Default +RTS -qn to the number of cores

Setting a -N value that is too large has a dramatic negative effect on
performance, but the new -qn flag can mitigate the worst of the effects
by limiting the number of GC threads.

So now, if you don't explcitly set +RTS -qn, and you set -N larger than
the number of cores (or use setNumCapabilities to do the same), we'll
default -qn to the number of cores.

These are the results from nofib/parallel on my 4-core (2 cores x 2
threads) i7 laptop, comparing -N8 before and after this change.

```
------------------------------------------------------------------------
        Program           Size    Allocs   Runtime   Elapsed  TotalMem
------------------------------------------------------------------------
   blackscholes          +0.0%     +0.0%    -72.5%    -72.0%     +9.5%
          coins          +0.0%     -0.0%    -73.7%    -72.2%     -0.8%
         mandel          +0.0%     +0.0%    -76.4%    -75.4%     +3.3%
        matmult          +0.0%    +15.5%    -26.8%    -33.4%     +1.0%
          nbody          +0.0%     +2.4%     +0.7%     0.076      0.0%
         parfib          +0.0%     -8.5%    -33.2%    -31.5%     +2.0%
        partree          +0.0%     -0.0%    -60.4%    -56.8%     +5.7%
           prsa          +0.0%     -0.0%    -65.4%    -60.4%      0.0%
         queens          +0.0%     +0.2%    -58.8%    -58.8%     -1.5%
            ray          +0.0%     -1.5%    -88.7%    -85.6%     -3.6%
       sumeuler          +0.0%     -0.0%    -47.8%    -46.9%      0.0%
------------------------------------------------------------------------
            Min          +0.0%     -8.5%    -88.7%    -85.6%     -3.6%
            Max          +0.0%    +15.5%     +0.7%    -31.5%     +9.5%
 Geometric Mean          +0.0%     +0.6%    -61.4%    -63.1%     +1.4%
```

Test Plan: validate, nofib/parallel benchmarks

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

Reviewed By: trofi, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2580

GHC Trac Issues: #9221

1 files changed, 12 insertions, 3 deletions

diff --git a/rts/Schedule.c b/rts/Schedule.c
index 611d70411f..3cbfc0eba0 100644
--- a/rts/Schedule.c
+++ b/rts/Schedule.c
@@ -1531,6 +1531,7 @@ scheduleDoGC (Capability **pcap, Task *task USED_IF_THREADS,
     uint32_t gc_type;
     uint32_t i;
     uint32_t need_idle;
+    uint32_t n_gc_threads;
     uint32_t n_idle_caps = 0, n_failed_trygrab_idles = 0;
     StgTSO *tso;
     rtsBool *idle_cap;
@@ -1561,9 +1562,17 @@ scheduleDoGC (Capability **pcap, Task *task USED_IF_THREADS,
         gc_type = SYNC_GC_SEQ;
     }
 
-    if (gc_type == SYNC_GC_PAR && RtsFlags.ParFlags.parGcThreads > 0) {
-        need_idle = stg_max(0, enabled_capabilities -
-                            RtsFlags.ParFlags.parGcThreads);
+    // If -qn is not set and we have more capabilities than cores, set the
+    // number of GC threads to #cores.  We do this here rather than in
+    // normaliseRtsOpts() because here it will work if the program calls
+    // setNumCapabilities.
+    n_gc_threads = RtsFlags.ParFlags.parGcThreads;
+    if (n_gc_threads == 0 && enabled_capabilities > getNumberOfProcessors()) {
+        n_gc_threads = getNumberOfProcessors();
+    }
+
+    if (gc_type == SYNC_GC_PAR && n_gc_threads > 0) {
+        need_idle = stg_max(0, enabled_capabilities - n_gc_threads);
     } else {
         need_idle = 0;
     }