linux-kernel - Re: Higer latency with dynamic tick (need for an io-ondemand govenor?)

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Date:	Sun, 20 Apr 2008 14:41:05 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Woodruff, Richard" <r-woodruff2@...com>
Cc:	"Thomas Gleixner" <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...e.hu>, <linux-kernel@...r.kernel.org>,
	<linux-pm@...ts.linux-foundation.org>, lenb@...nel.org
Subject: Re: Higer latency with dynamic tick (need for an io-ondemand govenor?)

"Woodruff, Richard" <r-woodruff2@...com> writes:

> When capturing some traces with dynamic tick we were noticing the
> interrupt latency seems to go up a good amount. If you look at the trace
> the gpio IRQ is now offset a good amount.  Good news I guess is its
> pretty predictable.
>
> * If we couple this with progressively higher latency C-States we see
> that IO speed can fall by a good amount, especially for PIO mixes.  Now
> if QOS is maintained you may or may-not care.
>
> I was wondering what thoughts of optimizing this might be.
>
> One thought was if an io-ondemand of some sort was used.  It could track
> interrupt statistics and be feed back into cpu-idle.  When there is a
> high interrupt load period it could shrink the acceptable latency and
> thus help choose a good a C-State which favors throughput.  Some moving
> average window could be used to track it.
>
> Perhaps a new interrupt attribute could be attached at irq request time
> to allow the tracking of bandwidth important devices.
>
> The attached is captured on a .22 kernel.  The same should be available
> in a bit on a .24 kernel.

Are you talking about x86? 

On older x86 this effect should have been handled by the C state
algorithm taking the bus master activity register into account (which
should also trigger for interrupts)

But I think the register has been nop'ed on newer platforms 
so indeed we'll need some way to handle this.

-Andi
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