RE: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

Dan Magenheimer
2009-11-02T15:47:19Z

> > 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