On Wed, Aug 18, 2010 at 06:41:02PM -0700, john stultz wrote: > On Wed, Aug 18, 2010 at 11:12 AM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > I'm completely ignorant about higher-resolution time sources. Any > > recommended reading? What resolution do they actually provide, what's > > the expense of reading them, how reliable are they, and how do the > > answers to those questions vary across different hardware and kernel > > versions? A quick look at drivers/clocksource/ doesn't suggest > > simple answers. > > Yea, there aren't simple answers. Clocksource hardware varies > drastically in resolution and access time across systems and > architectures. Further, clocksources may change while the system is > up, so we don't really expose the hardware resolution. > > On x86, access latency varies from ~50ns (TSC) to ~1.3us (ACPI PM). > (And that is ignoring the PIT, which can be 18us per call - luckily > almost no hardware uses that). The resolution similarly scales from > sub-ns (TSC @ > 1ghz cpus) to ~279ns (ACPI PM). Of course, across > architectures you will see even more variance. The race in question occurs when you manage to check mtime between two file data updates, with all three operations occurring within a clock tick. No idea if that's feasible in hundreds of nanoseconds. I'm also not sure how to judge the access latency. Certainly a microsecond is a lot compared to just reading a cached mtime value. Will we ever see them go backwards? (So if I know I wrote to file B after writing to file A, is there ever a case where I could end up with an earlier mtime on B than A?) --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
