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

@@ -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 */
 

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;
 

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;
 

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. */