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This assumption may >>> not hold when a long latency event (e.g. SMI or NMI) occurs between the >>> two reads. Reduce the risk of reading uncorrelated values by doing at >>> least four pairs of reads, using the tuple where the delta between the >>> enclosing TSC reads was smallest. From the fourth iteration onwards bail >>> if the new TSC delta isn't better (smaller) than the best earlier one. >>> >>> Signed-off-by: Jan Beulich >> >> When running virtualized, scheduling in the host would also constitute >> long latency events. I wonder whether, to compensate for that, we'd want >> more than 3 "base" iterations, as I would expect scheduling events to >> occur more frequently than e.g. SMI (and with a higher probability of >> multiple ones occurring in close succession). > > Should Xen be continually or periodically recalibrating? Rather than > trying to get it perfect at the start of day? I wouldn't call dealing with bad samples "getting it perfect". IOW I think recalibrating later may be an option, but independent of what I'm doing here. > You may also be able to find inspiration from the design or > implementation of the Precision Time Protocol which has to similarly > filter out outliers due to transmission delays. Thanks for the pointer. Jan