Re: [patch 2/2] MM: allow per-cpu vmstat_threshold and vmstat_worker configuration

[Marcelo Tosatti]

On Thu, May 25, 2017 at 10:24:46PM -0500, Christoph Lameter wrote:
> On Thu, 25 May 2017, Marcelo Tosatti wrote:
> 
> > Argument? We're showing you the data that this is causing a latency
> > problem for us.
> 
> Sorry I am not sure where the data shows a latency problem. There are
> interrupts and scheduler ticks. But what does this have to do with vmstat?
> 
> Show me your dpdk code running and trace the tick on / off events  as well
> as the vmstat invocations. Also show all system calls occurring on the cpu
> that runs dpdk. That is necessary to see what triggers vmstat and how the
> system reacts to the changes to the differentials.

Sure, i can get that to you. The question remains: Are you arguing
its not valid for a realtime application to use any system call
which changes a vmstat counter? 

Because if they are allowed, then its obvious something like
this is needed.

> Then please rerun the test by setting the vmstat_interval to 60.
> 
> Do another run with your modifications and show the difference.

Will do so.

> > > Something that crossed my mind was to add a new tunable to set
> > > the vmstat_interval for each CPU, this way we could essentially
> > > disable it to the CPUs where DPDK is running. What's the implications
> > > of doing this besides not getting up to date stats in /proc/vmstat
> > > (which I still have to confirm would be OK)? Can this break anything
> > > in the kernel for example?
> >
> > Well, you get incorrect statistics.
> 
> The statistics are never completely accurate. You will get less accurate
> statistics but they will be correct. The differentials may not be
> reflected in the counts shown via /proc but there is a cap on how
> inaccurate those can becore.