linux-kernel - Re: Ubuntu 32-bit, 32-bit PAE, 64-bit Kernel Benchmarks

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Message-ID: <alpine.LFD.2.00.1001160753390.5650@localhost>
Date:	Sat, 16 Jan 2010 07:57:40 -0500 (EST)
From:	"Robert P. J. Day" <rpjday@...shcourse.ca>
To:	Sitsofe Wheeler <sitsofe@...oo.com>
cc:	"H. Peter Anvin" <hpa@...or.com>,
	Yuhong Bao <yuhongbao_386@...mail.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: Ubuntu 32-bit, 32-bit PAE, 64-bit Kernel Benchmarks

On Sat, 16 Jan 2010, Sitsofe Wheeler wrote:

> On Fri, Jan 15, 2010 at 05:49:06PM -0800, H. Peter Anvin wrote:
> > On 12/30/2009 05:29 PM, Yuhong Bao wrote:
> > >
> > > Given that Linus was once talking about the performance penalties of
> > > PAE and HIGHMEM64G, perhaps you'd find these benchmarks done by
> > > Phoronix of interest:
> > > http://www.phoronix.com/scan.php?page=article&item=ubuntu_32_pae
> > >
> >
> > The big difference isn't between HIGHMEM4G (no PAE) and HIGHMEM64G
> > (PAE), it's between HIGHMEM and !HIGHMEM.  That cutoff is ~892 MB
> > for a stock 32-bit kernel.
>
> Thanks for the clarification - I had been wondering about why those
> settings had been benchmarked against each other...
>
> I took a mild interest because I have an EeePC 900 with 1G of RAM.
> The machine can do PAE but my understanding is that this would lead
> to a performance drop (I currently have VMSPLIT_3G so I can use all
> 1G of memory) so I run it without HIGHMEM.

  actually, it's 896M, not 892M, and i believe it's defined in
arch/x86/mm/init_32.c:

#define high_memory (-128UL << 20)
        BUILD_BUG_ON(VMALLOC_START     >= VMALLOC_END);
#undef high_memory

interesting that it's a hardcoded value -- is there reason that wasn't
configurable?  (he asked from a position of total ignorance.)

rday
--


========================================================================
Robert P. J. Day                               Waterloo, Ontario, CANADA

            Linux Consulting, Training and Kernel Pedantry.

Web page:                                          http://crashcourse.ca
Twitter:                                       http://twitter.com/rpjday
========================================================================
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