iperf: thread bug on linux 2.6.21 or later

[Stephen Hemminger <shemminger --at-- linux-foundation.org>]

The latest version of Linux kernel aggravates a pre-existing iperf thread
library bug. The iperf thread library assumes that calling usleep(0) will cause
the thread to yield so that other threads will run. This has never been a documented
behavior of Linux/Unix. The new high resolution timer option in the kernel causes
usleep(0) to be a nop so the thread keeps running (until it's quanta is exhausted).

Without this fix, iperf will get poor performance because the monitoring thread
may hog the cpu, keeping the sender/receiver threads from running.

The fix to iperf is easy, just use sched_yield() instead. The manual page for sched_yield
says to test for POSIX_PRIORITY_SCHEDULING as a Posix option.

--- compat/Thread.c.orig	2005-05-03 08:15:51.000000000 -0700
+++ compat/Thread.c	2007-06-04 10:41:11.000000000 -0700
@@ -405,9 +405,13 @@
 void thread_rest ( void ) {
 #if defined( HAVE_THREAD )
 #if defined( HAVE_POSIX_THREAD )
-    // TODO add checks for sched_yield or pthread_yield and call that
-    // if available
+
+#if defined( _POSIX_PRIORITY_SCHEDULING )
+    sched_yield();
+#else
     usleep( 0 );
+#endif
+
 #else // Win32
     SwitchToThread( );
 #endif