runtime: eliminate memstats.heap_reachable

We used to compute an estimate of the reachable heap size that was
different from the marked heap size. This ultimately caused more
problems than it solved, so we pulled it out, but memstats still has
both heap_reachable and heap_marked, and there are some leftover TODOs
about the problems with this estimate.

Clean this up by eliminating heap_reachable in favor of heap_marked
and deleting the stale TODOs.

Change-Id: I713bc20a7c90683d2b43ff63c0b21a440269cc4d
Reviewed-on: https://go-review.googlesource.com/29271
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -429,12 +429,11 @@ func (c *gcControllerState) startCycle() {
 	// error response).
 	if memstats.gc_trigger <= heapminimum {
 		memstats.heap_marked = uint64(float64(memstats.gc_trigger) / (1 + c.triggerRatio))
-		memstats.heap_reachable = memstats.heap_marked
 	}
 
 	// Re-compute the heap goal for this cycle in case something
 	// changed. This is the same calculation we use elsewhere.
-	memstats.next_gc = memstats.heap_reachable + memstats.heap_reachable*uint64(gcpercent)/100
+	memstats.next_gc = memstats.heap_marked + memstats.heap_marked*uint64(gcpercent)/100
 
 	// Ensure that the heap goal is at least a little larger than
 	// the current live heap size. This may not be the case if GC
@@ -558,10 +557,6 @@ func (c *gcControllerState) endCycle() {
 	// growth if we had the desired CPU utilization). The
 	// difference between this estimate and the GOGC-based goal
 	// heap growth is the error.
-	//
-	// TODO(austin): gc_trigger is based on heap_reachable, not
-	// heap_marked, which means the actual growth ratio
-	// technically isn't comparable to the trigger ratio.
 	goalGrowthRatio := float64(gcpercent) / 100
 	actualGrowthRatio := float64(memstats.heap_live)/float64(memstats.heap_marked) - 1
 	assistDuration := nanotime() - c.markStartTime
@@ -1624,14 +1619,14 @@ func gcMark(start_time int64) {
 
 	cachestats()
 
-	// Update the reachable heap stat.
-	memstats.heap_reachable = work.bytesMarked
+	// Update the marked heap stat.
+	memstats.heap_marked = work.bytesMarked
 
 	// Trigger the next GC cycle when the allocated heap has grown
-	// by triggerRatio over the reachable heap size. Assume that
-	// we're in steady state, so the reachable heap size is the
+	// by triggerRatio over the marked heap size. Assume that
+	// we're in steady state, so the marked heap size is the
 	// same now as it was at the beginning of the GC cycle.
-	memstats.gc_trigger = uint64(float64(memstats.heap_reachable) * (1 + gcController.triggerRatio))
+	memstats.gc_trigger = uint64(float64(memstats.heap_marked) * (1 + gcController.triggerRatio))
 	if memstats.gc_trigger < heapminimum {
 		memstats.gc_trigger = heapminimum
 	}
@@ -1644,15 +1639,13 @@ func gcMark(start_time int64) {
 	// cachestats (which flushes local statistics to these) and
 	// flushallmcaches (which modifies heap_live).
 	memstats.heap_live = work.bytesMarked
-	memstats.heap_marked = work.bytesMarked
 	memstats.heap_scan = uint64(gcController.scanWork)
 
 	minTrigger := memstats.heap_live + sweepMinHeapDistance*uint64(gcpercent)/100
 	if memstats.gc_trigger < minTrigger {
 		// The allocated heap is already past the trigger.
 		// This can happen if the triggerRatio is very low and
-		// the reachable heap estimate is less than the live
-		// heap size.
+		// the marked heap is less than the live heap size.
 		//
 		// Concurrent sweep happens in the heap growth from
 		// heap_live to gc_trigger, so bump gc_trigger up to ensure
@@ -1664,7 +1657,7 @@ func gcMark(start_time int64) {
 
 	// The next GC cycle should finish before the allocated heap
 	// has grown by GOGC/100.
-	memstats.next_gc = memstats.heap_reachable + memstats.heap_reachable*uint64(gcpercent)/100
+	memstats.next_gc = memstats.heap_marked + memstats.heap_marked*uint64(gcpercent)/100
 	if memstats.next_gc < memstats.gc_trigger {
 		memstats.next_gc = memstats.gc_trigger
 	}

src/runtime/mstats.go

@@ -111,10 +111,6 @@ type mstats struct {
 	// unlike heap_live, heap_marked does not change until the
 	// next mark termination.
 	heap_marked uint64
-
-	// heap_reachable is an estimate of the reachable heap bytes
-	// at the end of the previous GC.
-	heap_reachable uint64
 }
 
 var memstats mstats