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Date:   Thu, 30 May 2019 12:45:57 +0200
From:   Paolo Valente <>
To:     "Srivatsa S. Bhat" <>
        linux-block <>,,,
        kernel list <>,
        Jens Axboe <>, Jan Kara <>,
        Jeff Moyer <>, Theodore Ts'o <>,,,,
        Ulf Hansson <>,
        Linus Walleij <>
Subject: Re: CFQ idling kills I/O performance on ext4 with blkio cgroup

> Il giorno 30 mag 2019, alle ore 10:29, Srivatsa S. Bhat <> ha scritto:
> On 5/29/19 12:41 AM, Paolo Valente wrote:
>>> Il giorno 29 mag 2019, alle ore 03:09, Srivatsa S. Bhat <> ha scritto:
>>> On 5/23/19 11:51 PM, Paolo Valente wrote:
>>>>> Il giorno 24 mag 2019, alle ore 01:43, Srivatsa S. Bhat <> ha scritto:
>>>>> When trying to run multiple dd tasks simultaneously, I get the kernel
>>>>> panic shown below (mainline is fine, without these patches).
>>>> Could you please provide me somehow with a list *(bfq_serv_to_charge+0x21) ?
>>> Hi Paolo,
>>> Sorry for the delay! Here you go:
>>> (gdb) list *(bfq_serv_to_charge+0x21)
>>> 0xffffffff814bad91 is in bfq_serv_to_charge (./include/linux/blkdev.h:919).
>>> 914
>>> 915	extern unsigned int blk_rq_err_bytes(const struct request *rq);
>>> 916
>>> 917	static inline unsigned int blk_rq_sectors(const struct request *rq)
>>> 918	{
>>> 919		return blk_rq_bytes(rq) >> SECTOR_SHIFT;
>>> 920	}
>>> 921
>>> 922	static inline unsigned int blk_rq_cur_sectors(const struct request *rq)
>>> 923	{
>>> (gdb)
>>> For some reason, I've not been able to reproduce this issue after
>>> reporting it here. (Perhaps I got lucky when I hit the kernel panic
>>> a bunch of times last week).
>>> I'll test with your fix applied and see how it goes.
>> Great!  the offending line above gives me hope that my fix is correct.
>> If no more failures occur, then I'm eager (and a little worried ...)
>> to see how it goes with throughput :)
> Your fix held up well under my testing :)


> As for throughput, with low_latency = 1, I get around 1.4 MB/s with
> bfq (vs 1.6 MB/s with mq-deadline). This is a huge improvement
> compared to what it was before (70 KB/s).

That's beautiful news!

So, now we have the best of the two worlds: maximum throughput and
total control on I/O (including minimum latency for interactive and
soft real-time applications).  Besides, no manual configuration
needed.  Of course, this holds unless/until you find other flaws ... ;)

> With tracing on, the throughput is a bit lower (as expected I guess),
> about 1 MB/s, and the corresponding trace file
> (trace-waker-detection-1MBps) is available at:

Thank you for the new trace.  I've analyzed it carefully, and, as I
imagined, this residual 12% throughput loss is due to a couple of
heuristics that occasionally get something wrong.  Most likely, ~12%
is the worst-case loss, and if one repeats the tests, the loss may be
much lower in some runs.

I think it is very hard to eliminate this fluctuation while keeping
full I/O control.  But, who knows, I might have some lucky idea in the

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 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.


> Thank you so much for your tireless efforts in fixing this issue!

I did enjoy working on this with you: your test case and your support
enabled me to make important improvements.  So, thank you very much
for your collaboration so far,

> Regards,
> Srivatsa
> VMware Photon OS

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