linux-kernel - Re: [PATCH V3 00/11] block-throttle: add .high limit

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Message-Id: <278BCC7B-ED58-4FDF-9243-FAFC3F862E4D@unimore.it>
Date:   Tue, 4 Oct 2016 19:01:39 +0200
From:   Paolo Valente <paolo.valente@...more.it>
To:     Tejun Heo <tj@...nel.org>
Cc:     Vivek Goyal <vgoyal@...hat.com>, Shaohua Li <shli@...com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...com>, Kernel-team@...com,
        jmoyer@...hat.com, Mark Brown <broonie@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH V3 00/11] block-throttle: add .high limit

> Il giorno 04 ott 2016, alle ore 18:27, Tejun Heo <tj@...nel.org> ha scritto:
> 
> Hello,
> 
> On Tue, Oct 04, 2016 at 06:22:28PM +0200, Paolo Valente wrote:
>> Could you please elaborate more on this point?  BFQ uses sectors
>> served to measure service, and, on the all the fast devices on which
>> we have tested it, it accurately distributes
>> bandwidth as desired, redistributes excess bandwidth with any issue,
>> and guarantees high responsiveness and low latency at application and
>> system level (e.g., ~0 drop rate in video playback, with any background
>> workload tested).
> 
> The same argument as before.  Bandwidth is a very bad measure of IO
> resources spent.  For specific use cases (like desktop or whatever),
> this can work but not generally.
> 

Actually, we have already discussed this point, and IMHO the arguments
that (apparently) convinced you that bandwidth is the most relevant
service guarantee for I/O in desktops and the like, prove that
bandwidth is the most important service guarantee in servers too.

Again, all the examples I can think of seem to confirm it:
. file hosting: a good service must guarantee reasonable read/write,
i.e., download/upload, speeds to users
. file streaming: a good service must guarantee low drop rates, and
this can be guaranteed only by guaranteeing bandwidth and latency
. web hosting: high bandwidth and low latency needed here too
. clouds: high bw and low latency needed to let, e.g., users of VMs
enjoy high responsiveness and, for example, reasonable file-copy
time
...

To put in yet another way, with packet I/O in, e.g., clouds, there are
basically the same issues, and the main goal is again guaranteeing
bandwidth and low latency among nodes.

Could you please provide a concrete server example (assuming we still
agree about desktops), where I/O bandwidth does not matter while time
does?

>> Could you please suggest me some test to show how sector-based
>> guarantees fails?
> 
> Well, mix 4k random and sequential workloads and try to distribute the
> acteual IO resources.
> 

If I'm not mistaken, we have already gone through this example too,
and I thought we agreed on what service scheme worked best, again
focusing only on desktops.  To make a long story short(er), here is a
snippet from one of our last exchanges.

----------

On Sat, Apr 16, 2016 at 12:08:44AM +0200, Paolo Valente wrote:
> Maybe the source of confusion is the fact that a simple sector-based,
> proportional share scheduler always distributes total bandwidth
> according to weights. The catch is the additional BFQ rule: random
> workloads get only time isolation, and are charged for full budgets,
> so as to not affect the schedule of quasi-sequential workloads. So,
> the correct claim for BFQ is that it distributes total bandwidth
> according to weights (only) when all competing workloads are
> quasi-sequential. If some workloads are random, then these workloads
> are just time scheduled. This does break proportional-share bandwidth
> distribution with mixed workloads, but, much more importantly, saves
> both total throughput and individual bandwidths of quasi-sequential
> workloads.
> 
> We could then check whether I did succeed in tuning timeouts and
> budgets so as to achieve the best tradeoffs. But this is probably a
> second-order problem as of now.

[you] Ah, I see.  Yeah, that clears it up for me.  I'm gonna play with
cgroup settings and see how it actually behaves.

---------

Why does the above argument not work for a server too?  What am I
missing?

Thanks,
Paolo

> Thanks.
> 
> -- 
> tejun

--
Paolo Valente
Algogroup
Dipartimento di Scienze Fisiche, Informatiche e Matematiche
Via Campi 213/B
41125 Modena - Italy
http://algogroup.unimore.it/people/paolo/