Message-Id: <88488E82-E360-45B0-B010-209190D32892@linaro.org>
Date:   Fri, 16 Aug 2019 20:17:19 +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 19:59, Josef Bacik <josef@...icpanda.com> ha scritto:
> 
> On Fri, Aug 16, 2019 at 07:52:40PM +0200, Paolo Valente wrote:
>> 
>> 
>>> 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.
> 
> Ah ok, you are doing it on a mountpoint.

Yep

>  Are you using btrfs?

ext4

>  Cause otherwise
> you are going to have a sad time.

Could you elaborate more on this?  I/O seems to be controllable on ext4.

>  The other thing is you are using buffered,

Actually, the problem is suffered by sync random reads, which always
hit the disk in this test.

> which may or may not hit the disk.  This is what I use to test io.latency
> 
> https://patchwork.kernel.org/patch/10714425/
> 
> I had to massage it since it didn't apply directly, but running this against the
> actual block device, with O_DIRECT so I'm sure to be measure the actual impact
> of the controller, it all works out fine.

I'm not getting why non-direct sync reads, or buffered writes, should
be uncontrollable.  As a trivial example, BFQ in this tests controls
I/O as expected, and keeps latency extremely low.

What am I missing?

Thanks,
Paolo

>  Thanks,
> 
> Josef

Powered by blists - more mailing lists