Re: MTRR fixup?

On Wed, Sep 03, 2008 at 09:47:59PM +0100, David Malone wrote:
> On Wed, Sep 03, 2008 at 05:49:44AM +0200, Bernd Walter wrote:
> > Some boards (including my Intel DG33BU) seem to have problems setting
> > up the mtrr to cache all RAM.
> > My system runs fast with 2G and ist about 6 times slower in buildworld
> > with 6G RAM.
> > I will try a BIOS update once Intels tells me why their update ISO
> > just turn the system off instead of updating the BIOS - sigh.
> > But it seems that Linux is doing some kind of fixup for MTRR:
> > http://lkml.org/lkml/2008/1/18/170
> > Can we do something similar?
> 
> You may be able to fix this by just using the memcontrol command -
> it already lets you program the MTRRs.

Oh damn - a new fancy tool to play with ;-)

Interesting - the values look good:
[...]
0x0/0x80000000 ticso write-back active 
0x80000000/0x40000000 ticso write-back active 
0xc0000000/0x10000000 ticso write-back active 
0xcf800000/0x800000 BIOS uncacheable set-by-firmware active 
0xcf400000/0x400000 BIOS uncacheable set-by-firmware active 
0x100000000/0x80000000 ticso write-back active 
0x180000000/0x20000000 ticso write-back active 
0x0/0x1000000000 - uncacheable

I've already overwritten it for tests, but it was the same as left by
the BIOS.
If I set everything uncacheable the system slows down by a factor of
two - measured from top CPU usage seen in top.
If I set it back to write-back it returns to previous usage, but it is
still much slower than with 2G installed.
Maybe MTRR is a red hering...
But why are the Linux guys claim to fix this with MTRR settings?

-- 
B.Walter <bernd_at_bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.