commit bdfc5375f219d6def81effda4e57cb56d01fc917
Author: Sergei Trofimovich <slyfox@gentoo.org>
Date:   Tue Aug 30 12:10:54 2016 +0100

    rts: enable parallel GC scan of large (32M+) allocation area
    
    Parallel GC does not scan large allocation area (-A)
    effectively as it does not do work stealing from nursery
    by default.
    
    That leads to large imbalance when only one of threads
    overflows allocation area: most of GC threads finish
    quickly (as there is not much to collect) and sit idle
    waiting while single GC thread finishes scan of single
    allocation area for that thread.
    
    The patch enables work stealing for (equivalent of -qb0)
    allocation area of -A32M or higher.
    
    Tested on a highlighting-kate package from Trac #9221
    
    On 8-core machine the difference is around 5% faster
    of wall-clock time. On 24-core VM the speedup is 20%.
    
    Signed-off-by: Sergei Trofimovich <siarheit@google.com>
    
    Test Plan: measured wall time and GC parallelism on highlighting-kate build
    
    Reviewers: austin, bgamari, erikd, simonmar
    
    Reviewed By: bgamari, simonmar
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D2483
    
    GHC Trac Issues: #9221

diff --git a/rts/RtsFlags.c b/rts/RtsFlags.c
index fda33f0..7a719b9 100644
--- a/rts/RtsFlags.c
+++ b/rts/RtsFlags.c
@@ -237,1 +237,1 @@ void initRtsFlagsDefaults(void)
-    RtsFlags.ParFlags.parGcLoadBalancingGen = 1;
+    RtsFlags.ParFlags.parGcLoadBalancingGen = ~0u; /* auto, based on -A */
@@ -1398,2 +1390,19 @@ static void normaliseRtsOpts (void)
     }
 
+#ifdef THREADED_RTS
+    if (RtsFlags.ParFlags.parGcLoadBalancingGen == ~0u) {
+        StgWord alloc_area_bytes
+            = RtsFlags.GcFlags.minAllocAreaSize * BLOCK_SIZE;
+
+        // If allocation area is larger that CPU cache
+        // we can finish scanning quicker doing work-stealing
+        // scan. Trac #9221
+        // 32M looks big enough not to fit into L2 cache
+        // of popular modern CPUs.
+        if (alloc_area_bytes >= 32 * 1024 * 1024) {
+            RtsFlags.ParFlags.parGcLoadBalancingGen = 0;
+        } else {
+            RtsFlags.ParFlags.parGcLoadBalancingGen = 1;
+        }
+    }
+#endif