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 23 mag 2019, alle ore 04:30, Srivatsa S. Bhat <srivatsa@csail.mit.edu> ha scritto:
> 
> On 5/22/19 3:54 AM, Paolo Valente wrote:
>> 
>> 
>>> 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.
>> 
> 
> Ah, I see. Just to make sure that this fix doesn't overly optimize for
> total throughput (because of the testcase we've been using) and end up
> causing regressions in per-group I/O control, I ran a test with
> multiple simultaneous dd instances, each writing to a different
> portion of the filesystem (well separated, to induce seeks), and each
> dd task bound to its own blkio cgroup. I saw similar results with and
> without this patch, and the throughput was equally distributed among
> all the dd tasks.
> 

Thank you very much for pre-testing this change, this let me know in
advance that I shouldn't find issues when I'll test regressions, at
the end of this change phase.

>> 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/
>> 
> 
> Great article! :) Thank you for sharing it!

Thanks! I mentioned it just to better put things into context.

> 
>>> 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.
>> 
> 
> My main concern here is about Linux's I/O performance out-of-the-box,
> i.e., with all default settings, which are:
> 
> - cgroups and blkio enabled (systemd default)
> - blkio non-root cgroups in use (this is the implicit systemd behavior
>  if docker is installed; i.e., it runs tasks under user.slice)
> - I/O scheduler with blkio group sched support: bfq
> - bfq default configuration: low_latency = 1
> 
> If this yields a throughput that is 10x-30x slower than what is
> achievable, I think we should either fix the code (if possible) or
> change the defaults such that they don't lead to this performance
> collapse (perhaps default low_latency to 0 if bfq group scheduling
> is in use?)

Yeah, I thought of this after sending my last email yesterday.  Group
scheduling and low-latency heuristics may simply happen to fight
against each other in personal systems.  Let's proceed this way.  I'll
try first to make the BFQ low-latency mechanism clever enough to not
hinder throughput when groups are in place.  If I make it, then we
will get the best of the two worlds: group isolation and intra-group
low latency; with no configuration change needed.  If I don't make it,
I'll try to think of the best solution to cope with this non-trivial
situation.


>> However, I feel bad with such a low throughput :)  Would you be so
>> kind to provide me with a trace?
>> 
> Certainly! Short runs of dd resulted in a lot of variation in the
> throughput (between 60 KB/s - 1 MB/s), so I increased dd's runtime
> to get repeatable numbers (~70 KB/s). As a result, the trace file
> (trace-bfq-boost-injection-low-latency-71KBps) is quite large, and
> is available here:
> 
> https://www.dropbox.com/s/svqfbv0idcg17pn/bfq-traces.tar.gz?dl=0
> 

Thank you very much for your patience and professional help.

> Also, I'm very happy to run additional tests or experiments to help
> track down this issue. So, please don't hesitate to let me know if
> you'd like me to try anything else or get you additional traces etc. :)
> 

Here's to you!  :) I've attached a new small improvement that may
reduce fluctuations (path to apply on top of the others, of course).
Unfortunately, I don't expect this change to boost the throughput
though.

In contrast, I've thought of a solution that might be rather
effective: making BFQ aware (heuristically) of trivial
synchronizations between processes in different groups.  This will
require a little more work and time.


Thanks,
Paolo


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> Thank you!
> 
> Regards,
> Srivatsa
> VMware Photon OS


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