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    SubjectRe: 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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