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From: Paolo Valente <paolo.valente@linaro.org>
To: "Srivatsa S. Bhat" <srivatsa@csail.mit.edu>
Cc: linux-fsdevel@vger.kernel.org,
	linux-block <linux-block@vger.kernel.org>,
	linux-ext4@vger.kernel.org, cgroups@vger.kernel.org,
	kernel list <linux-kernel@vger.kernel.org>,
	Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
	Jeff Moyer <jmoyer@redhat.com>, Theodore Ts'o <tytso@mit.edu>,
	amakhalov@vmware.com, anishs@vmware.com, srivatsab@vmware.com,
	Ulf Hansson <ulf.hansson@linaro.org>,
	Linus Walleij <linus.walleij@linaro.org>
Subject: Re: CFQ idling kills I/O performance on ext4 with blkio cgroup controller
Date: Thu, 30 May 2019 12:45:57 +0200	[thread overview]
Message-ID: <7B74A790-BD98-412B-ADAB-3B513FB1944E@linaro.org> (raw)
In-Reply-To: <0d6e3c02-1952-2177-02d7-10ebeb133940@csail.mit.edu>

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> Il giorno 30 mag 2019, alle ore 10:29, Srivatsa S. Bhat <srivatsa@csail.mit.edu> ha scritto:
> 
> On 5/29/19 12:41 AM, Paolo Valente wrote:
>> 
>> 
>>> Il giorno 29 mag 2019, alle ore 03:09, Srivatsa S. Bhat <srivatsa@csail.mit.edu> ha scritto:
>>> 
>>> On 5/23/19 11:51 PM, Paolo Valente wrote:
>>>> 
>>>>> Il giorno 24 mag 2019, alle ore 01:43, Srivatsa S. Bhat <srivatsa@csail.mit.edu> 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 :)
> 

Great!

> 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:
> 
> https://www.dropbox.com/s/3roycp1zwk372zo/bfq-traces.tar.gz?dl=0
> 

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
future.

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.

[1] https://algo.ing.unimo.it/people/paolo/disk_sched/results.php
[2] https://www.linaro.org/blog/io-bandwidth-management-for-production-quality-services/

> 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,
Paolo


> Regards,
> Srivatsa
> VMware Photon OS


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  reply	other threads:[~2019-05-30 10:46 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-17 22:16 Srivatsa S. Bhat
2019-05-18 18:39 ` Paolo Valente
2019-05-18 19:28   ` Theodore Ts'o
2019-05-20  9:15     ` Jan Kara
2019-05-20 10:45       ` Paolo Valente
2019-05-21 16:48       ` Theodore Ts'o
2019-05-21 18:19         ` Josef Bacik
2019-05-21 19:10           ` Theodore Ts'o
2019-05-20 10:38     ` Paolo Valente
2019-05-21  7:38       ` Andrea Righi
2019-05-18 20:50   ` Srivatsa S. Bhat
2019-05-20 10:19     ` Paolo Valente
2019-05-20 22:45       ` Srivatsa S. Bhat
2019-05-21  6:23         ` Paolo Valente
2019-05-21  7:19           ` Srivatsa S. Bhat
2019-05-21  9:10           ` Jan Kara
2019-05-21 16:31             ` Theodore Ts'o
2019-05-21 11:25       ` Paolo Valente
2019-05-21 13:20         ` Paolo Valente
2019-05-21 16:21           ` Paolo Valente
2019-05-21 17:38             ` Paolo Valente
2019-05-21 22:51               ` Srivatsa S. Bhat
2019-05-22  8:05                 ` Paolo Valente
2019-05-22  9:02                   ` Srivatsa S. Bhat
2019-05-22  9:12                     ` Paolo Valente
2019-05-22 10:02                       ` Srivatsa S. Bhat
2019-05-22  9:09                   ` Paolo Valente
2019-05-22 10:01                     ` Srivatsa S. Bhat
2019-05-22 10:54                       ` Paolo Valente
2019-05-23  2:30                         ` Srivatsa S. Bhat
2019-05-23  9:19                           ` Paolo Valente
2019-05-23 17:22                             ` Paolo Valente
2019-05-23 23:43                               ` Srivatsa S. Bhat
2019-05-24  6:51                                 ` Paolo Valente
2019-05-24  7:56                                   ` Paolo Valente
2019-05-29  1:09                                   ` Srivatsa S. Bhat
2019-05-29  7:41                                     ` Paolo Valente
2019-05-30  8:29                                       ` Srivatsa S. Bhat
2019-05-30 10:45                                         ` Paolo Valente [this message]
2019-06-02  7:04                                           ` Srivatsa S. Bhat
2019-06-11 22:34                                             ` Srivatsa S. Bhat
2019-06-12 13:04                                               ` Jan Kara
2019-06-12 19:36                                                 ` Srivatsa S. Bhat
2019-06-13  6:02                                                   ` Greg Kroah-Hartman
2019-06-13 19:03                                                     ` Srivatsa S. Bhat
2019-06-13  8:20                                                   ` Jan Kara
2019-06-13 19:05                                                     ` Srivatsa S. Bhat
2019-06-13  8:37                                                   ` Jens Axboe
2019-06-13  5:46                                               ` Paolo Valente
2019-06-13 19:13                                                 ` Srivatsa S. Bhat
2019-05-23 23:32                           ` Srivatsa S. Bhat
2019-05-30  8:38                             ` Srivatsa S. Bhat

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