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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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