PAE Slowdown

[Jeff Kramer]

At 2:00 PM -0500 10/8/07, Richard Todd wrote:
>Jeff Kramer <jeffk at well.com> writes:
>
>>  Hey all,
>>
>>  I know that AMD64's the preferred way to run >4 gig systems, but I'm
>>  having a weird situation with 6.2-RELEASE-p8 and 6-STABLE as of last
>>  night.  When I compile the PAE kernel, my system performance drops
>>  like a rock.  It still boots and everything still runs, but for
>>  instance, running the Flops port my megaflops drop from the 950 MFLOPS
>>  range to 4 MFLOPS.  It feels about as fast as a 486.
>
>This may not be a PAE-related problem.  I say this because I noticed you
>have the same MB I have:
>
>>  ACPI APIC Table: <INTEL  DP965LT >
>
>Several Intel MBs, including the DP965LT, have a BIOS bug that rears
>its head when you have 4G (or more) of memory installed, where the
>BIOS sets the cache control registers incorrectly.  This cause a chunk
>of your main memory (on my system, the chunk between 448MB and 512MB)
>to be labeled uncachable, with the result being random slowdowns
>whenever the kernel or user processes happen to touch memory in that
>chunk.  This problem drove me crazy trying to figure out what the problem
>was until I stumbled on this report on a Linux users' forum explaining
>the situation.
>
>   http://forums.fedoraforum.org/showthread.php?t=157232
>
>Fortunately, the workaround is fairly straightforward, adding an rc.d script
>to twiddle the MTRRs.  Assuming this is your problem, if you could post the
>output of "memcontrol list" it should be possible to id which of the entires
>is bogus and needs to be removed.

Sweet, I just downgraded to the 1669 bios rev and it looks like it's 
running at full speed.  I'm compiling an SMP/PAE kernel now, but it 
looks like this was the fix!  Thanks, Richard!

-- 

Jeff Kramer
jeffk at well.com
http://www.jeffkramer.org/