runtime: reduce per-P memory footprint when greenteagc is disabled

There are two additional sources of memory overhead per P that come from
greenteagc. One is for ptrBuf, but on platforms other than Windows it
doesn't actually cost anything due to demand-paging (Windows also
demand-pages, but the memory is 'committed' so it still counts against
OS RSS metrics). The other is for per-sizeclass scan stats. However when
greenteagc is disabled, most of these scan stats are completely unused.

The worst-case memory overhead from these two sources is relatively
small (about 10 KiB per P), but for programs with a small memory
footprint running on a machine with a lot of cores, this can be
significant (single-digit percent).

This change does two things. First, it puts ptrBuf initialization behind
the greenteagc experiment, so now that memory is never allocated by
default. Second, it abstracts the implementation details of scan stat
collection and emission, such that we can have two different
implementations depending on the build tag. This lets us remove all the
unused stats when the greenteagc experiment is disabled, reducing the
memory overhead of the stats from ~2.6 KiB per P to 536 bytes per P.
This is enough to make the difference no longer noticable in our
benchmark suite.

Fixes #73931.

Change-Id: I4351f1cbb3f6743d8f5922d757d73442c6d6ad3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/678535
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/mgc.go

@@ -131,6 +131,7 @@ package runtime
 import (
 	"internal/cpu"
 	"internal/goarch"
+	"internal/goexperiment"
 	"internal/runtime/atomic"
 	"internal/runtime/gc"
 	"unsafe"
@@ -717,7 +718,7 @@ func gcStart(trigger gcTrigger) {
 			throw("p mcache not flushed")
 		}
 		// Initialize ptrBuf if necessary.
-		if p.gcw.ptrBuf == nil {
+		if goexperiment.GreenTeaGC && p.gcw.ptrBuf == nil {
 			p.gcw.ptrBuf = (*[gc.PageSize / goarch.PtrSize]uintptr)(persistentalloc(gc.PageSize, goarch.PtrSize, &memstats.gcMiscSys))
 		}
 	}
@@ -1233,14 +1234,7 @@ func gcMarkTermination(stw worldStop) {
 			})
 		}
 		if debug.gctrace > 1 {
-			for i := range pp.gcw.stats {
+			pp.gcw.flushScanStats(&memstats.lastScanStats)
-				memstats.lastScanStats[i].spansDenseScanned += pp.gcw.stats[i].spansDenseScanned
-				memstats.lastScanStats[i].spanObjsDenseScanned += pp.gcw.stats[i].spanObjsDenseScanned
-				memstats.lastScanStats[i].spansSparseScanned += pp.gcw.stats[i].spansSparseScanned
-				memstats.lastScanStats[i].spanObjsSparseScanned += pp.gcw.stats[i].spanObjsSparseScanned
-				memstats.lastScanStats[i].sparseObjsScanned += pp.gcw.stats[i].sparseObjsScanned
-			}
-			clear(pp.gcw.stats[:])
 		}
 		pp.pinnerCache = nil
 	})
@@ -1301,38 +1295,7 @@ func gcMarkTermination(stw worldStop) {
 		print("\n")
 
 		if debug.gctrace > 1 {
-			var (
+			dumpScanStats()
-				spansDenseScanned     uint64
-				spanObjsDenseScanned  uint64
-				spansSparseScanned    uint64
-				spanObjsSparseScanned uint64
-				sparseObjsScanned     uint64
-			)
-			for _, stats := range memstats.lastScanStats {
-				spansDenseScanned += stats.spansDenseScanned
-				spanObjsDenseScanned += stats.spanObjsDenseScanned
-				spansSparseScanned += stats.spansSparseScanned
-				spanObjsSparseScanned += stats.spanObjsSparseScanned
-				sparseObjsScanned += stats.sparseObjsScanned
-			}
-			totalObjs := sparseObjsScanned + spanObjsSparseScanned + spanObjsDenseScanned
-			totalSpans := spansSparseScanned + spansDenseScanned
-			print("scan: total ", sparseObjsScanned, "+", spanObjsSparseScanned, "+", spanObjsDenseScanned, "=", totalObjs, " objs")
-			print(", ", spansSparseScanned, "+", spansDenseScanned, "=", totalSpans, " spans\n")
-			for i, stats := range memstats.lastScanStats {
-				if stats == (sizeClassScanStats{}) {
-					continue
-				}
-				totalObjs := stats.sparseObjsScanned + stats.spanObjsSparseScanned + stats.spanObjsDenseScanned
-				totalSpans := stats.spansSparseScanned + stats.spansDenseScanned
-				if i == 0 {
-					print("scan: class L ")
-				} else {
-					print("scan: class ", gc.SizeClassToSize[i], "B ")
-				}
-				print(stats.sparseObjsScanned, "+", stats.spanObjsSparseScanned, "+", stats.spanObjsDenseScanned, "=", totalObjs, " objs")
-				print(", ", stats.spansSparseScanned, "+", stats.spansDenseScanned, "=", totalSpans, " spans\n")
-			}
 		}
 		printunlock()
 	}

