New implementation of BLACKHOLEs

This replaces the global blackhole_queue with a clever scheme that
enables us to queue up blocked threads on the closure that they are
blocked on, while still avoiding atomic instructions in the common
case.

Advantages:

 - gets rid of a locked global data structure and some tricky GC code
   (replacing it with some per-thread data structures and different
   tricky GC code :)

 - wakeups are more prompt: parallel/concurrent performance should
   benefit.  I haven't seen anything dramatic in the parallel
   benchmarks so far, but a couple of threading benchmarks do improve
   a bit.

 - waking up a thread blocked on a blackhole is now O(1) (e.g. if
   it is the target of throwTo).

 - less sharing and better separation of Capabilities: communication
   is done with messages, the data structures are strictly owned by a
   Capability and cannot be modified except by sending messages.

 - this change will utlimately enable us to do more intelligent
   scheduling when threads block on each other.  This is what started
   off the whole thing, but it isn't done yet (#3838).

I'll be documenting all this on the wiki in due course.

1 files changed, 0 insertions, 2 deletions

diff --git a/rts/Timer.c b/rts/Timer.c
index a5d42fbc9d..dddc75414d 100644
--- a/rts/Timer.c
+++ b/rts/Timer.c
@@ -76,8 +76,6 @@ handle_tick(int unused STG_UNUSED)
               ticks_to_gc = RtsFlags.GcFlags.idleGCDelayTime /
                   RtsFlags.MiscFlags.tickInterval;
               recent_activity = ACTIVITY_INACTIVE;
-              blackholes_need_checking = rtsTrue;
-              /* hack: re-use the blackholes_need_checking flag */
               wakeUpRts();
           }
       }