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Date:   Thu, 13 Jun 2019 02:37:29 -0600
From:   Jens Axboe <>
To:     "Srivatsa S. Bhat" <>,
        Jan Kara <>
Cc:     Paolo Valente <>,,
        linux-block <>,,,
        kernel list <>,
        Jeff Moyer <>, Theodore Ts'o <>,,,,
        Ulf Hansson <>,
        Linus Walleij <>,
        Greg Kroah-Hartman <>,
        Stable <>
Subject: Re: CFQ idling kills I/O performance on ext4 with blkio cgroup

On 6/12/19 1:36 PM, 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.
> Greg, do you have any thoughts on this? The context is that both CFQ
> and BFQ I/O schedulers have issues that cause I/O throughput to suffer
> upto 10x - 30x on certain workloads and system configurations, as
> reported in [1].
> In this thread, Paolo posted patches to fix BFQ performance on
> mainline. However CFQ suffers from the same performance collapse, but
> CFQ was removed from the kernel in v5.0. So obviously the usual stable
> backporting path won't work here for several reasons:
>    1. There won't be a mainline commit to backport from, as CFQ no
>       longer exists in mainline.
>    2. This is not a security/stability fix, and is likely to involve
>       invasive changes.
> I was wondering if there was a way to address the performance issues
> in CFQ in the older stable kernels (including the latest LTS 4.19),
> despite the above constraints, since the performance drop is much too
> significant. I guess not, but thought I'd ask :-)
> [1].

This issue has always been there. There will be no specific patches made
for stable for something that doesn't even exist in the newer kernels.

Jens Axboe

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