Same problem for me...
Solved with patch of Ingo Molnar from:
http://kerneltrap.org/Linux/Finding_Bugs_With_CFS

---
 compat/Thread.c |    3 ---
 src/Reporter.c  |   13 +++++++++----
 src/main.cpp    |    2 ++
 3 files changed, 11 insertions(+), 7 deletions(-)

Index: iperf-2.0.2/compat/Thread.c
===================================================================
--- iperf-2.0.2.orig/compat/Thread.c
+++ iperf-2.0.2/compat/Thread.c
@@ -405,9 +405,6 @@ int thread_numuserthreads( void ) {
 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 );
 #else // Win32
     SwitchToThread( );
 #endif
Index: iperf-2.0.2/src/Reporter.c
===================================================================
--- iperf-2.0.2.orig/src/Reporter.c
+++ iperf-2.0.2/src/Reporter.c
@@ -111,6 +111,7 @@ report_statistics multiple_reports[kRepo
 char buffer[64]; // Buffer for printing
 ReportHeader *ReportRoot = NULL;
 extern Condition ReportCond;
+extern Condition ReportDoneCond;
 int reporter_process_report ( ReportHeader *report );
 void process_report ( ReportHeader *report );
 int reporter_handle_packet( ReportHeader *report );
@@ -338,7 +339,7 @@ void ReportPacket( ReportHeader* agent, 
             // item
             while ( index == 0 ) {
                 Condition_Signal( &ReportCond );
-                thread_rest();
+                Condition_Wait( &ReportDoneCond );
                 index = agent->reporterindex;
             }
             agent->agentindex = 0;
@@ -346,7 +347,7 @@ void ReportPacket( ReportHeader* agent, 
         // Need to make sure that reporter is not about to be "lapped"
         while ( index - 1 == agent->agentindex ) {
             Condition_Signal( &ReportCond );
-            thread_rest();
+            Condition_Wait( &ReportDoneCond );
             index = agent->reporterindex;
         }
         
@@ -553,6 +554,7 @@ void reporter_spawn( thread_Settings *th
         }
         Condition_Unlock ( ReportCond );
 
+again:
         if ( ReportRoot != NULL ) {
             ReportHeader *temp = ReportRoot;
             //Condition_Unlock ( ReportCond );
@@ -575,9 +577,12 @@ void reporter_spawn( thread_Settings *th
                 // finished with report so free it
                 free( temp );
                 Condition_Unlock ( ReportCond );
+            	Condition_Signal( &ReportDoneCond );
+		if (ReportRoot)
+			goto again;
             }
-            // yield control of CPU is another thread is waiting
-            thread_rest();
+            Condition_Signal( &ReportDoneCond );
+            usleep(10000);
         } else {
             //Condition_Unlock ( ReportCond );
         }
Index: iperf-2.0.2/src/main.cpp
===================================================================
--- iperf-2.0.2.orig/src/main.cpp
+++ iperf-2.0.2/src/main.cpp
@@ -96,6 +96,7 @@ extern "C" {
     // records being accessed in a report and also to
     // serialize modification of the report list
     Condition ReportCond;
+    Condition ReportDoneCond;
 }
 
 // global variables only accessed within this file
@@ -141,6 +142,7 @@ int main( int argc, char **argv ) {
 
     // Initialize global mutexes and conditions
     Condition_Initialize ( &ReportCond );
+    Condition_Initialize ( &ReportDoneCond );
     Mutex_Initialize( &groupCond );
     Mutex_Initialize( &clients_mutex );
 
-