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Date:	Thu, 15 Nov 2007 16:41:16 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	William Lee Irwin III <wli@...omorphy.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>, Andi Kleen <ak@...e.de>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Why preallocate pmd in x86 32-bit PAE?

William Lee Irwin III wrote:
> 
> There may be bigger fish to fry in terms of per-process overhead, if
> you're trying to cut that down. The trouble with trying to address
> some of those is that there is mutual antagonism between compactness
> and expansibility in the process address space layout, so you'll end
> up instantiating a lot more than you want barring some sort of provision
> for a compact address space layout. Pagetable sharing is a far more
> powerful resource scalability method, though it also needs cooperation
> in user address space layout to reap its gains.
> 
> There are other overheads, of course, though they're more typically
> per-something besides processes.
> 

I think Jeremy's question was due to trying to reduce the 32/64-bit 
differences.  Performance-wise, it might add a small amount to user 
setup time (a typical 32-bit process will need all four, for the main 
binary, libraries, stack and kernel, respectively) but it is probably 
not significant (although I'd like to see numbers just in case).

	-hpa

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