On Wed, 2010-08-18 at 22:31 -0400, J. Bruce Fields wrote: > 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 think this is what Andi meant that you'll always race with time and that version counters are the only real solution here. > 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?) You should not. However, there have been bugs in the past, and there will probably be a few more in the future. There are also theoretical issues with SMP systems where the TSCs are not perfectly synced, but the window for those races should be small (ie: smaller then can be detected - otherwise we'll throw out the TSC). thanks -john -- 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
