diff --git a/mps/code/config.h b/mps/code/config.h
index 1c1df84f751..cf39d861457 100644
--- a/mps/code/config.h
+++ b/mps/code/config.h
@@ -422,8 +422,9 @@
 #define ARENA_MINIMUM_COLLECTABLE_SIZE ((Size)1000000)
 
 /* ARENA_DEFAULT_COLLECTION_RATE is an estimate of the MPS's
- * collection rate (in bytes per second), for use in the case where
- * there isn't enough data to use a measured value. */
+ * collection rate (in work per second; see <design/type/#work>), for
+ * use in the case where there isn't enough data to use a measured
+ * value. */
 
 #define ARENA_DEFAULT_COLLECTION_RATE (25000000.0)
 
diff --git a/mps/code/eventdef.h b/mps/code/eventdef.h
index 081b45de7c2..225c21f5f65 100644
--- a/mps/code/eventdef.h
+++ b/mps/code/eventdef.h
@@ -36,7 +36,7 @@
  */
 
 #define EVENT_VERSION_MAJOR  ((unsigned)1)
-#define EVENT_VERSION_MEDIAN ((unsigned)4)
+#define EVENT_VERSION_MEDIAN ((unsigned)5)
 #define EVENT_VERSION_MINOR  ((unsigned)0)
 
 
@@ -447,7 +447,7 @@
   PARAM(X,  1, W, condemned) \
   PARAM(X,  2, W, notCondemned) \
   PARAM(X,  3, W, foundation) \
-  PARAM(X,  4, W, rate) \
+  PARAM(X,  4, W, quantumWork) \
   PARAM(X,  5, D, mortality) \
   PARAM(X,  6, D, finishingTime)
 
@@ -674,7 +674,7 @@
   PARAM(X,  4, W, notCondemned) /* collectible but not condemned bytes */ \
   PARAM(X,  5, W, foundation)   /* foundation size */ \
   PARAM(X,  6, W, white)        /* white reference set */ \
-  PARAM(X,  7, W, rate)         /* segs to scan per increment */
+  PARAM(X,  7, W, quantumWork)  /* work constituting a quantum */
 
 #define EVENT_VMCompact_PARAMS(PARAM, X) \
   PARAM(X,  0, W, vmem0)        /* pre-collection reserved size */ \
diff --git a/mps/code/global.c b/mps/code/global.c
index d92d9b403a5..d15e5cd0da7 100644
--- a/mps/code/global.c
+++ b/mps/code/global.c
@@ -189,7 +189,7 @@ Bool GlobalsCheck(Globals arenaGlobals)
     CHECKD_NOSIG(Ring, &arena->greyRing[rank]);
   CHECKD_NOSIG(Ring, &arena->chainRing);
 
-  CHECKL(arena->tracedSize >= 0.0);
+  CHECKL(arena->tracedWork >= 0.0);
   CHECKL(arena->tracedTime >= 0.0);
   /* no check for arena->lastWorldCollect (Clock) */
 
@@ -287,7 +287,7 @@ Res GlobalsInit(Globals arenaGlobals)
   arena->finalPool = NULL;
   arena->busyTraces = TraceSetEMPTY;    /* <code/trace.c> */
   arena->flippedTraces = TraceSetEMPTY; /* <code/trace.c> */
-  arena->tracedSize = 0.0;
+  arena->tracedWork = 0.0;
   arena->tracedTime = 0.0;
   arena->lastWorldCollect = ClockNow();
   arena->insideShield = FALSE;          /* <code/shield.c> */
@@ -705,7 +705,7 @@ void (ArenaPoll)(Globals globals)
   Arena arena;
   Clock start;
   Count quanta;
-  Size tracedSize;
+  Work tracedWork;
 
   AVERT(Globals, globals);
 
@@ -726,11 +726,11 @@ void (ArenaPoll)(Globals globals)
   EVENT3(ArenaPoll, arena, start, 0);
 
   do {
-    tracedSize = TracePoll(globals);
-    if (tracedSize > 0) {
+    tracedWork = TracePoll(globals);
+    if (tracedWork > 0) {
       quanta += 1;
     }
-  } while (PolicyPollAgain(arena, start, tracedSize));
+  } while (PolicyPollAgain(arena, start, tracedWork));
 
   /* Don't count time spent checking for work, if there was no work to do. */
   if(quanta > 0) {
@@ -779,7 +779,7 @@ static Bool arenaShouldCollectWorld(Arena arena,
 
