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

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Date:	Tue, 5 May 2009 23:33:39 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	akataria@...are.com
Cc:	"H. Peter Anvin" <hpa@...or.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

> IMO, one of the main motives of HRT implementation apart from getting
> higher precision timers was that we now don't necessarily need to rely

Timer frequency and HZ are two entirely different things nowdyas

> on a high timer frequency. If you see problems with Desktop feel and
> responsiveness don't you think there would be other problem which might
> be causing that ?  Your argument about the "desktop feel and
> responsiveness" doesn't explain what actual problem did you see.

People spent months poking at the differences before HZ=1000 became the
default. It wasn't due for amusement values - but this is irrelevant
anyway on a modern kernel as HZ=1000 is simply a precision setting that
affects things like poll()

HZ on a tickless system has no meaningful relationship to wakup rates -
which are what I assume you actually care about.

So do you want to change the precision of poll() and other
functionality ? or do you want to change the wakeup rates and
corresponding virtualisation overhead ?

If the latter then HZ is not the thing to touch.

What are you *actually* trying to achieve ?
What measurements have you done that make you think HZ is relevant in a
tickless kernel ?

Alan
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