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

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Message-ID: <alpine.DEB.2.02.1405091659350.6261@ionos.tec.linutronix.de>
Date:	Fri, 9 May 2014 17:05:12 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Christoph Lameter <cl@...ux.com>
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, 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

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