 Bool ArenaStep(Globals globals, double interval, double multiplier)
 {
-  Size scanned;
+  Work work;
   Bool stepped;
   Clock start, end, now;
   Clock clocks_per_sec;
@@ -812,12 +812,12 @@ Bool ArenaStep(Globals globals, double interval, double multiplier)
 
   /* loop while there is work to do and time on the clock. */
   do {
-    scanned = TracePoll(globals);
+    work = TracePoll(globals);
     now = ClockNow();
-    if (scanned > 0) {
+    if (work > 0) {
       stepped = TRUE;
     }
-  } while ((scanned > 0) && (now < end));
+  } while ((work > 0) && (now < end));
 
   if (stepped) {
     ArenaAccumulateTime(arena, start);
diff --git a/mps/code/mpm.h b/mps/code/mpm.h
index ba7e85f2f65..3e888ad28de 100644
--- a/mps/code/mpm.h
+++ b/mps/code/mpm.h
@@ -399,7 +399,7 @@ extern void TraceDestroyFinished(Trace trace);
 extern Res TraceAddWhite(Trace trace, Seg seg);
 extern Res TraceCondemnZones(Trace trace, ZoneSet condemnedSet);
 extern Res TraceStart(Trace trace, double mortality, double finishingTime);
-extern Size TracePoll(Globals globals);
+extern Work TracePoll(Globals globals);
 
 extern Rank TraceRankForAccess(Arena arena, Seg seg);
 extern void TraceSegAccess(Arena arena, Seg seg, AccessSet mode);
@@ -667,7 +667,7 @@ extern Res PolicyAlloc(Tract *tractReturn, Arena arena, LocusPref pref,
 extern Bool PolicyStartTrace(Trace *traceReturn, Arena arena);
 extern double PolicyCollectionTime(Arena arena);
 extern Bool PolicyPoll(Arena arena);
-extern Bool PolicyPollAgain(Arena arena, Clock start, Size tracedSize);
+extern Bool PolicyPollAgain(Arena arena, Clock start, Work tracedWork);
 
 
 /* Locus interface */
diff --git a/mps/code/mpmst.h b/mps/code/mpmst.h
index ac5c12c1498..86b6ec94890 100644
--- a/mps/code/mpmst.h
+++ b/mps/code/mpmst.h
@@ -486,7 +486,7 @@ typedef struct TraceStruct {
   Size condemned;               /* condemned bytes */
   Size notCondemned;            /* collectable but not condemned */
   Size foundation;              /* initial grey set size */
-  Size rate;                    /* segs to scan per increment */
+  Work quantumWork;             /* collection work constituting a quantum */
   STATISTIC_DECL(Count greySegCount); /* number of grey segs */
   STATISTIC_DECL(Count greySegMax); /* max number of grey segs */
   STATISTIC_DECL(Count rootScanCount); /* number of roots scanned */
@@ -777,7 +777,7 @@ typedef struct mps_arena_s {
   TraceMessage tMessage[TraceLIMIT];  /* <design/message-gc/> */
 
   /* policy fields */
-  double tracedSize;
+  double tracedWork;
   double tracedTime;
   Clock lastWorldCollect;
 
diff --git a/mps/code/mpmtypes.h b/mps/code/mpmtypes.h
index 3a494760cb6..c744fb41fb6 100644
--- a/mps/code/mpmtypes.h
+++ b/mps/code/mpmtypes.h
@@ -38,6 +38,7 @@ typedef Word Size;                      /* <design/type/#size> */
 typedef Word Count;                     /* <design/type/#count> */
 typedef Word Index;                     /* <design/type/#index> */
 typedef Word Align;                     /* <design/type/#align> */
+typedef Word Work;                      /* <design/type/#work> */
 typedef unsigned Shift;                 /* <design/type/#shift> */
 typedef unsigned Serial;                /* <design/type/#serial> */
 typedef Addr Ref;                       /* <design/type/#ref> */
diff --git a/mps/code/policy.c b/mps/code/policy.c
index 1052c67f2dc..ff570e4899b 100644
--- a/mps/code/policy.c
+++ b/mps/code/policy.c
@@ -280,9 +280,8 @@ double PolicyCollectionTime(Arena arena)
   collectableSize = ArenaCollectable(arena);
   /* The condition arena->tracedTime >= 1.0 ensures that the division
    * can't overflow. */
-  if (arena->tracedSize >= ARENA_MINIMUM_COLLECTABLE_SIZE
-      && arena->tracedTime >= 1.0)
-    collectionRate = arena->tracedSize / arena->tracedTime;
+  if (arena->tracedTime >= 1.0)
+    collectionRate = arena->tracedWork / arena->tracedTime;
   else
     collectionRate = ARENA_DEFAULT_COLLECTION_RATE;
   collectionTime = collectableSize / collectionRate;
@@ -313,10 +312,10 @@ Bool PolicyPoll(Arena arena)
  * should return to the mutator.
  *
  * start is the clock time when the MPS was entered.
- * tracedSize is the amount of work done by the last call to TracePoll.
+ * tracedWork is the amount of work done by the last call to TracePoll.
  */
 
