On Tue, Jul 26, 2011 at 06:04:16PM -0400, Tom Lane wrote: > Noah Misch <n...@2ndquadrant.com> writes: > > On Tue, Jul 26, 2011 at 05:05:15PM -0400, Tom Lane wrote: > >> Dirty cache line, maybe not, but what if the assembly code commands the > >> CPU to load those variables into CPU registers before doing the > >> comparison? If they're loaded with maxMsgNum coming in last (or at > >> least after resetState), I think you can have the problem without any > >> assumptions about cache line behavior at all. You just need the process > >> to lose the CPU at the right time. > > > True. If the compiler places the resetState load first, you could hit the > > anomaly by "merely" setting a breakpoint on the next instruction, waiting > > for > > exactly MSGNUMWRAPAROUND messages to enqueue, and letting the backend > > continue. > > I think, though, we should either plug that _and_ the cache incoherency > > case or > > worry about neither. > > How do you figure that? The poor-assembly-code-order risk is both a lot > easier to fix and a lot higher probability. Admittedly, it's still way > way down there, but you only need a precisely-timed sleep, not a > precisely-timed sleep *and* a cache line that somehow remained stale.
I think both probabilities are too low to usefully distinguish. An sinval wraparound takes a long time even in a deliberate test setup: almost 30 hours @ 10k messages/sec. To get a backend to sleep that long, you'll probably need something like SIGSTOP or a debugger attach. The sleep has to fall within the space of no more than a few instructions. Then, you'd need to release the process at the exact moment for it to observe wrapped equality. In other words, you get one split-millisecond opportunity every 30 hours of process sleep time. If your backends don't have multi-hour sleeps, it can't ever happen. Even so, all the better if we settle on an approach that has neither hazard. -- Noah Misch http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
