CONFIG_X86_PM_TIMER is slow

[dean gaudet]

when using CONFIG_X86_PM_TIMER i'm finding that gettimeofday() calls take 
2.8us on a p-m 1.4GHz box... which is an order of magnitude slower than 
TSC-based solutions.

on one workload i'm seeing a 7% perf improvement by booting "acpi=off" to 
force it to use tsc instead of the PM timer... (the workload calls 
gettimeofday too frequently, but i can't change that).

i'm curious why other folks haven't run into this -- is it because most 
systems have HPET timer as well and that's not nearly as bad as PM timer?

-dean
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