From: Paolo Valente <paolo.valente@linaro.org>
To: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>,
Fabio Checconi <fchecconi@gmail.com>,
Arianna Avanzini <avanzini.arianna@gmail.com>,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
Ulf Hansson <ulf.hansson@linaro.org>,
Linus Walleij <linus.walleij@linaro.org>,
Mark Brown <broonie@kernel.org>
Subject: Re: [PATCH RFC 09/22] block, cfq: replace CFQ with the BFQ-v0 I/O scheduler
Date: Sat, 16 Apr 2016 00:08:44 +0200 [thread overview]
Message-ID: <D84E57F8-A4EB-4EFB-8F0A-45C29E860E1D@linaro.org> (raw)
In-Reply-To: <20160415192930.GL12583@htj.duckdns.org>
Il giorno 15/apr/2016, alle ore 21:29, Tejun Heo <tj@kernel.org> ha scritto:
> Hello, Paolo.
>
> On Fri, Apr 15, 2016 at 06:17:55PM +0200, Paolo Valente wrote:
>>> I don't think that is true with time based scheduling. If you
>>> allocate 50% of time, it'll get close to 50% of IO time which
>>> translates to bandwidth which is lower than 50% but still in the
>>> ballpark.
>>
>> But this is the same minimal service guarantee that you get with BFQ
>> in any case. I'm sorry for being so confusing to not make this central
>> point clear :(
>
> lol sorry about being dumb.
>
dumb? my problem is your remaining patience ...
>>> That is very different from "we can't guarantee anything if
>>> the other workloads are highly variable”.
>>
>> If you have 50% of the time, but
>> . you don’t know anything about your workload properties, and
>> . the device speed can vary by two orders of magnitude,
>> then you can't provide any bandwidth guarantee, with any scheduler. Of
>> course I'm neglecting the minimal, trivial guarantee "getting a fraction
>> of the minimum possible speed of the device".
>
> Oh, the guarantee is about "getting close to half of the available IO
> resource", what that translates to depends on the underlying hardware
> and the workload of course.
>
yes
>> If you have 50% of the time allocated for a quasi-sequential workload,
>> then bandwidth and latencies may vary by an uncontrollable 30 or 40%,
>> depending on what you and the other groups do.
>
> Yes, may be but it won't dive to 5% depending on what others are
> doing.
>
exact
>> With the same device, if you have 50% of the bandwidth allocated with
>> BFQ for a quasi-sequential workload, then you can provide bandwidth
>> and latencies that may vary at most by a (still uncontrollable) 3 or
>> 4%, depending on what you and the other groups do.
>>
>> This improvement is shown, e.g., in my--admittedly boring--numerical
>> example, and is confirmed by my experimental results so far.
>
> I don't think the above is true. Are you saying that the following
> two cases would lead to the same outcome for cgroup A?
>
> cgroup A (50) cgroup B (50)
> case 1 sequential sequential
> case 2 sequential random (to a certain degree)
>
> The aggregate bandwidths for case 1 and 2 would be wildly different
> depending on the randomness of the second workload. What cgroup A
> would be able to get would fluctuate accordingly, no?
>
Your example is definitely to the point.
The answer to your question is no. In fact, in both cases cgroup A
will get exactly the same service slots, and in each slot exactly the
same number of sectors transferred. In particular, cgroup B will
systematically hit the timeout in the second case.
In other words, in case 2 cgroup A is guaranteed the same bandwidth
that it would get, in case 1, if cgroup B was quasi-sequential and so
slow to get served for a full time slice every time it got access to
the resource.
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.
>>> So, I get that for a lot of workload, especially interactive ones, IO
>>> patterns are quasi-sequential and bw based scheduling is beneficial
>>> and we don't care that much about fairness in general; however, it's
>>> problematic that it would make the behavior of proportional control
>>> quite surprising.
>>
>> If I have somehow convinced you with what I wrote above, then I hope
>> we might agree that a surprising behavior of BFQ with cgroups would be
>> just a matter of bugs.
>
> I think I might still need more help. What am I missing?
>
I hope that what I wrote above did help.
Thanks,
Paolo
> Thanks.
