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

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Date:	Mon, 11 Oct 2010 14:11:40 -0700
From:	"Tim Pepper" <lnxninja@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Tim Pepper <lnxninja@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Marcio Saito <marcio@...lades.com>,
	Jiri Slaby <jirislaby@...il.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org, John Stultz <johnstul@...ibm.com>,
	Avantika Mathur <mathur@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC] [PATCH] allow low HZ values?

On Mon 11 Oct at 22:32:06 +0200 tglx@...utronix.de said:
> 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.

Yep.  Long term no hz is definitely the goal.  I'm not sufficiently
connected to the -rt space I guess to have followed that there's somebody
again looking in that direction.  The rfc patch was mostly just a minimal
is there anything simple we can do in the meantime exercise.

> 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.

I'd not heard of this in between NOHZ-y idea...sounds promising.
We'd talked about different non-idle no hz approaches in the past year
or so, some of which were on the veeery complicated side of the spectrum.

> Sigh, nothing
> happened ever except for repeating the same crap patches over and
> over.

I'll check out what Frederic is doing.  Thanks for the pointer and
apologies for the noise.

-- 
Tim Pepper  <lnxninja@...ux.vnet.ibm.com>
IBM Linux Technology Center
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