Serverworks LE, 4GB RAM, and MTRR

From: Marvin Justice (
Date: Mon Jul 30 2001 - 18:37:02 EST

Slow performance on Serverworks LE boards with 4GB of RAM seem to be related
to mtrr misconfiguration. Here is the /proc/mtrr for a Tyan 2510 ( 2.4.7-ac2):

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= 64MB: write-back, count=1
reg06: base=0xfc000000 (4032MB), size= 64MB: uncachable, count=1

Also, the framebuffer is 4MB starting at 0xfd000000 (4048MB) on this system.
The last entry seems to be the culprit. Why should there be 64MB uncachable
starting at 4032?

Back in April there was a thread concering the mtrr setup for the LE chipset.
A patch for mtrr.c was submitted (but never accepted, apparently) that allows
write-combining (which is currently disabled for all Serverworks LE) for
revisions >5. If I modify mtrr.c to allow write-combining the system works
normally with 4G. /proc/mtrr is unchanged but the following line shows up in
the syslog when the X-server is started:

mtrr: type mismatch for fd000000,400000 old: uncachable new: write-combining

The slowness of the system without write-combining is independent of whether
X is started.

Marvin Justice
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This archive was generated by hypermail 2b29 : Tue Jul 31 2001 - 21:00:48 EST