Re: RFD: nanoseconds, rdtsc and SMP

Philip Gladstone (philip@raptor.com)
Thu, 25 Feb 1999 10:52:00 -0500

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Alan Cox wrote:
>
> > On the i386, at least currently, they will all operate in sync, but
> > may not start out from the same value (they are *supposed* to, but it
> > turns out to be buggy in practice.)
>
> They aren't even required to by the MP 1.1 spec at least, and I dont
> remember seeing such a requirement in MP 1.4. It was just a convenient
> "works all the time" assumption

In fact, it might be more convenient if the values were 'far apart' as
that
would allow a userspace program to determine which timer it read with
RDTSC.
This, coupled with a shared memory segment, would allow gettimeofday
(and friends)
to be implemented in user space. [The segment would contain the
conversion
factors for each CPU, together with a base and range for each CPU
(updated
each second (or so). The only remaining trick is to ensure that the
timers
never get close (avoids ambiguity about which processor value was read).
I think that this is solvable.]

The only problem is that any user space program that uses RDTSC directly
would probably get confused, but I'm not sure that there is much that
can be done about that.

Philip

-- 
Philip Gladstone                           +1 781 530 2461
Axent Technologies, Waltham, MA
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