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

[Paolo Valente <paolo.valente@linaro.org>]

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> Il giorno 22 mag 2019, alle ore 12:01, Srivatsa S. Bhat <srivatsa@csail.mit.edu> ha scritto:
> 
> On 5/22/19 2:09 AM, Paolo Valente wrote:
>> 
>> First, thank you very much for testing my patches, and, above all, for
>> sharing those huge traces!
>> 
>> According to the your traces, the residual 20% lower throughput that you
>> record is due to the fact that the BFQ injection mechanism takes a few
>> hundredths of seconds to stabilize, at the beginning of the workload.
>> During that setup time, the throughput is equal to the dreadful ~60-90 KB/s
>> that you see without this new patch.  After that time, there
>> seems to be no loss according to the trace.
>> 
>> The problem is that a loss lasting only a few hundredths of seconds is
>> however not negligible for a write workload that lasts only 3-4
>> seconds.  Could you please try writing a larger file?
>> 
> 
> I tried running dd for longer (about 100 seconds), but still saw around
> 1.4 MB/s throughput with BFQ, and between 1.5 MB/s - 1.6 MB/s with
> mq-deadline and noop.

Ok, then now the cause is the periodic reset of the mechanism.

It would be super easy to fill this gap, by just gearing the mechanism
toward a very aggressive injection.  The problem is maintaining
control.  As you can imagine from the performance gap between CFQ (or
BFQ with malfunctioning injection) and BFQ with this fix, it is very
hard to succeed in maximizing the throughput while at the same time
preserving control on per-group I/O.

On the bright side, you might be interested in one of the benefits
that BFQ gives in return for this ~10% loss of throughput, in a
scenario that may be important for you (according to affiliation you
report): from ~500% to ~1000% higher throughput when you have to serve
the I/O of multiple VMs, and to guarantee at least no starvation to
any VM [1].  The same holds with multiple clients or containers, and
in general with any set of entities that may compete for storage.

[1] https://www.linaro.org/blog/io-bandwidth-management-for-production-quality-services/

> But I'm not too worried about that difference.
> 
>> In addition, I wanted to ask you whether you measured BFQ throughput
>> with traces disabled.  This may make a difference.
>> 
> 
> The above result (1.4 MB/s) was obtained with traces disabled.
> 
>> After trying writing a larger file, you can try with low_latency on.
>> On my side, it causes results to become a little unstable across
>> repetitions (which is expected).
>> 
> With low_latency on, I get between 60 KB/s - 100 KB/s.
> 

Gosh, full regression.  Fortunately, it is simply meaningless to use
low_latency in a scenario where the goal is to guarantee per-group
bandwidths.  Low-latency heuristics, to reach their (low-latency)
goals, modify the I/O schedule compared to the best schedule for
honoring group weights and boosting throughput.  So, as recommended in
BFQ documentation, just switch low_latency off if you want to control
I/O with groups.  It may still make sense to leave low_latency on
in some specific case, which I don't want to bother you about.

However, I feel bad with such a low throughput :)  Would you be so
kind to provide me with a trace?

Thanks,
Paolo

> Regards,
> Srivatsa
> VMware Photon OS


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