On Fri, 2007-11-30 at 14:29 +1100, Nick Piggin wrote: > On Friday 30 November 2007 14:15, Zhang, Yanmin wrote: > > On Fri, 2007-11-30 at 13:46 +1100, Nick Piggin wrote: > > > On Wednesday 28 November 2007 09:57, Arjan van de Ven wrote: > > > > > sounds like a bad idea; volanomark (well, technically the jvm behind > > > > it) is abusing sched_yield() by assuming it does something it really > > > > doesn't do, and as it happens some of the earlier 2.6 schedulers > > > > accidentally happened to behave in a way that was nice for this > > > > benchmark. > > > > > > OK, why is this still happening? Haven't we been asking JVMs to use > > > futexes or posix locking for years and years now? Are there any sane > > > jvms that _don't_ use yield? > > > > I think it's an issue of volanomark (a kind of java application) instead of > > JVM. > > volanomark itself and not the jvm is calling sched_yield()? Do we have > any non-toy threaded java apps? (what's JAVA in the kernel-perf tests?) I run lots of well-known benchmarks and volanoMark is the one who gets the largest impact from sched_yield.
As for real-applications which use sched_yield, mostly, they are not open sources. Yesterday, I got to know someone was using sched_yield in his network C programs, but he didn't want to share the sources with me. > > > > > > Todays kernel has a different behavior somewhat (and before people > > > > scream "regression"; sched_yield() behavior isn't really specified and > > > > doesn't make any sense at all, whatever you get is what you get.... > > > > it's pretty much an insane defacto behavior that is incredibly tied to > > > > which decisions the scheduler makes how, and no app can depend on that > > > > > > It is a performance regression. Is there any reason *not* to use the > > > "compat" yield by default? > > > > There is no, so I suggest to set sched_compat_yield=1 by default. > > If sched_compat_yield=0, kernel almost does nothing but returns. When > > sched_compat_yield=1, it is closer to the meaning of sched_yield man page. > > sched_yield() is really only defined for posix realtime scheduling > AFAIK, which talks about priority lists. > > SCHED_OTHER is defined to be a single priority, below the rest of the > realtime priorities. So at first you *might* say that the process > should then be made to run only after all other SCHED_OTHER processes, > however there is no such ordering requirement for SCHED_OTHER > scheduling. The SCHED_OTHER scheduler can run any task at any time. > > That said, I think people would *expect* that call be much closer to > the compat behaviour than the current default. And that's definitely > what Linux has done in the past. So there really does need to be a > good reason to change it like this IMO. That's indeed what I am thinking. I am running many testing(SPECjbb/SPECjbb2005/cpu2000/iozone/dbench/tbench...) to see if there is any regression if sched_compat_yield=1. I think there is no regression and the testing is just to double-check. > > > > > As you say, for SCHED_OTHER tasks, yield > > > can do almost anything. We may as well do something that isn't a > > > regression... > > > > I just found SCHED_OTHER in man sched_setscheduler. Is it SCHED_NORMAL in > > the latest kernel? > > Yes, SCHED_NORMAL is SCHED_OTHER. Don't know why it got renamed... Thanks. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
