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Date: Sat, 15 Sep 2007 19:50:13 +0200
From: Kris Kennaway <kris@FreeBSD.org>
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Subject: Massive performance loss from OS::sleep hack
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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