linux-kernel - Re: CFQ idling kills I/O performance on ext4 with blkio cgroup controller

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Date:   Thu, 13 Jun 2019 10:20:53 +0200
From:   Jan Kara <jack@...e.cz>
To:     "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>
Cc:     Jan Kara <jack@...e.cz>, Paolo Valente <paolo.valente@...aro.org>,
        linux-fsdevel@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        linux-ext4@...r.kernel.org, cgroups@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Jeff Moyer <jmoyer@...hat.com>,
        Theodore Ts'o <tytso@....edu>, amakhalov@...are.com,
        anishs@...are.com, srivatsab@...are.com,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Stable <stable@...r.kernel.org>
Subject: Re: CFQ idling kills I/O performance on ext4 with blkio cgroup
 controller

On Wed 12-06-19 12:36:53, Srivatsa S. Bhat wrote:
> 
> [ Adding Greg to CC ]
> 
> On 6/12/19 6:04 AM, Jan Kara wrote:
> > On Tue 11-06-19 15:34:48, Srivatsa S. Bhat wrote:
> >> On 6/2/19 12:04 AM, Srivatsa S. Bhat wrote:
> >>> On 5/30/19 3:45 AM, Paolo Valente wrote:
> >>>>
> >> [...]
> >>>> At any rate, since you pointed out that you are interested in
> >>>> out-of-the-box performance, let me complete the context: in case
> >>>> low_latency is left set, one gets, in return for this 12% loss,
> >>>> a) at least 1000% higher responsiveness, e.g., 1000% lower start-up
> >>>> times of applications under load [1];
> >>>> b) 500-1000% higher throughput in multi-client server workloads, as I
> >>>> already pointed out [2].
> >>>>
> >>>
> >>> I'm very happy that you could solve the problem without having to
> >>> compromise on any of the performance characteristics/features of BFQ!
> >>>
> >>>
> >>>> I'm going to prepare complete patches.  In addition, if ok for you,
> >>>> I'll report these results on the bug you created.  Then I guess we can
> >>>> close it.
> >>>>
> >>>
> >>> Sounds great!
> >>>
> >>
> >> Hi Paolo,
> >>
> >> Hope you are doing great!
> >>
> >> I was wondering if you got a chance to post these patches to LKML for
> >> review and inclusion... (No hurry, of course!)
> >>
> >> Also, since your fixes address the performance issues in BFQ, do you
> >> have any thoughts on whether they can be adapted to CFQ as well, to
> >> benefit the older stable kernels that still support CFQ?
> > 
> > Since CFQ doesn't exist in current upstream kernel anymore, I seriously
> > doubt you'll be able to get any performance improvements for it in the
> > stable kernels...
> > 
> 
> I suspected as much, but that seems unfortunate though. The latest LTS
> kernel is based on 4.19, which still supports CFQ. It would have been
> great to have a process to address significant issues on older
> kernels too.

Well, you could still tune the performance difference by changing
slice_idle and group_idle tunables for CFQ (in
/sys/block/<device>/queue/iosched/).  Changing these to lower values will
reduce the throughput loss when switching between cgroups at the cost of
lower accuracy of enforcing configured IO proportions among cgroups.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR