runtime: disentangle next_gc from GC trigger

Back in Go 1.4, memstats.next_gc was both the heap size at which GC
would trigger, and the size GC kept the heap under. When we switched
to concurrent GC in Go 1.5, we got somewhat confused and made this
variable the trigger heap size, while gcController.heapGoal became the
goal heap size.

memstats.next_gc is exposed to the user via MemStats.NextGC, while
gcController.heapGoal is not. This is unfortunate because 1) the heap
goal is far more useful for diagnostics, and 2) the trigger heap size
is just part of the GC trigger heuristic, which means it wouldn't be
useful to an application even if it tried to use it.

We never noticed this mess because MemStats.NextGC is practically
undocumented. Now that we're trying to document MemStats, it became
clear that this field had diverged from its original usefulness.

Clean up this mess by shuffling things back around so that next_gc is
the goal heap size and the new (unexposed) memstats.gc_trigger field
is the trigger heap size. This eliminates gcController.heapGoal.

Updates #15849.

Change-Id: I2cbbd43b1d78bdf613cb43f53488bd63913189b7
Reviewed-on: https://go-review.googlesource.com/29270
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>
Reviewed-by: Rick Hudson <[email protected]>

Author: aclements

src/runtime/mgc.go

@@ -180,7 +180,13 @@ func gcinit() {
 		datap.gcdatamask = progToPointerMask((*byte)(unsafe.Pointer(datap.gcdata)), datap.edata-datap.data)
 		datap.gcbssmask = progToPointerMask((*byte)(unsafe.Pointer(datap.gcbss)), datap.ebss-datap.bss)
 	}
-	memstats.next_gc = heapminimum
+	memstats.gc_trigger = heapminimum
+	// Compute the goal heap size based on the trigger:
+	//   trigger = marked * (1 + triggerRatio)
+	//   marked = trigger / (1 + triggerRatio)
+	//   goal = marked * (1 + GOGC/100)
+	//        = trigger / (1 + triggerRatio) * (1 + GOGC/100)
+	memstats.next_gc = uint64(float64(memstats.gc_trigger) / (1 + gcController.triggerRatio) * (1 + float64(gcpercent)/100))
 	work.startSema = 1
 	work.markDoneSema = 1
 }
@@ -218,6 +224,9 @@ func setGCPercent(in int32) (out int32) {
 	if gcController.triggerRatio > float64(gcpercent)/100 {
 		gcController.triggerRatio = float64(gcpercent) / 100
 	}
+	// This is either in gcinit or followed by a STW GC, both of
+	// which will reset other stats like memstats.gc_trigger and
+	// memstats.next_gc to appropriate values.
 	unlock(&mheap_.lock)
 	return out
 }
@@ -303,7 +312,7 @@ const (
 // when to trigger concurrent garbage collection and how much marking
 // work to do in mutator assists and background marking.
 //
-// It uses a feedback control algorithm to adjust the memstats.next_gc
+// It uses a feedback control algorithm to adjust the memstats.gc_trigger
 // trigger based on the heap growth and GC CPU utilization each cycle.
 // This algorithm optimizes for heap growth to match GOGC and for CPU
 // utilization between assist and background marking to be 25% of
@@ -359,10 +368,6 @@ type gcControllerState struct {
 	// that assists and background mark workers started.
 	markStartTime int64
 
-	// heapGoal is the goal memstats.heap_live for when this cycle
-	// ends. This is computed at the beginning of each cycle.
-	heapGoal uint64
-
 	// dedicatedMarkWorkersNeeded is the number of dedicated mark
 	// workers that need to be started. This is computed at the
 	// beginning of each cycle and decremented atomically as
@@ -390,8 +395,9 @@ type gcControllerState struct {
 	// triggerRatio is the heap growth ratio at which the garbage
 	// collection cycle should start. E.g., if this is 0.6, then
 	// GC should start when the live heap has reached 1.6 times
-	// the heap size marked by the previous cycle. This is updated
-	// at the end of of each cycle.
+	// the heap size marked by the previous cycle. This should be
+	// ≤ GOGC/100 so the trigger heap size is less than the goal
+	// heap size. This is updated at the end of of each cycle.
 	triggerRatio float64
 
 	_ [sys.CacheLineSize]byte
@@ -416,28 +422,29 @@ func (c *gcControllerState) startCycle() {
 	c.idleMarkTime = 0
 
 	// If this is the first GC cycle or we're operating on a very
-	// small heap, fake heap_marked so it looks like next_gc is
+	// small heap, fake heap_marked so it looks like gc_trigger is
 	// the appropriate growth from heap_marked, even though the
 	// real heap_marked may not have a meaningful value (on the
 	// first cycle) or may be much smaller (resulting in a large
 	// error response).
-	if memstats.next_gc <= heapminimum {
-		memstats.heap_marked = uint64(float64(memstats.next_gc) / (1 + c.triggerRatio))
+	if memstats.gc_trigger <= heapminimum {
+		memstats.heap_marked = uint64(float64(memstats.gc_trigger) / (1 + c.triggerRatio))
 		memstats.heap_reachable = memstats.heap_marked
 	}
 
-	// Compute the heap goal for this cycle
-	c.heapGoal = memstats.heap_reachable + memstats.heap_reachable*uint64(gcpercent)/100
+	// 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
 
 	// 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
 	// start is delayed or if the allocation that pushed heap_live
-	// over next_gc is large or if the trigger is really close to
+	// over gc_trigger is large or if the trigger is really close to
 	// GOGC. Assist is proportional to this distance, so enforce a
 	// minimum distance, even if it means going over the GOGC goal
 	// by a tiny bit.
-	if c.heapGoal < memstats.heap_live+1024*1024 {
-		c.heapGoal = memstats.heap_live + 1024*1024
+	if memstats.next_gc < memstats.heap_live+1024*1024 {
+		memstats.next_gc = memstats.heap_live + 1024*1024
 	}
 
 	// Compute the total mark utilization goal and divide it among
@@ -467,7 +474,7 @@ func (c *gcControllerState) startCycle() {
 		print("pacer: assist ratio=", c.assistWorkPerByte,
 			" (scan ", memstats.heap_scan>>20, " MB in ",
 			work.initialHeapLive>>20, "->",
-			c.heapGoal>>20, " MB)",
+			memstats.next_gc>>20, " MB)",
 			" workers=", c.dedicatedMarkWorkersNeeded,
 			"+", c.fractionalMarkWorkersNeeded, "\n")
 	}
@@ -516,7 +523,7 @@ func (c *gcControllerState) revise() {
 	}
 
 	// Compute the heap distance remaining.
-	heapDistance := int64(c.heapGoal) - int64(memstats.heap_live)
+	heapDistance := int64(memstats.next_gc) - int64(memstats.heap_live)
 	if heapDistance <= 0 {
 		// This shouldn't happen, but if it does, avoid
 		// dividing by zero or setting the assist negative.
@@ -552,7 +559,7 @@ func (c *gcControllerState) endCycle() {
 	// difference between this estimate and the GOGC-based goal
 	// heap growth is the error.
 	//
-	// TODO(austin): next_gc is based on heap_reachable, not
+	// 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
@@ -585,7 +592,7 @@ func (c *gcControllerState) endCycle() {
 		// Print controller state in terms of the design
 		// document.
 		H_m_prev := memstats.heap_marked
-		H_T := memstats.next_gc
+		H_T := memstats.gc_trigger
 		h_a := actualGrowthRatio
 		H_a := memstats.heap_live
 		h_g := goalGrowthRatio
@@ -881,7 +888,7 @@ const (
 // If forceTrigger is true, it ignores the current heap size, but
 // checks all other conditions. In general this should be false.
 func gcShouldStart(forceTrigger bool) bool {
-	return gcphase == _GCoff && (forceTrigger || memstats.heap_live >= memstats.next_gc) && memstats.enablegc && panicking == 0 && gcpercent >= 0
+	return gcphase == _GCoff && (forceTrigger || memstats.heap_live >= memstats.gc_trigger) && memstats.enablegc && panicking == 0 && gcpercent >= 0
 }
 
