Turn on -n4m with -A16m or greater

Nursery chunks help reduce the cost of GC when capabilities are unevenly
loaded, by ensuring that we use more of the available nursery.

The rationale for enabling this at -A16m is that any negative effects
due to loss of cache locality are less likely to be an issue at -A16m
and above.  It's a conservative guess.  If we had a lot of benchmark
data we could probably do better.

Results for nofib/parallel at -N4 -A32m with and without -n4m:

```
------------------------------------------------------------------------
        Program           Size    Allocs   Runtime   Elapsed  TotalMem
------------------------------------------------------------------------
   blackscholes           0.0%     -9.5%     -9.0%    -15.0%     -2.2%
          coins           0.0%     -4.7%     -3.6%     -0.6%    -13.6%
         mandel           0.0%     -0.3%     +7.7%    +13.1%     +0.1%
        matmult           0.0%     +1.5%    +10.0%     +7.7%     +0.1%
          nbody           0.0%     -4.1%     -2.9%     0.085      0.0%
         parfib           0.0%     -1.4%     +1.0%     +1.5%     +0.2%
        partree           0.0%     -0.3%     +0.8%     +2.9%     -0.8%
           prsa           0.0%     -0.5%     -2.1%     -7.6%      0.0%
         queens           0.0%     -3.2%     -1.4%     +2.2%     +1.3%
            ray           0.0%     -5.6%    -14.5%     -7.6%     +0.8%
       sumeuler           0.0%     -0.4%     +2.4%     +1.1%      0.0%
------------------------------------------------------------------------
            Min           0.0%     -9.5%    -14.5%    -15.0%    -13.6%
            Max           0.0%     +1.5%    +10.0%    +13.1%     +1.3%
 Geometric Mean          +0.0%     -2.6%     -1.3%     -0.5%     -1.4%
```

Not conclusive, but slightly better.  This matters a lot more when you
have more cores.

Test Plan: validate, nofib/paralel

Reviewers: niteria, ezyang, nh2, trofi, austin, erikd, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2581

GHC Trac Issues: #9221

3 files changed, 19 insertions, 14 deletions

diff --git a/docs/users_guide/runtime_control.rst b/docs/users_guide/runtime_control.rst
index 0ffb1d8206..54c7508ba8 100644
--- a/docs/users_guide/runtime_control.rst
+++ b/docs/users_guide/runtime_control.rst
@@ -327,7 +327,7 @@ performance.
 
 .. rts-flag:: -n ⟨size⟩
 
-    :default: 0
+    :default: 4m with ``-A16m`` or larger, otherwise 0.
 
     .. index::
        single: allocation area, chunk size
diff --git a/rts/RtsFlags.c b/rts/RtsFlags.c
index 4bd544ee29..d86b154342 100644
--- a/rts/RtsFlags.c
+++ b/rts/RtsFlags.c
@@ -1454,6 +1454,24 @@ static void normaliseRtsOpts (void)
         errorUsage();
     }
 
+    if (RtsFlags.GcFlags.maxHeapSize != 0 &&
+        RtsFlags.GcFlags.heapSizeSuggestion >
+        RtsFlags.GcFlags.maxHeapSize) {
+        RtsFlags.GcFlags.maxHeapSize = RtsFlags.GcFlags.heapSizeSuggestion;
+    }
+
+    if (RtsFlags.GcFlags.maxHeapSize != 0 &&
+        RtsFlags.GcFlags.minAllocAreaSize >
+        RtsFlags.GcFlags.maxHeapSize) {
+        errorBelch("maximum heap size (-M) is smaller than minimum alloc area size (-A)");
+        RtsFlags.GcFlags.minAllocAreaSize = RtsFlags.GcFlags.maxHeapSize;
+    }
+
+    // If we have -A16m or larger, use -n4m.
+    if (RtsFlags.GcFlags.minAllocAreaSize >= (16*1024*1024) / BLOCK_SIZE) {
+        RtsFlags.GcFlags.nurseryChunkSize = (4*1024*1024) / BLOCK_SIZE;
+    }
+
     if (RtsFlags.ParFlags.parGcLoadBalancingGen == ~0u) {
         StgWord alloc_area_bytes
             = RtsFlags.GcFlags.minAllocAreaSize * BLOCK_SIZE;
diff --git a/rts/sm/Storage.c b/rts/sm/Storage.c
index 4d0c8d5260..357e0180d2 100644
--- a/rts/sm/Storage.c
+++ b/rts/sm/Storage.c
@@ -140,19 +140,6 @@ initStorage (void)
   ASSERT(LOOKS_LIKE_CLOSURE_PTR(&stg_dummy_ret_closure));
   ASSERT(!HEAP_ALLOCED(&stg_dummy_ret_closure));
 
-  if (RtsFlags.GcFlags.maxHeapSize != 0 &&
-      RtsFlags.GcFlags.heapSizeSuggestion >
-      RtsFlags.GcFlags.maxHeapSize) {
-      RtsFlags.GcFlags.maxHeapSize = RtsFlags.GcFlags.heapSizeSuggestion;
-  }
-
-  if (RtsFlags.GcFlags.maxHeapSize != 0 &&
-      RtsFlags.GcFlags.minAllocAreaSize >
-      RtsFlags.GcFlags.maxHeapSize) {
-      errorBelch("maximum heap size (-M) is smaller than minimum alloc area size (-A)");
-      RtsFlags.GcFlags.minAllocAreaSize = RtsFlags.GcFlags.maxHeapSize;
-  }
-
   initBlockAllocator();
 
 #if defined(THREADED_RTS)