linux-kernel - Re: [PATCH] x86: Reduce the default HZ value

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Date:	Tue, 5 May 2009 22:44:17 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	akataria@...are.com, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Reduce the default HZ value

> What is the benefit of this?

I believe the "benefit" is that a certain posters proprietary
virtualisation product works better.

> I can see at least one immediate downside: some timeout values in the 
> kernel are still maintained in units of HZ (like poll, I believe), and 
> so with a lower HZ value we'll have higher roundoff errors.

And HZ=100 actually causes real problems for some video work (not in
Europe where its just peachy). We switched to 1000Hz a very long time ago
because it improved desktop feel and responsiveness. We switched to
tickless to keep that behaviour with good power and idle behaviour.

If we still have problems that give virtualisers hiccups then they will
be giving real processor power management the same grief so I don't think
meddling with HZ to harm the desktop is the right move - but if there are
improvements to tickless (eg timer granularity and bunching under
virtualisation) that would be far better to explore.

Alan
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