Re: Observation on NOHZ_FULL

[Andrea Righi <andrea.righi@canonical.com>]

On Wed, Feb 07, 2024 at 02:04:12PM +0100, Uladzislau Rezki wrote:
> > > 
> > > I repeated some tests in a more isolated environment and posted the
> > > results here:
> > > https://discourse.ubuntu.com/t/enable-low-latency-features-in-the-generic-ubuntu-kernel-for-24-04/42255
> > > 
> > > Highlights:
> > > 
> > >  - stress-ng --matrix seems quite unpredictable to be used a benchmarks
> > >    in this scenario (the bogo-ops/s are very susceptible to any kind of
> > >    interference, so even if in the long runs NO_HZ_FULL still seems to
> > >    provide some benefits looking at the average, we also need to
> > >    consider that there might be a significant error in the measurements,
> > >    standard deviation was pretty high)
> > > 
> > >  - fio doing short writes (in page cache) seems to perform like 2x
> > >    better in terms of iops with nohz_full, respect to the other cases
> > >    and it performs 2x slower with large IO writes (not sure why... need
> > >    to investigate more)
> > > 
> > >  - with lazy RCU enabled hrtimer_interrupt() takes like 2x more to
> > >    return, respect to the other cases (is this expected?)
> > 
> > This last is surprising at first glance, but I could be missing
> > something.  Joel, Uladzislau, thoughts?
> > 
> Could you please share the steps how you run "fio" tests?

For short writes I was running something like this (on a 8 cores system):

 $ fio --rw=write --bs=1M --size=32M --numjobs=8 --name=worker --time_based --runtime=300

Larger writes:

 $ fio --rw=write --bs=1M --size=1G --numjobs=8 --name=worker --time_based --runtime=300

Thanks,
-Andrea