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

[Robert Hancock <hancockr@shaw.ca>]

Peter Rabbitson wrote:
> 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

There was a patch floating around recently to detect the case where the 
MTRRs don't map all of RAM as write-back, automatically cap the memory 
used by the kernel to what is mapped and print some loud warnings..

-- 
Robert Hancock      Saskatoon, SK, Canada
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