Re: Terrible IO performance when using 4GB of RAM on a 32 bit machine

[Robert Hancock <hancockr@shaw.ca>]

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/