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

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Date:   Thu, 13 Jun 2019 02:37:29 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>,
        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>,
        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 6/12/19 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]. https://lore.kernel.org/lkml/8d72fcf7-bbb4-2965-1a06-e9fc177a8938@csail.mit.edu/

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