Ingo Molnar wrote:
* George Anzinger <[email protected]> wrote:
The TSC is such a fast and, usually, accurate answer, I think it
deserves a little effort to save it. With your new clock code I think
we could use per cpu TSC counters, read the full 64 bits and, in real
corner cases, even per cpu conversion "constants" and solve this
problem.
the problem is, this is the same issue as 'boot-time TSC syncing', but
in disguise: to get any 'per CPU TSC offset' you need to do exactly the
same type of careful all-CPUs-dance to ensure that the TSCs were sampled
at around the same moment in time!
The box where i have these small TSC inconsistencies shows that it's the
bootup synchronization of TSCs that failed to be 100% accurate. Even a 2
usecs error in synchronization can show up as a time-warp - regardless
of whether we keep per-CPU TSC offsets or whether we clear these offsets
back to 0. So it is not a solution to do another type of synchronization
dance. The only solution is to fix the boot-time synchronization (where
the hardware keeps TSCs synchronized all the time), or to switch TSCs
off where this is not possible.
I was rather thinking of only comparing a cup's TSC with the SAME cup's TSC. We only use the TSC to
bridge from the latest update of the the clock to now and when we update the clock we capture (i.e.
update this cup's last TSC value) the TSC on that CPU. If we also capture the system time then we
have a set of (last TSC, sys clock) for each CPU. If we further, keep 64-bits of TSC, we don't
really require each cpu to update the clock very often, or, we could force such and update from time
to time as needed. This would not require the TSC to be synced at all and even if they had
different rates we could add that to the per cpu data and cover that too.
What this does require is that we do a really good job of figuring the TSC multiplier, something we
don't do at all well today (rc5-rt5 on my box is fast by ~15 sec/ day). We might also want to
verify that we are not letting monotonic time be other than monotonic :)
As you might suspect, I have some ideas about how to do a much better job of figuring out the TSC
multiplier.
--
George Anzinger [email protected]
HRT (High-res-timers): http://sourceforge.net/projects/high-res-timers/
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