-Bool PolicyPollAgain(Arena arena, Clock start, Size tracedSize)
+Bool PolicyPollAgain(Arena arena, Clock start, Work tracedWork)
 {
   Globals globals;
   double nextPollThreshold;
@@ -325,7 +324,7 @@ Bool PolicyPollAgain(Arena arena, Clock start, Size tracedSize)
   globals = ArenaGlobals(arena);
   UNUSED(start);
   
-  if (tracedSize == 0) {
+  if (tracedWork == 0) {
     /* No work was done.  Sleep until NOW + a bit. */
     nextPollThreshold = globals->fillMutatorSize + ArenaPollALLOCTIME;
   } else {
diff --git a/mps/code/trace.c b/mps/code/trace.c
index 9e7d880f2b6..d34d953bf40 100644
--- a/mps/code/trace.c
+++ b/mps/code/trace.c
@@ -109,7 +109,7 @@ void ScanStateInit(ScanState ss, TraceSet ts, Arena arena,
   STATISTIC(ss->preservedInPlaceCount = (Count)0);
   ss->preservedInPlaceSize = (Size)0; /* see .message.data */
   STATISTIC(ss->copiedSize = (Size)0);
-  ss->scannedSize = (Size)0; /* see .workclock */
+  ss->scannedSize = (Size)0; /* see .work */
   ss->sig = ScanStateSig;
 
   AVERT(ScanState, ss);
@@ -277,7 +277,7 @@ static void traceUpdateCounts(Trace trace, ScanState ss,
       break;
     }
     case traceAccountingPhaseSegScan: {
-      trace->segScanSize += ss->scannedSize; /* see .workclock */
+      trace->segScanSize += ss->scannedSize; /* see .work */
       trace->segCopiedSize += ss->copiedSize;
       STATISTIC(++trace->segScanCount);
       break;
@@ -697,14 +697,14 @@ found:
   trace->condemned = (Size)0;   /* nothing condemned yet */
   trace->notCondemned = (Size)0;
   trace->foundation = (Size)0;  /* nothing grey yet */
-  trace->rate = (Size)0;        /* no scanning to be done yet */
+  trace->quantumWork = (Work)0; /* no work done yet */
   STATISTIC(trace->greySegCount = (Count)0);
   STATISTIC(trace->greySegMax = (Count)0);
   STATISTIC(trace->rootScanCount = (Count)0);
   trace->rootScanSize = (Size)0;
   trace->rootCopiedSize = (Size)0;
   STATISTIC(trace->segScanCount = (Count)0);
-  trace->segScanSize = (Size)0; /* see .workclock */
+  trace->segScanSize = (Size)0; /* see .work */
   trace->segCopiedSize = (Size)0;
   STATISTIC(trace->singleScanCount = (Count)0);
   STATISTIC(trace->singleScanSize = (Size)0);
@@ -1713,8 +1713,10 @@ Res TraceStart(Trace trace, double mortality, double finishingTime)
     /* integer, so try to make sure it fits. */
     if(nPolls >= (double)LONG_MAX)
       nPolls = (double)LONG_MAX;
-    /* rate equals scanning work per number of polls available */
-    trace->rate = (trace->foundation + sSurvivors) / (unsigned long)nPolls + 1;
+    /* One quantum of work equals scanning work divided by number of
+     * polls, plus one to ensure it's not zero. */
+    trace->quantumWork
+      = (trace->foundation + sSurvivors) / (unsigned long)nPolls + 1;
   }
 
   /* TODO: compute rate of scanning here. */
@@ -1722,11 +1724,11 @@ Res TraceStart(Trace trace, double mortality, double finishingTime)
   EVENT8(TraceStart, trace, mortality, finishingTime,
          trace->condemned, trace->notCondemned,
          trace->foundation, trace->white,
-         trace->rate);
+         trace->quantumWork);
 
   STATISTIC_STAT(EVENT7(TraceStatCondemn, trace,
                         trace->condemned, trace->notCondemned,
-                        trace->foundation, trace->rate,
+                        trace->foundation, trace->quantumWork,
                         mortality, finishingTime));
 
   trace->state = TraceUNFLIPPED;
@@ -1737,11 +1739,11 @@ Res TraceStart(Trace trace, double mortality, double finishingTime)
 }
 