>
> --
> tejun
next prev parent reply other threads:[~2016-04-15 22:08 UTC|newest]
Thread overview: 103+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-01 22:12 [PATCH RFC 00/22] Replace the CFQ I/O Scheduler with BFQ Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 01/22] block, cfq: remove queue merging for close cooperators Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 02/22] block, cfq: remove close-based preemption Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 03/22] block, cfq: remove deep seek queues logic Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 04/22] block, cfq: remove SSD-related logic Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 05/22] block, cfq: get rid of hierarchical support Paolo Valente
2016-02-10 23:04 ` Tejun Heo
2016-02-01 22:12 ` [PATCH RFC 06/22] block, cfq: get rid of queue preemption Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 07/22] block, cfq: get rid of workload type Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 08/22] block, cfq: get rid of latency tunables Paolo Valente
2016-02-10 23:05 ` Tejun Heo
2016-02-01 22:12 ` [PATCH RFC 09/22] block, cfq: replace CFQ with the BFQ-v0 I/O scheduler Paolo Valente
2016-02-11 22:22 ` Tejun Heo
2016-02-12 0:35 ` Mark Brown
2016-02-17 15:57 ` Tejun Heo
2016-02-17 16:02 ` Mark Brown
2016-02-17 17:04 ` Tejun Heo
2016-02-17 18:13 ` Jonathan Corbet
2016-02-17 19:45 ` Tejun Heo
2016-02-17 19:56 ` Jonathan Corbet
2016-02-17 20:14 ` Tejun Heo
2016-02-17 9:02 ` Paolo Valente
2016-02-17 17:02 ` Tejun Heo
2016-02-20 10:23 ` Paolo Valente
2016-02-20 11:02 ` Paolo Valente
2016-03-01 18:46 ` Tejun Heo
2016-03-04 17:29 ` Linus Walleij
2016-03-04 17:39 ` Christoph Hellwig
2016-03-04 18:10 ` Austin S. Hemmelgarn
2016-03-11 11:16 ` Christoph Hellwig
2016-03-11 13:38 ` Austin S. Hemmelgarn
2016-03-05 12:18 ` Linus Walleij
2016-03-11 11:17 ` Christoph Hellwig
2016-03-11 11:24 ` Nikolay Borisov
2016-03-11 11:49 ` Christoph Hellwig
2016-03-11 14:53 ` Linus Walleij
2016-03-09 6:55 ` Paolo Valente
2016-04-13 19:54 ` Tejun Heo
2016-04-14 5:03 ` Mark Brown
2016-03-09 6:34 ` Paolo Valente
2016-04-13 20:41 ` Tejun Heo
2016-04-14 10:23 ` Paolo Valente
2016-04-14 16:29 ` Tejun Heo
2016-04-15 14:20 ` Paolo Valente
2016-04-15 15:08 ` Tejun Heo
2016-04-15 16:17 ` Paolo Valente
2016-04-15 19:29 ` Tejun Heo
2016-04-15 22:08 ` Paolo Valente [this message]
2016-04-15 22:45 ` Tejun Heo
2016-04-16 6:03 ` Paolo Valente
2016-04-15 14:49 ` Linus Walleij
2016-02-01 22:12 ` [PATCH RFC 10/22] block, bfq: add full hierarchical scheduling and cgroups support Paolo Valente
2016-02-11 22:28 ` Tejun Heo
2016-02-17 9:07 ` Paolo Valente
2016-02-17 17:14 ` Tejun Heo
2016-02-17 17:45 ` Tejun Heo
2016-04-20 9:32 ` Paolo
2016-04-22 18:13 ` Tejun Heo
2016-04-22 18:19 ` Paolo Valente
2016-04-22 18:41 ` Tejun Heo
2016-04-22 19:05 ` Paolo Valente
2016-04-22 19:32 ` Tejun Heo
2016-04-23 7:07 ` Paolo Valente
2016-04-25 19:24 ` Tejun Heo
2016-04-25 20:30 ` Paolo
2016-05-06 20:20 ` Paolo Valente
2016-05-12 13:11 ` Paolo
2016-07-27 16:13 ` [PATCH RFC V8 00/22] Replace the CFQ I/O Scheduler with BFQ Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 01/22] block, cfq: remove queue merging for close cooperators Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 02/22] block, cfq: remove close-based preemption Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 03/22] block, cfq: remove deep seek queues logic Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 04/22] block, cfq: remove SSD-related logic Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 05/22] block, cfq: get rid of hierarchical support Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 06/22] block, cfq: get rid of queue preemption Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 07/22] block, cfq: get rid of workload type Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 08/22] block, cfq: get rid of latency tunables Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 09/22] block, cfq: replace CFQ with the BFQ-v0 I/O scheduler Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 10/22] block, bfq: add full hierarchical scheduling and cgroups support Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 11/22] block, bfq: improve throughput boosting Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 12/22] block, bfq: modify the peak-rate estimator Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 13/22] block, bfq: add more fairness with writes and slow processes Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 14/22] block, bfq: improve responsiveness Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 15/22] block, bfq: reduce I/O latency for soft real-time applications Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 16/22] block, bfq: preserve a low latency also with NCQ-capable drives Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 17/22] block, bfq: reduce latency during request-pool saturation Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 18/22] block, bfq: add Early Queue Merge (EQM) Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 19/22] block, bfq: reduce idling only in symmetric scenarios Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 20/22] block, bfq: boost the throughput on NCQ-capable flash-based devices Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 21/22] block, bfq: boost the throughput with random I/O on NCQ-capable HDDs Paolo Valente
2016-07-27 16:13 ` [PATCH RFC V8 22/22] block, bfq: handle bursts of queue activations Paolo Valente
2016-07-28 16:50 ` [PATCH RFC V8 00/22] Replace the CFQ I/O Scheduler with BFQ Paolo
2016-02-01 22:12 ` [PATCH RFC 11/22] block, bfq: improve throughput boosting Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 12/22] block, bfq: modify the peak-rate estimator Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 13/22] block, bfq: add more fairness to boost throughput and reduce latency Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 14/22] block, bfq: improve responsiveness Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 15/22] block, bfq: reduce I/O latency for soft real-time applications Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 16/22] block, bfq: preserve a low latency also with NCQ-capable drives Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 17/22] block, bfq: reduce latency during request-pool saturation Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 18/22] block, bfq: add Early Queue Merge (EQM) Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 19/22] block, bfq: reduce idling only in symmetric scenarios Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 20/22] block, bfq: boost the throughput on NCQ-capable flash-based devices Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 21/22] block, bfq: boost the throughput with random I/O on NCQ-capable HDDs Paolo Valente
2016-02-01 22:12 ` [PATCH RFC 22/22] block, bfq: handle bursts of queue activations Paolo Valente
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