[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation
On 11/02/2009 05:46 PM, Dan Magenheimer wrote: 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. In this case we should provide a facility for this. Providing a global monotonic counter may be easier than providing a monotonic clock. Hence my question. 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 usingFor 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 doubt it. A discontinuity has occured, but what do we know about it? nothing. 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. We have that already. The question is how to implement it in a vsyscall. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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