 
-/* traceWorkClock -- a measure of the work done for this trace
+/* traceWork -- a measure of the work done for this trace
  *
- * .workclock: Segment and root scanning work is the regulator.  */
+ * .work: Segment and root scanning work is the measure.  */
 
-#define traceWorkClock(trace) ((trace)->segScanSize + (trace)->rootScanSize)
+#define traceWork(trace) ((Work)((trace)->segScanSize + (trace)->rootScanSize))
 
 
 /* TraceAdvance -- progress a trace by one step */
@@ -1749,11 +1751,11 @@ Res TraceStart(Trace trace, double mortality, double finishingTime)
 void TraceAdvance(Trace trace)
 {
   Arena arena;
-  Size oldWork, newWork;
+  Work oldWork, newWork;
 
   AVERT(Trace, trace);
   arena = trace->arena;
-  oldWork = traceWorkClock(trace);
+  oldWork = traceWork(trace);
 
   switch (trace->state) {
   case TraceUNFLIPPED:
@@ -1783,9 +1785,9 @@ void TraceAdvance(Trace trace)
     break;
   }
 
-  newWork = traceWorkClock(trace);
+  newWork = traceWork(trace);
   AVER(newWork >= oldWork);
-  arena->tracedSize += newWork - oldWork;
+  arena->tracedWork += newWork - oldWork;
 }
 
 
@@ -1844,11 +1846,11 @@ failCondemn:
  * trace (if any) by one quantum. Return a measure of the work done.
  */
 
-Size TracePoll(Globals globals)
+Work TracePoll(Globals globals)
 {
   Trace trace;
   Arena arena;
-  Size oldScannedSize, scannedSize, pollEnd;
+  Work oldWork, newWork, work, endWork;
 
   AVERT(Globals, globals);
   arena = GlobalsArena(globals);
@@ -1862,20 +1864,22 @@ Size TracePoll(Globals globals)
   }
 
   AVER(arena->busyTraces == TraceSetSingle(trace));
-  oldScannedSize = traceWorkClock(trace);
-  pollEnd = oldScannedSize + trace->rate;
+  oldWork = traceWork(trace);
+  endWork = oldWork + trace->quantumWork;
   do {
     TraceAdvance(trace);
   } while (trace->state != TraceFINISHED
-           && (ArenaEmergency(arena) || traceWorkClock(trace) < pollEnd));
-  scannedSize = traceWorkClock(trace) - oldScannedSize;
+           && (ArenaEmergency(arena) || traceWork(trace) < endWork));
+  newWork = traceWork(trace);
+  AVER(newWork >= oldWork);
+  work = newWork - oldWork;
   if (trace->state == TraceFINISHED) {
     TraceDestroyFinished(trace);
     /* A trace finished, and hopefully reclaimed some memory, so clear any
      * emergency. */
     ArenaSetEmergency(arena, FALSE);
   }
-  return scannedSize;
+  return work;
 }
 
 
@@ -1913,7 +1917,7 @@ Res TraceDescribe(Trace trace, mps_lib_FILE *stream, Count depth)
                "  condemned $U\n", (WriteFU)trace->condemned,
                "  notCondemned $U\n", (WriteFU)trace->notCondemned,
                "  foundation $U\n", (WriteFU)trace->foundation,
-               "  rate $U\n", (WriteFU)trace->rate,
+               "  quantumWork $U\n", (WriteFU)trace->quantumWork,
                "  rootScanSize $U\n", (WriteFU)trace->rootScanSize,
                "  rootCopiedSize $U\n", (WriteFU)trace->rootCopiedSize,
                "  segScanSize $U\n", (WriteFU)trace->segScanSize,
diff --git a/mps/design/type.txt b/mps/design/type.txt
index 25943f41a4b..dc3c3b8cf99 100644
--- a/mps/design/type.txt
+++ b/mps/design/type.txt
@@ -664,6 +664,15 @@ _`.word.ops`: ``WordIsAligned()``, ``WordAlignUp()``,
 ``WordAlignDown()`` and ``WordRoundUp()``.
 
 
+``typedef MPS_T_WORD Work``
+
+_`.work`: ``Work`` is an unsigned integral type representing
+accumulated work done by the collector.
+
+_`.work.impl`: Work is currently implemented as a count of bytes
+scanned by the collector.
+
+
 ``typedef Word ZoneSet``
 
 _`.zoneset`: ``ZoneSet`` is a conservative approximation to a set of