58PM -0700, Tejun Heo <tj@kernel.org> wrote:
> [...]
> Please see the top comment in blk-iocost.c and documentation for
> more details.
I admit I did't grasp the explanations in the cgroup-v2.rst, perhaps
some of the explanations from blk-iocost.c would be useful there as
well.

IIUC, the controls are supposed to be abstracted and generic to express
high-level ideas and be independent of particular details.
Here a bunch of parameters is introduced whose tuning may become a
complex optimization task.

What is the metric that is the QoS controller striving to guarantee?
How does it differ from the io.latency policy?


> [...] 
> + * 2-2. Vrate Adjustment
> + * [...] When this delay becomes noticeable, it's a clear
> + * indication that the device is saturated and we lower the vrate.  This
> + * saturation signal is fairly conservative as it only triggers when both
> + * hardware and software queues are filled up, and is used as the default
> + * busy signal.
(The following paragraph is based only on naïve understanding of the
block layer.) So the device's vrate is lowered, causing its vtime
growing slower, i.e.  postponing issuing an IO later for all cgroups
accessing the device. But what's the purpose of this? If the queues fill
up, wouldn't be all naturally pushed back by the longer queue time
anyway? And wouldn't slowing down the device's vtime just cause queueing
elsewhere?

Thanks,
Michal

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  reply	other threads:[~2019-10-09 15:36 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-28 22:05 [PATCHSET v3 block/for-linus] IO cost model based work-conserving porportional controller Tejun Heo
2019-08-28 22:05 ` [PATCH 01/10] blkcg: pass @q and @blkcg into blkcg_pol_alloc_pd_fn() Tejun Heo
2019-08-28 22:05 ` [PATCH 02/10] blkcg: make ->cpd_init_fn() optional Tejun Heo
2019-08-28 22:05 ` [PATCH 03/10] blkcg: separate blkcg_conf_get_disk() out of blkg_conf_prep() Tejun Heo
2019-08-28 22:05 ` [PATCH 04/10] block/rq_qos: add rq_qos_merge() Tejun Heo
2019-08-28 22:05 ` [PATCH 05/10] block/rq_qos: implement rq_qos_ops->queue_depth_changed() Tejun Heo
2019-08-28 22:05 ` [PATCH 06/10] blkcg: s/RQ_QOS_CGROUP/RQ_QOS_LATENCY/ Tejun Heo
2019-08-28 22:05 ` [PATCH 07/10] blk-mq: add optional request->alloc_time_ns Tejun Heo
2019-08-28 22:05 ` [PATCH 08/10] blkcg: implement blk-iocost Tejun Heo
2019-08-29 15:53   ` [PATCH] blkcg: fix missing free on error path of blk_iocost_init() Tejun Heo
2019-09-10 12:55   ` [PATCH 08/10] blkcg: implement blk-iocost Michal Koutný
2019-09-10 16:08     ` Tejun Heo
2019-09-11  8:18       ` Paolo Valente
2019-09-11 14:16         ` Tejun Heo
2019-09-11 15:54           ` Tejun Heo
2019-09-11 16:44           ` Paolo Valente
2019-10-03 14:51       ` Michal Koutný
2019-10-03 16:45         ` Tejun Heo
2019-10-09 15:36           ` Michal Koutný [this message]
2019-10-14 15:36             ` Tejun Heo
2019-11-01 16:15               ` Michal Koutný
2019-11-01 16:56                 ` Paolo Valente
2019-08-28 22:05 ` [PATCH 09/10] blkcg: add tools/cgroup/iocost_monitor.py Tejun Heo
2019-08-28 22:06 ` [PATCH 10/10] blkcg: add tools/cgroup/iocost_coef_gen.py Tejun Heo
2019-08-29  3:29 ` [PATCHSET v3 block/for-linus] IO cost model based work-conserving porportional controller Jens Axboe
     [not found] ` <20190829082248.6464-1-hdanton@sina.com>
2019-08-29 15:43   ` [PATCH 07/10] blk-mq: add optional request->alloc_time_ns Tejun Heo
     [not found] ` <20190829133928.16192-1-hdanton@sina.com>
2019-08-29 15:46   ` [PATCH 08/10] blkcg: implement blk-iocost Tejun Heo
2019-08-29 15:54 ` [PATCHSET v3 block/for-linus] IO cost model based work-conserving porportional controller Paolo Valente
2019-08-29 15:56   ` Tejun Heo
  -- strict thread matches above, loose matches on Subject: below --
2019-07-10 20:51 [PATCHSET v2 " Tejun Heo
2019-07-10 20:51 ` [PATCH 08/10] blkcg: implement blk-iocost Tejun Heo

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