linux-kernel - Re: io.latency controller apparently not working

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Message-Id: <1842D618-3E31-47FE-8B9C-F26BF1F5349C@linaro.org>
Date:   Fri, 16 Aug 2019 19:52:40 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        noreply-spamdigest via bfq-iosched 
        <bfq-iosched@...glegroups.com>, Tejun Heo <tj@...nel.org>
Subject: Re: io.latency controller apparently not working



> Il giorno 16 ago 2019, alle ore 15:21, Josef Bacik <josef@...icpanda.com> ha scritto:
> 
> On Fri, Aug 16, 2019 at 12:57:41PM +0200, Paolo Valente wrote:
>> Hi,
>> I happened to test the io.latency controller, to make a comparison
>> between this controller and BFQ.  But io.latency seems not to work,
>> i.e., not to reduce latency compared with what happens with no I/O
>> control at all.  Here is a summary of the results for one of the
>> workloads I tested, on three different devices (latencies in ms):
>> 
>>             no I/O control        io.latency         BFQ
>> NVMe SSD     1.9                   1.9                0.07
>> SATA SSD     39                    56                 0.7
>> HDD          4500                  4500               11
>> 
>> I have put all details on hardware, OS, scenarios and results in the
>> attached pdf.  For your convenience, I'm pasting the source file too.
>> 
> 
> Do you have the fio jobs you use for this?

The script mentioned in the draft (executed with the command line
reported in the draft), executes one fio instance for the target
process, and one fio instance for each interferer.  I couldn't do with
just one fio instance executing all jobs, because the weight parameter
doesn't work in fio jobfiles for some reason, and because the ioprio
class cannot be set for individual jobs.

In particular, the script generates a job with the following
parameters for the target process:

 ioengine=sync
 loops=10000
 direct=0
 readwrite=randread
 fdatasync=0
 bs=4k
 thread=0
 filename=/mnt/scsi_debug/largefile_interfered0
 iodepth=1
 numjobs=1
 invalidate=1

and a job with the following parameters for each of the interferers,
in case, e.g., of a workload made of reads:

 ioengine=sync
 direct=0
 readwrite=read
 fdatasync=0
 bs=4k
 filename=/mnt/scsi_debug/largefileX
 invalidate=1

Should you fail to reproduce this issue by creating groups, setting
latencies and starting fio jobs manually, what if you try by just
executing my script?  Maybe this could help us spot the culprit more
quickly.

>  I just tested on Jens's most recent
> tree and io.latency appears to be doing what its supposed to be doing.  We've
> also started testing 5.2 in production and it's still working in production as
> well.

I tested 5.2 too, same negative outcome.

Thanks,
Paolo

>  The only thing I've touched recently was around wakeups and shouldn't
> have broken everything.  I'm not sure why it's not working for you, but a fio
> script will help me narrow down what's going on.  Thanks,
> 
> Josef