Messages in this thread | | | From | "Luck, Tony" <> | Date | Fri, 17 Oct 2008 11:58:31 -0700 | Subject | RE: [RFC patch 15/15] LTTng timestamp x86 |
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> 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.
-Tony
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