Re: io.latency controller apparently not working

[Josef Bacik <josef@toxicpanda.com>]

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@toxicpanda.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.  Are you using btrfs?  Cause otherwise
you are going to have a sad time.  The other thing is you are using buffered,
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.  Thanks,

Josef