On Tue, 2013-02-19 at 10:25 -0800, John Stultz wrote: > So describe how the perf time domain is different then > CLOCK_MONOTONIC_RAW.
The primary difference is that the trace/sched/perf time domain is not strictly monotonic, it is only locally monotonic -- that is two time stamps taken on the same cpu are guaranteed to be monotonic. Furthermore, to make it useful, there's an actual bound on the inter-cpu drift (implemented by limiting the drift to CLOCK_MONOTONIC). Additionally -- to increase use -- we also added a monotonic sync point when cpu A queries time of cpu B. > My concern here is that we're basically creating a kernel interface > that > exports implementation-defined semantics (again: whatever perf does > right now). And I think folks want to do this, because adding > CLOCK_PERF > is easier then trying to: > > 1) Get a lock-free method for accessing CLOCK_MONOTONIC_RAW > > 2) Having perf interpolate its timestamps to CLOCK_MONOTONIC, or > CLOCKMONOTONIC_RAW when it exports the data Mostly cheaper, not easier. Given unstable TSC, MONOTONIC will have to fall back to another clock source (hpet, acpi_pm and other assorted crap). In order to avoid this, we'd had to relax the requirements. Using anything other than TSC is simply not an option. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
