linux-kernel - Re: Dynamic configure max

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Message-ID: <87prbg3ohq.fsf@basil.nowhere.org>
Date:	Fri, 31 Jul 2009 16:40:01 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Robert Hancock <hancockrwd@...il.com>,
	Corrado Zoccolo <czoccolo@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-acpi@...r.kernel.org,
	venkatesh.pallipadi@...el.com
Subject: Re: Dynamic configure max_cstate

Andreas Mohr <andi@...as.de> writes:

> Instead we should strive for a far-reaching _generic_ mechanism
> which gathers average latencies of various I/O activities/devices
> and then uses some formula to determine the maximum (not necessarily ACPI)
> idle latency that we're willing to endure (e.g. average device I/O reply latency
> divided by 10 or so).

The interrupt heuristics in the menu cpuidle governour are already
attempting this, based on interrupt rates (or rather
wakeup rates) which are supposed to roughly correspond with IO rates
and scheduling events together.

Apparently that doesn't work in this case. The challenge would
be to find out why and improve the menu algorithm to deal with it.
I doubt a completely new mechanism is needed or makes sense.

> And in addition to this, we should also take into account (read: skip)
> any idle states which kill busmaster DMA completely
> (in case of busmaster DMA I/O activities, that is)

This is already done.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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