Re: [PATCH 08/10] blkcg: implement blk-iocost

From: Paolo Valente <paolo.valente@linaro.org>
To: "Michal Koutný" <mkoutny@suse.com>
Cc: Tejun Heo <tj@kernel.org>, Johannes Weiner <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, 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: Fri, 1 Nov 2019 17:56:48 +0100	[thread overview]
Message-ID: <F45F2906-AB10-473D-B515-793E8FB10E6A@linaro.org> (raw)
In-Reply-To: <20191101161506.GA28212@blackbody.suse.cz>

> Il giorno 1 nov 2019, alle ore 17:15, Michal Koutný <mkoutny@suse.com> ha scritto:
> 
> Hello
> 

Hi Michal,

> (I realize it's likely late for the remark but I'd like to bring it up
> anyway.)
> 
> On Mon, Oct 14, 2019 at 08:36:43AM -0700, Tejun Heo <tj@kernel.org> wrote:
>> We likely can talk on the subject
>> for a really long time probalby because there's no clearly technically
>> better choice here, so...
> I agree with you that functionally the two options are equal and also
> from configuration POV they seem both sensible.
> 
> I checked where BFQ stores its per-device parameters and its under the
> sysfs directory of given device's iosched directory. So from the user
> perspective it'd be more consistent if all similar tunables resided
> under that location.
> 
> (OTOH, I admit I'm not that familiar with block layer internals to
> identify the overlap between IO scheduler and IO controller.)
> 

If useful for you to know, BFQ parameters are not meant to changed
(apart from the low_latency tunable, if one wants full control on
weights).  Parameters are a testing aid, to use in case of an anomaly.
After solving the anomaly, default values should be used again.

Thanks,
Paolo

>> Yeah, it's kinda unfortunate that it requires this many parameters but
>> at least my opinion is that that's reflecting the inherent
>> complexities of the underlying devices and how workloads interact with
>> them.
> After I learnt about the existence of BFQ tunables, I'm no longer
> concerned by the complexity of the parameter space.
> 
> Thanks for the explanations of QoS purpose.
> 
>> For QoS parameters, Andy is currently working on a method to determine
>> the set of parametesr which are at the edge of total work cliff -
>> ie. the point where tighetning QoS params further starts reducing the
>> total amount of work the device can do significantly.
> The QoS description in the Documentation/ describes the interpretation
> of the individual parameters, however, this purpose and how it works was
> not clear to be from that. I think the QoS policy would deserve similar
> description in the Documentation/.
> 
>> Nothing can issue IOs indefinitely without some of them completing and
>> the total amount of work a workload can do is conjoined with the
>> completion latencies. [...]
> I may reply to this point later. However, if that provably works, I'm
> likely missing something in my understanding, so that'd be irrelevant.
> 
> Cheers,
> Michal