linux-kernel - Re: [RFC] [PATCH] allow low HZ values?

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Message-ID: <871v7v32kn.fsf@basil.nowhere.org>
Date:	Tue, 12 Oct 2010 16:31:20 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Tim Pepper <lnxninja@...ux.vnet.ibm.com>,
	Marcio Saito <marcio@...lades.com>,
	John Stultz <johnstul@...ibm.com>,
	Jiri Slaby <jirislaby@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Avantika Mathur <mathur@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, jblunck@...radead.org
Subject: Re: [RFC] [PATCH] allow low HZ values?

Thomas Gleixner <tglx@...utronix.de> writes:

> On Mon, 11 Oct 2010, Tim Pepper wrote:
>
>> I'm not necessarily wanting to open up the age old question of "what is
>> a good HZ", but we were doing some testing on timer tick overheads for
>> HPC applications and this came up...
>
> Yeah. This comes always up when the timer tick overhead on HPC is
> tested. And this patch is again the fundamentally wrong answer.

That's a unfair description of the proposal.

> We have told HPC folks for years that we need a kind of "NOHZ" mode
> for HPC where we can transparently switch off the tick when only one
> user space bound thread is active and switch back to normal once this
> thing terminates or goes into the kernel via a syscall. Sigh, nothing
> happened ever except for repeating the same crap patches over and
> over.

Jan Blunck posted a patch for this exactly few months ago.
Unfortunately it didn't get the accounting right, but other than
that it seemed like a reasonable starting point.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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