 // gcStart transitions the GC from _GCoff to _GCmark (if mode ==
@@ -979,7 +986,7 @@ func gcStart(mode gcMode, forceTrigger bool) {
 
 	if mode == gcBackgroundMode { // Do as much work concurrently as possible
 		gcController.startCycle()
-		work.heapGoal = gcController.heapGoal
+		work.heapGoal = memstats.next_gc
 
 		// Enter concurrent mark phase and enable
 		// write barriers.
@@ -1624,13 +1631,13 @@ func gcMark(start_time int64) {
 	// by triggerRatio over the reachable heap size. Assume that
 	// we're in steady state, so the reachable heap size is the
 	// same now as it was at the beginning of the GC cycle.
-	memstats.next_gc = uint64(float64(memstats.heap_reachable) * (1 + gcController.triggerRatio))
-	if memstats.next_gc < heapminimum {
-		memstats.next_gc = heapminimum
+	memstats.gc_trigger = uint64(float64(memstats.heap_reachable) * (1 + gcController.triggerRatio))
+	if memstats.gc_trigger < heapminimum {
+		memstats.gc_trigger = heapminimum
 	}
-	if int64(memstats.next_gc) < 0 {
+	if int64(memstats.gc_trigger) < 0 {
 		print("next_gc=", memstats.next_gc, " bytesMarked=", work.bytesMarked, " heap_live=", memstats.heap_live, " initialHeapLive=", work.initialHeapLive, "\n")
-		throw("next_gc underflow")
+		throw("gc_trigger underflow")
 	}
 
 	// Update other GC heap size stats. This must happen after
@@ -1640,19 +1647,26 @@ func gcMark(start_time int64) {
 	memstats.heap_marked = work.bytesMarked
 	memstats.heap_scan = uint64(gcController.scanWork)
 
-	minNextGC := memstats.heap_live + sweepMinHeapDistance*uint64(gcpercent)/100
-	if memstats.next_gc < minNextGC {
+	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.
 		//
 		// Concurrent sweep happens in the heap growth from
-		// heap_live to next_gc, so bump next_gc up to ensure
+		// heap_live to gc_trigger, so bump gc_trigger up to ensure
 		// that concurrent sweep has some heap growth in which
 		// to perform sweeping before we start the next GC
 		// cycle.
-		memstats.next_gc = minNextGC
+		memstats.gc_trigger = minTrigger
+	}
+
+	// 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
+	if memstats.next_gc < memstats.gc_trigger {
+		memstats.next_gc = memstats.gc_trigger
 	}
 
 	if trace.enabled {
@@ -1693,7 +1707,7 @@ func gcSweep(mode gcMode) {
 	// Concurrent sweep needs to sweep all of the in-use pages by
 	// the time the allocated heap reaches the GC trigger. Compute
 	// the ratio of in-use pages to sweep per byte allocated.
-	heapDistance := int64(memstats.next_gc) - int64(memstats.heap_live)
+	heapDistance := int64(memstats.gc_trigger) - int64(memstats.heap_live)
 	// Add a little margin so rounding errors and concurrent
 	// sweep are less likely to leave pages unswept when GC starts.
 	heapDistance -= 1024 * 1024

src/runtime/mstats.go

@@ -46,7 +46,7 @@ type mstats struct {
 
 	// Statistics about garbage collector.
 	// Protected by mheap or stopping the world during GC.
-	next_gc         uint64 // next gc (in heap_live time)
+	next_gc         uint64 // goal heap_live for when next GC ends
 	last_gc         uint64 // last gc (in absolute time)
 	pause_total_ns  uint64
 	pause_ns        [256]uint64 // circular buffer of recent gc pause lengths
@@ -68,6 +68,13 @@ type mstats struct {
 
 	tinyallocs uint64 // number of tiny allocations that didn't cause actual allocation; not exported to go directly
 
+	// gc_trigger is the heap size that triggers marking.
+	//
+	// When heap_live ≥ gc_trigger, the mark phase will start.
+	// This is also the heap size by which proportional sweeping
+	// must be complete.
+	gc_trigger uint64
+
 	// heap_live is the number of bytes considered live by the GC.
 	// That is: retained by the most recent GC plus allocated
 	// since then. heap_live <= heap_alloc, since heap_alloc