From b4460043e6a9f0fcc6b513094c68c3835a7b8b0d Mon Sep 17 00:00:00 2001
From: David Lovemore <dl@ravenbrook.com>
Date: Wed, 27 Aug 2014 11:21:18 +0100
Subject: [PATCH] Added more control over write barrier. in particular
 separated scans after a hit.

Copied from Perforce
 Change: 186988
 ServerID: perforce.ravenbrook.com
---
 mps/code/config.h | 14 +++++++++++---
 mps/code/mpmst.h  |  2 +-
 mps/code/seg.c    |  2 +-
 mps/code/trace.c  | 16 ++++++++--------
 4 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/mps/code/config.h b/mps/code/config.h
index 59a44ee3780..f9ce6fb9396 100644
--- a/mps/code/config.h
+++ b/mps/code/config.h
@@ -637,10 +637,18 @@
 
 /* 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
+/* Number of bits needed to keep the seg scan count */
+#define SEG_SCANS_BITS 10
+/* The number of unecessary scans performed, before raising the write
+ * barrier to remember the refset summary. */
+#define SEG_SCANS_INIT 3
+/* The number of unecessary scans performed, before raising the write
+ * barrier to remember the refset summary, after a necessary scan */
+#define SEG_SCANS_AFTER_NEEDED_SCAN 3
+/* The number of unecessary scans performed, before raising the write
+ * barrier to remember the refset summary, after a barrier hit */
+#define SEG_SCANS_AFTER_HIT 1
 
 #endif /* config_h */
 
diff --git a/mps/code/mpmst.h b/mps/code/mpmst.h
index ba0be7aba58..39625a470f6 100644
--- a/mps/code/mpmst.h
+++ b/mps/code/mpmst.h
@@ -285,6 +285,7 @@ typedef struct SegStruct {      /* segment structure */
   TraceSet white : TraceLIMIT;  /* traces for which seg is white */
   TraceSet nailed : TraceLIMIT; /* traces for which seg has nailed objects */
   RankSet rankSet : RankLIMIT;  /* ranks of references in this seg */
+  unsigned scans : SEG_SCANS_BITS; /* use write barrier after this many scans */
 } SegStruct;
 
 
@@ -300,7 +301,6 @@ 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 e5923c170f7..99e3c48e47c 100644
--- a/mps/code/seg.c
+++ b/mps/code/seg.c
@@ -160,6 +160,7 @@ static Res SegInit(Seg seg, Pool pool, Addr base, Size size,
   seg->grey = TraceSetEMPTY;
   seg->pm = AccessSetEMPTY;
   seg->sm = AccessSetEMPTY;
+  seg->scans = SEG_SCANS_INIT;
   seg->depth = 0;
   seg->firstTract = NULL;
 
@@ -1082,7 +1083,6 @@ 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 bbe09b2d2c6..2856d3339cb 100644
--- a/mps/code/trace.c
+++ b/mps/code/trace.c
@@ -1105,7 +1105,6 @@ 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. */
@@ -1138,16 +1137,14 @@ static Res traceScanSegRes(TraceSet ts, Rank rank, Arena arena, Seg seg)
     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;
-      }
+      if (seg->scans > 0)
+        seg->scans--;
     } else {
-      ((GCSeg)seg)->unnecessaryScans = 0;
+      if (seg->scans < SEG_SCANS_AFTER_NEEDED_SCAN)
+        seg->scans = SEG_SCANS_AFTER_NEEDED_SCAN;
     }
     
-    if (considerBarrier) {
+    if (seg->scans == 0) {
       if(res != ResOK || !wasTotal) {
         /* scan was partial, so... */
         /* scanned summary should be ORed into segment summary. */
@@ -1217,6 +1214,9 @@ void TraceSegAccess(Arena arena, Seg seg, AccessSet mode)
 
   EVENT3(TraceAccess, arena, seg, mode);
 
+  if ((mode & SegSM(seg) & AccessWRITE) != 0)     /* write barrier? */
+    seg->scans = SEG_SCANS_AFTER_HIT;
+
   if((mode & SegSM(seg) & AccessREAD) != 0) {   /* read barrier? */
     Trace trace;
     TraceId ti;