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Date:   Thu, 13 Jun 2019 12:05:46 -0700
From:   "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>
To:     Jan Kara <jack@...e.cz>
Cc:     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 6/13/19 1:20 AM, Jan Kara wrote:
> 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.
> 

Good point, and seems fair enough, thank you!

Regards,
Srivatsa
VMware Photon OS

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