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From: Andi Kleen <andi@firstfloor.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Pepper <lnxninja@linux.vnet.ibm.com>,
        Marcio Saito <marcio@cyclades.com>, John Stultz <johnstul@us.ibm.com>,
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        Avantika Mathur <mathur@linux.vnet.ibm.com>,
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Subject: Re: [RFC] [PATCH] allow low HZ values?
References: <20101011201121.GA953@tpepper-t61p.dolavim.us>
	<alpine.LFD.2.00.1010112227190.2909@localhost6.localdomain6>
Date: Tue, 12 Oct 2010 16:31:20 +0200
In-Reply-To: <alpine.LFD.2.00.1010112227190.2909@localhost6.localdomain6>
	(Thomas Gleixner's message of "Mon, 11 Oct 2010 22:32:06 +0200
	(CEST)")
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Thomas Gleixner <tglx@linutronix.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@linux.intel.com -- Speaking for myself only.