linux-kernel - Re: 2.6.20-rc6-mm3

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Message-ID: <20070206210959.GC25430@elte.hu>
Date:	Tue, 6 Feb 2007 22:09:59 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: 2.6.20-rc6-mm3

* Daniel Walker <dwalker@...sta.com> wrote:

> > it's quite easy to explain: because of the new dynticks feature. 
> > Both 'timer' and 'LOC' counts go way down.
> 
> I don't have that enabled tho .. This is with HRT/dynamic tick both 
> off..

your kernel utilizes the kernelin a more optimal way: the new 
clockevents code now utilizes the local APIC timer irq (represented by 
the LOC field) for periodic interrupts. The local APIC timer irq has a 
cost of ~2 usecs per IRQ, while the PIT irq is ~10 usecs per irq. With 
HZ=1000 this means savings of ~8000 usecs per second - i.e. 8 msecs per 
second, which is 0.8% more raw CPU power available - which isnt that 
bad.

we could make this clearer by renaming 'LOC' (which stands for 'LOCal 
timer interupts' and was added [and misnamed] by yours truly many moons 
ago) to 'apic-timer' and 'timer' to 'PIT-timer' but /that/ would be more 
of a userspace visible change than the change in the counter rates.

	Ingo
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