linux-kernel - Re: [PATCH] suppress needless timer reprogramming {tick-sched.c}

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Date:	Wed, 18 Jun 2008 09:28:41 -0700
From:	David Brownell <david-b@...bell.net>
To:	"Woodruff, Richard" <r-woodruff2@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Tony Lindgren <tony@...mide.com>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"TK, Pratheesh Gangadhar" <pratheesh@...com>
Subject: Re: [PATCH] suppress needless timer reprogramming {tick-sched.c}

On Wednesday 18 June 2008, Woodruff, Richard wrote:
> >
> > As measured using ETM trace this drops my reprogramming penalty from
> > almost 60% CPU load down to 15% during high interrupt rate.  If you like
> > I can send traces to show this.

More than doubling the available-for-real-use CPU time!  Impressive.

I'd imagine that many systems using reprogrammable 32K timers for
scheduling could win from such a patch, although not necessarily
quite so visibly as on this ARM unless their CPU clocks were also
in the 500+ MHz range.

Seems like a "law of unintended consequences" here.  Use a 32K
timer since it takes less power than a multi-MHz one, and so the
faster clocks can be gated off more comprehensively (in hardware)
to give more power savings.  Then throw NO_HZ on top to get one
more increment of power savings ... whoops, it was assuming that
it was dirt cheap to reprogram timers, major badness!

- Dave

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