linux-kernel - Re: [PATCH RFC] block, bfq: set default slice

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Date:   Fri, 2 Nov 2018 21:18:26 +0300
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Holger Hoffstätte <holger@...lied-asynchrony.com>
Cc:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        linux-block <linux-block@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] block, bfq: set default slice_idle to zero for
 non-rotational devices



> Il giorno 1 nov 2018, alle ore 22:06, Holger Hoffstätte <holger@...lied-asynchrony.com> ha scritto:
> 
> On 11/01/18 18:43, Konstantin Khlebnikov wrote:
>> With default 8ms idle slice BFQ is up to 10 times slower than CFQ
>> for massive random read workloads for common SATA SSD.
>> For now zero idle slice gives better out of box experience.
>> CFQ employs this since commit 41c0126b3f22 ("block: Make CFQ default
>> to IOPS mode on SSDs")
> 
> Well, that's interesting because 3 years ago I made the same suggestion
> and was told that BFQ's heuristics automagically make it not idle when
> rotational=0.

Yep, that automagic is probably 50% of the good of BFQ.

If one just sets slice_idle=0, then throughput is always maximum with
random I/O; but there is no control on I/O any longer.

At any rate, Konstantin, if you have some use case where BFQ fails,
I'll be very glad to analyze it, and hopefully improve BFQ.  Just one
request: use at least a 4.19.

Thanks,
Paolo

> Did you actually benchmark this? I just tried and don't
> get a noticeable performance difference with slice_idle=0 compared to
> deadline.
> 
> Discussion link:
> https://groups.google.com/forum/#!msg/bfq-iosched/iRMw2n3kYLY/6l9cIm3TBgAJ
> 
> curious..
> 
> Holger