Massive performance loss from OS::sleep hack

[Kurt Miller]

Kip Macy wrote:
> Yes, if we could hack the jvm to work around this without calling
> sleep that would be better yet. However, making java work well is more
> important than keeping the interface clean.

One possible alternative is to not honor thread priorities in the
jdk by default. In hotspot/src/os/linux/vm/os_linux.cpp I see this
comment:

// Note: LinuxThreads only honor thread priority for real time threads.
// sched_priority is ignored if policy is SCHED_OTHER. This function is
// equivalent to a "noop" on current Linux platforms.
OSReturn os::set_native_priority(Thread* thread, int newpri) {

which leads me to believe linux doesn't honor thread priorities
in the jdk. Perhaps someone could verify that on a linux box.
If linux and/or windows doesn't actually honor thread priorities
I don't see why we need to.

IIRC, there weren't any specific JCK tests that validated a higher
priority thread actually got more time then a lower one (I could
be wrong on that since there was > 30k tests in the jck).

So we could default UseThreadPriorities to false and make the os_sleep()
call only be used when UseThreadPriorities is true (i.e. the user
explicitly sets -XX:+UseThreadPriorities).

Can someone check out linux to see if it indeed supports or ignores
thread priorities for java programs?

-Kurt