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
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/