Re: Timestamp counters on SMP

From: Mark Hahn (hahn@coffee.psychology.mcmaster.ca)
Date: Tue Mar 28 2000 - 12:19:03 EST


> see comments there. In general, I don't think you should count on TSC's
> being syncronized at all - some people have computers with TSC's of
> different rates (me included).

in this case, TSC is completely and utterly useless to user-space.
there are only three sensible alternatives:
    - TSC's are incoherent and TSC's are turned off in user-space.
    - TSC's can be relied upon, and the kernel syncs them.
    - the kernel saves/restores TSC contents as necessary to make
    user-space see coherent time. ouch!

the sanity of SMP with different clocks is ambiguously in any case:
the MPS standard says that OS's should merely try to function,
but doesn't mandate any particular or sensible user-space behavior.

regards, mark hahn.

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