SLOW machine when HIGHMEM enabled (1gb memory, kernel 2.4.22)

[Matt Hahnfeld]

I have an ASUS P4P800-VM motherboard with 2 sticks of 512mb PC3200 and
a single 2.4 ghz P4 processor.  The kernel is vanilla 2.4.22 configured
for SMP (hyperthreading).

When I use a kernel with high memory support off, I get good
performance (despite not being able to use some of my memory).  When I
enable CONFIG_HIGHMEM4G the remaining memory is detected, but the
machine takes a big performance hit and starts running very slow --
ie. kernel compilation looks like it would take 5 days instead of 5
minutes.  /proc/meminfo doesn't look particularly strange and no
strange log messages show up -- everything just runs slow...

CONFIG_HIGHMEM64G produces the same results...

Any suggestions?

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