Re: vmstat: On demand vmstat workers V4

Thomas Gleixner
2014-05-09T15:05:15Z

On Fri, 9 May 2014, Christoph Lameter wrote:
> On Fri, 9 May 2014, Thomas Gleixner wrote:
> > I think we agreed long ago, that for the whole HPC FULL_NOHZ stuff you
> > have to sacrify at least one CPU for housekeeping purposes of all
> > kinds, timekeeping, statistics and whatever.
>
> Ok how do I figure out that cpu? I'd rather have a specific cpu that
> never changes.

I followed the full nohz development only losely, but back then when
all started here at my place with frederic, we had a way to define the
housekeeper cpu. I think we lazily had it hardwired to 0 :)

That probably changed, but I'm sure there is still a way to define a
housekeeper. And we should simply force the timekeeping on that
housekeeper. That comes with the price, that the housekeeper is not
allowed to go deep idle, but I bet that in HPC scenarios this does not
matter at all simply because the whole machine is under full load.

Frederic?

> > So if you have a housekeeper, then it makes absolutely no sense at all
> > to move it around in circles.
> >
> > Can you please enlighten me why we need this at all?
>
> The vmstat kworker thread checks every 2 seconds if there are vmstat
> updates that need to be folded into the global statistics. This is not
> necessary if the application is running and no OS services are being used.
> Thus we could switch off vmstat updates and avoid taking the processor
> away from the application.
>
> This has also been noted by multiple other people at was brought up at the
> mm summit by others who noted the same issues.

I understand why you want to get this done by a housekeeper, I just
did not understand why we need this whole move it around business is
required.

Thanks,

tglx