linux-kernel - Re: Switching to MQ by default may generate some bug reports

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Date:   Thu, 3 Aug 2017 11:47:59 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Paolo Valente <paolo.valente@...aro.org>
Subject: Re: Switching to MQ by default may generate some bug reports

On Thu, Aug 03, 2017 at 05:57:50PM +0800, Ming Lei wrote:
> On Thu, Aug 3, 2017 at 5:42 PM, Mel Gorman <mgorman@...hsingularity.net> wrote:
> > On Thu, Aug 03, 2017 at 05:17:21PM +0800, Ming Lei wrote:
> >> Hi Mel Gorman,
> >>
> >> On Thu, Aug 3, 2017 at 4:51 PM, Mel Gorman <mgorman@...hsingularity.net> wrote:
> >> > Hi Christoph,
> >> >
> >> > I know the reasons for switching to MQ by default but just be aware that it's
> >> > not without hazards albeit it the biggest issues I've seen are switching
> >> > CFQ to BFQ. On my home grid, there is some experimental automatic testing
> >> > running every few weeks searching for regressions. Yesterday, it noticed
> >> > that creating some work files for a postgres simulator called pgioperf
> >> > was 38.33% slower and it auto-bisected to the switch to MQ. This is just
> >> > linearly writing two files for testing on another benchmark and is not
> >> > remarkable. The relevant part of the report is
> >>
> >> We saw some SCSI-MQ performance issue too, please see if the following
> >> patchset fixes your issue:
> >>
> >> http://marc.info/?l=linux-block&m=150151989915776&w=2
> >>
> >
> > That series is dealing with problems with legacy-deadline vs mq-none where
> > as the bulk of the problems reported in this mail are related to
> > legacy-CFQ vs mq-BFQ.
> 
> The serials deals with none and all mq schedulers, and you can see
> the improvement on mq-deadline in cover letter, :-)
> 

Would it be expected to fix a 2x to 4x slowdown as experienced by BFQ
that was not observed on other schedulers?

-- 
Mel Gorman
SUSE Labs