[RFC] Improve performance of multi-threaded debugging

[Kevin Buettner <kevinb@redhat.com>]

As things stand now, the thread list is fetched each time rda checks
the status of the program.  This doesn't sound like such a burden until
you realize that some decent sized data structures are read via the
ptrace() interface.  On slower machines, it can take a significant amount
of time to single step or continue primarily due to this overhead.

The patch below fetches the thread list only when it knows for certain
that something has changed.

The only fly in the ointment is that the signal based event model only
knows about thread creation, but not about thread death.  So it won't
catch thread death until some new thread is created.  I'm not sure
what the implications of this are in practice.

Comments?

	* thread-db.c (ALWAYS_UPDATE_THREAD_LIST): Define to be 0.
	(handle_thread_db_event): Update thread list upon receipt of
	TD_CREATE or TD_DEATH events.
	(thread_db_check_child_state): Potentially disable, depending upon
	value of ALWAYS_UPDATE_THREAD_LIST, the thread list update.
	(thread_db_check_child_state): Update thread list for signal based
	event model too.

Index: thread-db.c
===================================================================
RCS file: /cvs/cvsfiles/devo/rda/unix/thread-db.c,v
retrieving revision 1.25.2.1
diff -u -p -r1.25.2.1 thread-db.c
--- thread-db.c	3 Aug 2005 05:20:58 -0000	1.25.2.1
+++ thread-db.c	14 Sep 2005 21:31:48 -0000
@@ -48,6 +48,8 @@
 int thread_db_noisy = 0;
 int proc_service_noisy = 0;
 
+#define ALWAYS_UPDATE_THREAD_LIST 0
+
 /*
  * A tiny local symbol table.
  *
@@ -1779,6 +1781,7 @@ handle_thread_db_event (struct child_pro
   struct gdbserv_thread *thread = process->event_thread;
   lwpid_t lwp;
   union wait w;
+  int do_update = 0;
 
   /* We need to be actually using the event interface.  */
   if (! using_thread_db_events)
@@ -1812,13 +1815,16 @@ handle_thread_db_event (struct child_pro
 	  break;
 	}
 
-      /* The only messages we're concerned with are TD_CREATE and
-	 TD_DEATH.
+      if (msg.event == TD_CREATE || msg.event == TD_DEATH)
+	do_update = 1;
+    }
 
-	 Every time thread_db_check_child_state gets a wait status
-	 from waitpid, we call update_thread_list, so our list is
-	 always up to date; we don't actually need to do anything with
-	 these messages for our own sake.  */
+  if (do_update)
+    {
+#if !ALWAYS_UPDATE_THREAD_LIST
+      /* Update the thread list.  */
+      update_thread_list (process);
+#endif
     }
 
   /* Disable the event breakpoints while we step the thread across them.  */
@@ -2122,9 +2128,11 @@ thread_db_check_child_state (struct chil
 		     process->stop_signal,
 		     (unsigned long) debug_get_pc (process->serv, process->pid));
 
+#if ALWAYS_UPDATE_THREAD_LIST
 	  /* Update the thread list, and attach to (and thereby stop)
              any new threads we find.  */
 	  update_thread_list (process);
+#endif
 
 	  process->event_thread = thread_list_lookup_by_lid (process->pid);
 
@@ -2170,6 +2178,12 @@ thread_db_check_child_state (struct chil
 		    process->stop_signal = restart_signal;
 		  else				/* not main thread */
 		    process->stop_signal = 0;
+
+#if !ALWAYS_UPDATE_THREAD_LIST
+		  /* Update the thread list.  */
+		  update_thread_list (process);
+#endif
+
 		}
 	      process->signal_to_send = process->stop_signal;
 	      currentvec->continue_program (serv);