Massive performance loss from OS::sleep hack

[Nick Johnson]

I looked up the bug # in Sun's bug database, and although it isn't there 
anymore, there is another bug marked as a duplicate:

http://bugs.sun.com/view_bug.do?bug_id=4319085

I don't pretend to understand the details, but it looks like that bit of 
code was introduced to work around a Solaris kernel bug related to threads 
not yielding/suspending.

I wonder if that might have been a problem a long time ago with libc_r 
that no longer troubles us.  Fortunately, the bug indicates that it is 
very easy to repeat (though that may be more true on Solaris than 
FreeBSD), so it could be easy to test whether just using pthread_yield() 
is now sufficient.

   Nick

On Sat, 15 Sep 2007, Kris Kennaway wrote:

> Hi,
> 
> I have been running the volano java benchmark
> (http://www.volano.com/benchmarks.html) on an 8-core i386 system, and out of
> the box jdk15 on FreeBSD performs extremely poorly.  The system is more than
> 90% idle, and profiling shows that the ~800 threads in the benchmark are
> spending most of their time doing short nanosleep() calls.
> 
> 
> I traced it to the following FreeBSD-specific hack in the jdk:
> 
> // XXXBSD: understand meaning and workaround related to yield
> ...
> // XXXBSD: done differently in 1.3.1, take a look
> int os::sleep(Thread* thread, jlong millis, bool interruptible) {
>   assert(thread == Thread::current(),  "thread consistency check");
> ...
> 
>   if (millis <= 0) {
>     // NOTE: workaround for bug 4338139
>     if (thread->is_Java_thread()) {
>       ThreadBlockInVM tbivm((JavaThread*) thread);
> // BSDXXX: Only use pthread_yield here and below if the system thread
> // scheduler gives time slices to lower priority threads when yielding.
> #ifdef __FreeBSD__
>       os_sleep(MinSleepInterval, interruptible);
> #else
>       pthread_yield();
> #endif
> 
> When I removed this hack (i.e. revert to pthread_yield()) I got an immediate
> 7-fold performance increase, which brings FreeBSD performance on par with
> Solaris.
> 
> What is the reason why this code is necessary?  Does FreeBSD's sched_yield()
> really have different semantics to the other operating systems, or was this a
> libkse bug that was being worked around?
> 
> Kris
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> 

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