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    SubjectRe: Terrible IO performance when using 4GB of RAM on a 32 bit machine
    Robert Hancock wrote:
    > Peter Rabbitson wrote:
    >> H. Peter Anvin wrote:
    >>>
    >>> What does /proc/mtrr look like in the two cases?
    >>>
    >>
    >> Identical for mem=3900 and without it.
    >>
    >> reg00: base=0x00000000 ( 0MB), size=2048MB: write-back, count=1
    >> reg01: base=0x80000000 (2048MB), size=1024MB: write-back, count=1
    >> reg02: base=0xc0000000 (3072MB), size= 512MB: write-back, count=1
    >> reg03: base=0xe0000000 (3584MB), size= 256MB: write-back, count=1
    >> reg04: base=0xf0000000 (3840MB), size= 128MB: write-back, count=1
    >> reg05: base=0xf8000000 (3968MB), size= 32MB: write-back, count=1
    >
    > Looks like another case of bad MTRRs on an Intel motherboard? The BIOS
    > is marking only memory up to 4000MB as cacheable, but the actual memory
    > extends up to about 4031MB. Therefore anything that accesses the top
    > 31MB of memory will run very slow.
    >

    Ah, it all makes sense now. In this case I assume mem=4000 is perfectly
    safe and usable for the time being. In the beginning I tried with
    mem=4g, which obviously did not work. If anyone is interested in adding
    an exception/workaround for this particular motherboard, I'd be happy to
    help with testing. I have added more information about the system:
    current kernel config [1], output of `lspci -vv`[2], dmesg with mem=4000[3].

    Thank you!

    Peter

    [1] http://rabbit.us/pool/4g/config-2.6.21.5.arzamas.6.txt
    [2] http://rabbit.us/pool/4g/lspci_4000.txt
    [3] http://rabbit.us/pool/4g/dmesg_4000.txt
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