Per-thread allocation counters and limits

This tracks the amount of memory allocation by each thread in a
counter stored in the TSO.  Optionally, when the counter drops below
zero (it counts down), the thread can be sent an asynchronous
exception: AllocationLimitExceeded.  When this happens, given a small
additional limit so that it can handle the exception.  See
documentation in GHC.Conc for more details.

Allocation limits are similar to timeouts, but

  - timeouts use real time, not CPU time.  Allocation limits do not
    count anything while the thread is blocked or in foreign code.

  - timeouts don't re-trigger if the thread catches the exception,
    allocation limits do.

  - timeouts can catch non-allocating loops, if you use
    -fno-omit-yields.  This doesn't work for allocation limits.

I couldn't measure any impact on benchmarks with these changes, even
for nofib/smp.

1 files changed, 8 insertions, 0 deletions

diff --git a/includes/rts/Flags.h b/includes/rts/Flags.h
index bf6a7f3c5c..ec542701df 100644
--- a/includes/rts/Flags.h
+++ b/includes/rts/Flags.h
@@ -56,6 +56,14 @@ struct GC_FLAGS {
     rtsBool doIdleGC;
 
     StgWord heapBase;           /* address to ask the OS for memory */
+
+    StgWord allocLimitGrace;    /* units: *blocks*
+                                 * After an AllocationLimitExceeded
+                                 * exception has been raised, how much
+                                 * extra space is given to the thread
+                                 * to handle the exception before we
+                                 * raise it again.
+                                 */
 };
 
 struct DEBUG_FLAGS {