Subject: Re: iperf: performance regression (was b44 driver problem?)

From: Stephen Hemminger <[email-masked]>
Date: Mon, 4 Jun 2007 10:51:58 -0700

On Mon, 04 Jun 2007 19:32:48 +0200
Thomas Gleixner <[email-masked]> wrote:

> On Mon, 2007-06-04 at 09:59 -0700, Stephen Hemminger wrote:
> > > > gettimeofday({1180973726, 982754}, NULL) = 0
> > > > recv(4, "\0\0\0\0\0\0\0\1\0\0\23\211\0\0\0\0\0\0\0\0\377\377\364"..., 8192, 0) = 8192
> > > > gettimeofday({1180973726, 983790}, NULL) = 0
> > > 
> > > Well, gettimeofday() is not affected by the highres code, but
> > > 
> > > > nanosleep({0, 0}, NULL) = 0
> > > > nanosleep({0, 0}, NULL) = 0
> > > 
> > > is. The nanosleep call with a relative timeout of 0 returns immediately
> > > with highres enabled, while it sleeps at least until the next tick
> > > arrives when highres is off. Are there more of those stupid sleeps in
> > > the code ?
> > 
> > GLIBC pthread_mutex does it, YES it is a problem!
> > Looks like the old behavior is required for ABI compatibility.
> >
> > iperf server has several threads. One thread is using pthread_mutex_lock
> > to wait for the other thread.  It looks like pthread_mutex_lock is using
> > nanosleep as yield().
> 
> I doubt that. This is in the iperf code itself.
> 
> 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
>     usleep( 0 );
> 
> ----------^^^^
> 
> It results in a nanosleep({0,0}, NULL)
> 
> 	tglx
> 

Yes, the following patch makes iperf work better than ever.
But are other broken applications going to have same problem.
Sounds like the old "who runs first" fork() problems.

--- iperf-2.0.2/compat/Thread.c.orig	2005-05-03 08:15:51.000000000 -0700
+++ iperf-2.0.2/compat/Thread.c	2007-06-04 10:54:21.000000000 -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

-- 
Stephen Hemminger <[email-masked]>