FWIW, I had X11 start using CPU cycles today - near 100%. I hadn't done anything X11 related since yesterday. Running xlsclients produced no output. I could quit the process normally, no problem. And when I fire up a new one, nothing unusual happens. Attached is a process sample in case it is useful. - Harald Sampling process 84193 for 3 seconds with 1 millisecond of run time between samples Sampling completed, processing symbols... Analysis of sampling X11.bin (pid 84193) every 1 millisecond Call graph: 2247 Thread_1186174 DispatchQueue_1: com.apple.main-thread (serial) 2247 start 2247 main 2247 mach_msg_server 2247 mach_startup_server 2247 _Xstart_x11_server 2247 do_start_x11_server 2247 server_main 2247 X11ApplicationMain 2247 -[NSApplication run] 2245 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] 2245 _DPSNextEvent 2245 BlockUntilNextEventMatchingListInMode 2245 ReceiveNextEventCommon 2245 RunCurrentEventLoopInMode 2245 CFRunLoopRunSpecific 2245 __CFRunLoopRun 2240 mach_msg 2240 mach_msg_trap 2 __CFMachPortPerform 2 PullEventsFromWindowServerOnConnection(unsigned int, unsigned char) 2 CGEventCreateNextEvent 2 CGSGetNextEventRecordInternal 2 snarfEvents 1 _CGSGetPortStreamInline 1 mach_msg 1 mach_msg_trap 1 snarfEvents 1 __CFRunLoopDoObservers 1 _handleWindowsNeedUpdateNote 1 -[NSApplication updateWindows] 1 objc_msgSend 1 __CFRunLoopDoSources0 1 CFSetApplyFunction 1 CFBasicHashApply 1 __CFSetApplyFunction_block_invoke_1 1 __CFRunLoopCollectSources0 1 mach_port_extract_member 2 -[X11Application sendEvent:] 2 -[X11Application(Private) sendX11NSEvent:] 1 objc_msgSend 1 xp_find_window 1 CGSFindWindowAndOwner 2247 Thread_1186215 DispatchQueue_2: com.apple.libdispatch-manager (serial) 2247 start_wqthread 2247 _pthread_wqthread 2247 _dispatch_worker_thread2 2247 _dispatch_queue_invoke 2247 _dispatch_mgr_invoke 2247 kevent 2247 Thread_1186221 2247 thread_start 2247 _pthread_start 2247 server_thread 2247 dix_main 2247 Dispatch 2237 WaitForSomething 1851 select$DARWIN_EXTSN 153 WaitForSomething 142 BlockHandler 129 QuartzBlockHandler 67 -[NSAutoreleasePool release] 44 _CFAutoreleasePoolPop 19 _CFAutoreleasePoolPop 15 pthread_setspecific 5 pthread_equal 3 pthread_getspecific 2 objc_collectingEnabled 7 -[NSAutoreleasePool release] 7 NSPopAutoreleasePool 4 _CFExecutableLinkedOnOrAfter 3 NSPopAutoreleasePool 5 DYLD-STUB$$pthread_setspecific 2 OSAtomicCompareAndSwap64 1 DYLD-STUB$$_CFExecutableLinkedOnOrAfter 1 DYLD-STUB$$pthread_getspecific 18 OSAtomicCompareAndSwap64 18 __compare_and_swap64 16 -[NSAutoreleasePool init] 12 _CFAutoreleasePoolPush 8 pthread_setspecific 3 _CFAutoreleasePoolPush 1 objc_collectingEnabled 4 -[NSAutoreleasePool init] 11 objc_msgSend_vtable14 5 +[NSAutoreleasePool allocWithZone:] 3 OSAtomicCompareAndSwap64 2 +[NSAutoreleasePool allocWithZone:] 4 QuartzBlockHandler 4 objc_msgSend 1 DYLD-STUB$$_CFAutoreleasePoolPop 1 DYLD-STUB$$objc_removeAssociatedObjects 1 __compare_and_swap64 1 objc_msgSend_vtable1 6 RootlessBlockHandler 3 BlockHandler 2 DYLD-STUB$$objc_msgSend 1 +[NSObject(NSObject) alloc] 1 objc_msgSend_vtable0 67 GetTimeInMillis 62 gettimeofday 52 __gettimeofday 30 __nanotime 22 __gettimeofday 6 gettimeofday 4 __commpage_gettimeofday 4 GetTimeInMillis 1 DYLD-STUB$$__commpage_gettimeofday 7 __bzero 6 WakeupHandler 4 WakeupHandler 1 NoopDDA 1 QuartzWakeupHandler 4 DYLD-STUB$$gettimeofday 4 __error 2 memset 1 select$1050 9 Dispatch 1 DYLD-STUB$$__error 2247 Thread_1186222 2247 thread_start 2247 _pthread_start 2247 xpbproxy_x_thread 2247 xpbproxy_run 2247 CFRunLoopRun 2247 CFRunLoopRunSpecific 2247 __CFRunLoopRun 2247 mach_msg 2247 mach_msg_trap 2247 Thread_1186233 2247 thread_start 2247 _pthread_start 2247 __NSThread__main__ 2247 +[NSURLConnection(NSURLConnectionReallyInternal) _resourceLoadLoop:] 2247 CFRunLoopRunSpecific 2247 __CFRunLoopRun 2247 mach_msg 2247 mach_msg_trap 2247 Thread_1186241: com.apple.CFSocket.private 2247 thread_start 2247 _pthread_start 2247 __CFSocketManager 2247 select$DARWIN_EXTSN 2247 Thread_1186247 2247 thread_start 2247 _pthread_start 2247 DarwinProcessFDAdditionQueue_thread 2247 _pthread_cond_wait 2247 __semwait_signal 2247 Thread_1186358 2247 thread_start 2247 _pthread_start 2247 _xp_async_thread 2247 _xp_async_dequeue 2247 _pthread_cond_wait 2247 __semwait_signal 2247 Thread_2057760 2247 start_wqthread 2247 _pthread_wqthread 2247 __workq_kernreturn Total number in stack (recursive counted multiple, when >=5): 6 _pthread_start 6 thread_start Sort by top of stack, same collapsed (when >= 5): mach_msg_trap 6735 __semwait_signal 4494 select$DARWIN_EXTSN 4098 __workq_kernreturn 2247 kevent 2247 WaitForSomething 153 __nanotime 30 pthread_setspecific 23 __gettimeofday 22 _CFAutoreleasePoolPop 19 __compare_and_swap64 19 objc_msgSend_vtable14 11 Dispatch 9 -[NSAutoreleasePool release] 7 __bzero 7 RootlessBlockHandler 6 gettimeofday 6 objc_msgSend 6 DYLD-STUB$$pthread_setspecific 5 OSAtomicCompareAndSwap64 5 pthread_equal 5 Sample analysis of process 84193 written to file /dev/stdout