linux-kernel - Re: CPA patchset

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Message-ID: <20080110105726.GD28209@elte.hu>
Date:	Thu, 10 Jan 2008 11:57:26 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <ak@...e.de>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Venki Pallipadi <venkatesh.pallipadi@...el.com>,
	suresh.b.siddha@...el.com, Arjan van de Ven <arjan@...radead.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: CPA patchset


* Andi Kleen <ak@...e.de> wrote:

> > > >   WBINVD isnt particular fast (takes a few msecs), but why is 
> > > >   that a problem? Drivers dont do high-frequency ioremap-ing. 
> > > >   It's typically only done at driver/device startup and that's 
> > > >   it.
> > > 
> > > Actually graphics drivers can do higher frequency allocation of WC 
> > > memory (with PAT) support.
> > 
> > but that's not too smart: why dont they use WB plus cflush instead?
> 
> Because they need to access it WC for performance.

I think you have it fundamentally backwards: the best for performance is 
WB + cflush. What would WC offer for performance that cflush cannot do?

also, it's irrelevant to change_page_attr() call frequency. Just map in 
everything from the card and use it. In graphics, if you remap anything 
on the fly and it's not a slowpath you've lost the performance game even 
before you began it.

	Ingo
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