Messages in this thread | | | Date | Mon, 2 Nov 2009 07:46:06 -0800 (PST) | From | Dan Magenheimer <> | Subject | RE: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation |
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> > I don't have any public data available for this DB usage, > > Sorry, that doesn't explain anything.
Well for now just consider the DB usage as another use of profiling. But one can easily draw scenarios where a monotonic timestamp is also used to guarantee transaction ordering.
> > Search for "flight recorder". This feature is intended to > > be enabled all the time, but with non-vsyscall gettimeofday > > the performance impact is unacceptably high, so they are using > > For profiling work fast timestamping is of course great, but surely > there is no monotonicity requirement?
Yes and no. Monotonicity is a poor substitute for a more generic mechanism that might provide an indication that a discontinuity has occurred (forward or backward); if an app could get both the timestamp AND some kind of "continuity generation counter" (basically a much more sophisticated form of TSC_AUX that changes whenever the timestamp is coming from a different source), perhaps all problems could be solved.
> I don't think we'll be able to provide monotonicity with vsyscall on > tsc-broken hosts, so we'll be limited to correcting the tsc frequency > after migration for good-tsc hosts.
True, though clock_gettime(CLOCK_MONOTONIC) can provide the monotonicity where it is required.
Dan
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