From d2439c4be7afb9bb809b95fd4b217cdb85f04b4a Mon Sep 17 00:00:00 2001
From: David Lovemore <dl@ravenbrook.com>
Date: Thu, 21 Aug 2014 15:53:57 +0100
Subject: [PATCH] Added experimental control over write barrier eagerness.

The write barrier is only raised after three unnecessary scans.

Copied from Perforce
 Change: 186975
 ServerID: perforce.ravenbrook.com
---
 mps/code/config.h |  6 ++++++
 mps/code/mpmst.h  |  1 +
 mps/code/seg.c    |  1 +
 mps/code/trace.c  | 13 ++++++++++++-
 4 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/mps/code/config.h b/mps/code/config.h
index ac5d7a849ec..59a44ee3780 100644
--- a/mps/code/config.h
+++ b/mps/code/config.h
@@ -635,6 +635,12 @@
     { 36 * 1024, 0.45 }  /* second gen, after which dynamic */ \
   }
 
+/* Experimental Scan Barrier threshold
+ *
+ * The number of unecessary scans performed, before raising the write
+ * barrier to remember the refset summary.
+ */
+#define TRACE_SCAN_BARRIER_THRESHOLD 3
 
 #endif /* config_h */
 
diff --git a/mps/code/mpmst.h b/mps/code/mpmst.h
index 219858f0f54..ba0be7aba58 100644
--- a/mps/code/mpmst.h
+++ b/mps/code/mpmst.h
@@ -300,6 +300,7 @@ typedef struct GCSegStruct {    /* GC segment structure */
   RingStruct greyRing;          /* link in list of grey segs */
   RefSet summary;               /* summary of references out of seg */
   Buffer buffer;                /* non-NULL if seg is buffered */
+  unsigned unnecessaryScans;    /* consecutive unnecessary scans performed */
   Sig sig;                      /* <design/sig/> */
 } GCSegStruct;
 
diff --git a/mps/code/seg.c b/mps/code/seg.c
index ef930071850..e5923c170f7 100644
--- a/mps/code/seg.c
+++ b/mps/code/seg.c
@@ -1082,6 +1082,7 @@ static Res gcSegInit(Seg seg, Pool pool, Addr base, Size size,
 
   gcseg->summary = RefSetEMPTY;
   gcseg->buffer = NULL;
+  gcseg->unnecessaryScans = 0;
   RingInit(&gcseg->greyRing);
   gcseg->sig = GCSegSig;
 
diff --git a/mps/code/trace.c b/mps/code/trace.c
index 4f5b1477d81..bbe09b2d2c6 100644
--- a/mps/code/trace.c
+++ b/mps/code/trace.c
@@ -1105,6 +1105,7 @@ static Res traceScanSegRes(TraceSet ts, Rank rank, Arena arena, Seg seg)
   } else {      /* scan it */
     ScanStateStruct ssStruct;
     ScanState ss = &ssStruct;
+    Bool considerBarrier = FALSE;
     ScanStateInit(ss, ts, arena, rank, white);
 
     /* Expose the segment to make sure we can scan it. */
@@ -1135,8 +1136,18 @@ static Res traceScanSegRes(TraceSet ts, Rank rank, Arena arena, Seg seg)
      * scan, consistent with the recorded SegSummary?
      */
     AVER(RefSetSub(ScanStateUnfixedSummary(ss), SegSummary(seg)));
+    if (ZoneSetInter(ScanStateUnfixedSummary(ss), white) == ZoneSetEMPTY) {
+      /* a scan was not necessary */
+      if (((GCSeg)seg)->unnecessaryScans < TRACE_SCAN_BARRIER_THRESHOLD) {
+        ((GCSeg)seg)->unnecessaryScans++;
+      } else {
+        considerBarrier = TRUE;
+      }
+    } else {
+      ((GCSeg)seg)->unnecessaryScans = 0;
+    }
     
-    if (arena->incremental) {
+    if (considerBarrier) {
       if(res != ResOK || !wasTotal) {
         /* scan was partial, so... */
         /* scanned summary should be ORed into segment summary. */