src/runtime/mgcmark_greenteagc.go

@@ -763,3 +763,57 @@ func heapBitsSmallForAddrInline(spanBase, addr, elemsize uintptr) uintptr {
 	}
 	return read
 }
+
+type sizeClassScanStats struct {
+	spansDenseScanned     uint64
+	spanObjsDenseScanned  uint64
+	spansSparseScanned    uint64
+	spanObjsSparseScanned uint64
+	sparseObjsScanned     uint64
+}
+
+func dumpScanStats() {
+	var (
+		spansDenseScanned     uint64
+		spanObjsDenseScanned  uint64
+		spansSparseScanned    uint64
+		spanObjsSparseScanned uint64
+		sparseObjsScanned     uint64
+	)
+	for _, stats := range memstats.lastScanStats {
+		spansDenseScanned += stats.spansDenseScanned
+		spanObjsDenseScanned += stats.spanObjsDenseScanned
+		spansSparseScanned += stats.spansSparseScanned
+		spanObjsSparseScanned += stats.spanObjsSparseScanned
+		sparseObjsScanned += stats.sparseObjsScanned
+	}
+	totalObjs := sparseObjsScanned + spanObjsSparseScanned + spanObjsDenseScanned
+	totalSpans := spansSparseScanned + spansDenseScanned
+	print("scan: total ", sparseObjsScanned, "+", spanObjsSparseScanned, "+", spanObjsDenseScanned, "=", totalObjs, " objs")
+	print(", ", spansSparseScanned, "+", spansDenseScanned, "=", totalSpans, " spans\n")
+	for i, stats := range memstats.lastScanStats {
+		if stats == (sizeClassScanStats{}) {
+			continue
+		}
+		totalObjs := stats.sparseObjsScanned + stats.spanObjsSparseScanned + stats.spanObjsDenseScanned
+		totalSpans := stats.spansSparseScanned + stats.spansDenseScanned
+		if i == 0 {
+			print("scan: class L ")
+		} else {
+			print("scan: class ", gc.SizeClassToSize[i], "B ")
+		}
+		print(stats.sparseObjsScanned, "+", stats.spanObjsSparseScanned, "+", stats.spanObjsDenseScanned, "=", totalObjs, " objs")
+		print(", ", stats.spansSparseScanned, "+", stats.spansDenseScanned, "=", totalSpans, " spans\n")
+	}
+}
+
+func (w *gcWork) flushScanStats(dst *[gc.NumSizeClasses]sizeClassScanStats) {
+	for i := range w.stats {
+		dst[i].spansDenseScanned += w.stats[i].spansDenseScanned
+		dst[i].spanObjsDenseScanned += w.stats[i].spanObjsDenseScanned
+		dst[i].spansSparseScanned += w.stats[i].spansSparseScanned
+		dst[i].spanObjsSparseScanned += w.stats[i].spanObjsSparseScanned
+		dst[i].sparseObjsScanned += w.stats[i].sparseObjsScanned
+	}
+	clear(w.stats[:])
+}

src/runtime/mgcmark_nogreenteagc.go

@@ -6,6 +6,8 @@
 
 package runtime
 
+import "internal/runtime/gc"
+
 func (s *mspan) markBitsForIndex(objIndex uintptr) markBits {
 	bytep, mask := s.gcmarkBits.bitp(objIndex)
 	return markBits{bytep, mask, objIndex}
@@ -78,3 +80,33 @@ func (w *gcWork) tryGetSpan(steal bool) objptr {
 func scanSpan(p objptr, gcw *gcWork) {
 	throw("unimplemented")
 }
+
+type sizeClassScanStats struct {
+	sparseObjsScanned uint64
+}
+
+func dumpScanStats() {
+	var sparseObjsScanned uint64
+	for _, stats := range memstats.lastScanStats {
+		sparseObjsScanned += stats.sparseObjsScanned
+	}
+	print("scan: total ", sparseObjsScanned, " objs\n")
+	for i, stats := range memstats.lastScanStats {
+		if stats == (sizeClassScanStats{}) {
+			continue
+		}
+		if i == 0 {
+			print("scan: class L ")
+		} else {
+			print("scan: class ", gc.SizeClassToSize[i], "B ")
+		}
+		print(stats.sparseObjsScanned, " objs\n")
+	}
+}
+
+func (w *gcWork) flushScanStats(dst *[gc.NumSizeClasses]sizeClassScanStats) {
+	for i := range w.stats {
+		dst[i].sparseObjsScanned += w.stats[i].sparseObjsScanned
+	}
+	clear(w.stats[:])
+}

src/runtime/mstats.go

@@ -49,14 +49,6 @@ type mstats struct {
 	enablegc bool
 }
 
-type sizeClassScanStats struct {
-	spansDenseScanned     uint64
-	spanObjsDenseScanned  uint64
-	spansSparseScanned    uint64
-	spanObjsSparseScanned uint64
-	sparseObjsScanned     uint64
-}
-
 var memstats mstats
 
 // A MemStats records statistics about the memory allocator.