Is the TSC timecounter safe on SMP system?

Andrew Gallatin gallatin at
Fri Aug 13 09:49:21 PDT 2004

I have a system where the TSC timecounter is quite a bit more accurate
(or perhaps its just much cheaper) than the ACPI timecounter.  This is a
single CPU, HTT system running an SMP kernel.

A simple program which calls gettimeofday() in a tight loop, looking
for the microseconds to change sees ~998,000 microsecond updates/sec
with kern.timecounter.hardware=TSC, and 28,500 updates/sec with ACPI-safe.

1) Is it safe to switch to TSC?

2) If yes, would it be safe to switch to TSC if this was a real
   SMP system with multiple physical cpus?



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