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SubjectRe: Terrible IO performance when using 4GB of RAM on a 32 bit machine
Peter Rabbitson wrote:
> H. Peter Anvin wrote:
>> Peter Rabbitson wrote:
>>> I have captured dmesg output without mem[5], with mem=3900M[6] and
>>> mem=2048M[7].
>>>
>>
>> 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.

--
Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

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