RE: [RFC patch 15/15] LTTng timestamp x86

From: Luck, Tony
Date: Fri Oct 17 2008 - 14:59:22 EST

> Hrm, on such systems
> - *large* amount of cpus
> - no synchronized TSCs
> What would be the best approach to order events ?

There isn't a perfect solution for this. My feeling is
that your best hope is with per-cpu buffers logged with
the local TSC ... together with some fancy heuristics to
post-process the logs to come up with the best approximation
to the actual ordering.

If you have a tight upper bound estimate for the
errors in converting from "per-cpu" TSC values to "global
system time" then the post processing tool will be able
to identify events for which the order is uncertain.

> Do you think we should consider using HPET, event though it's
> painfully slow ? Would it be faster than cache-line bouncing
> on such large boxes ? With a frequency around 10MHz, that
> would give a 100ns precision, which should be enough
> to order events.

This sounds like a poor choice. Makes all traces very
slow. 100ns precision isn't all that good ... we can
probably do almost as well estimating the delta between
TSC on different cpus.

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