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SubjectRe: CFQ idling kills I/O performance on ext4 with blkio cgroup controller
On 5/30/19 3:45 AM, Paolo Valente wrote:
>> Il giorno 30 mag 2019, alle ore 10:29, Srivatsa S. Bhat <> ha scritto:
>> 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 ... ;)

Indeed, that's awesome! :)

>> 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.

Ah, I see.

> 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 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!

> [1]
> [2]
>> 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

My pleasure! :)

VMware Photon OS

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