linux-kernel - Re: vmstat: On demand vmstat workers V4

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Date:	Fri, 9 May 2014 10:28:27 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Tejun Heo <tj@...nel.org>, John Stultz <johnstul@...ibm.com>,
	Mike Frysinger <vapier@...too.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, hughd@...gle.com,
	viresh.kumar@...aro.org
Subject: Re: vmstat: On demand vmstat workers V4

On Fri, 9 May 2014, Thomas Gleixner wrote:

> > 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 :)

Yes that would be the easiest and simplest. We dedicate cpu 0 to OS
services around
here anyways.

> 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.

Excellent. Yes. Good.

> >
> > 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.

This came about because of another objection against having it simply
fixed to a processor. After all that processor may be disabled etc etc.

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