runtime: clean up gctrace format

Go 1.6 simplified the GC phases. The "synchronize Ps" phase no longer
exists and "root scan" and "mark" phases have been combined.

Update the gctrace line implementation and documentation to remove the
unused phases.

Fixes #13536.

Change-Id: I4fc37a3ce1ae3a99d48c0be2df64cbda3e05dee6
Reviewed-on: https://go-review.googlesource.com/18458
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/extern.go

@@ -66,7 +66,7 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	length of the pause. Setting gctrace=2 emits the same summary but also
 	repeats each collection. The format of this line is subject to change.
 	Currently, it is:
-		gc # @#s #%: #+...+# ms clock, #+...+# ms cpu, #->#-># MB, # MB goal, # P
+		gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # P
 	where the fields are as follows:
 		gc #        the GC number, incremented at each GC
 		@#s         time in seconds since program start
@@ -75,9 +75,9 @@ It is a comma-separated list of name=val pairs setting these named variables:
 		#->#-># MB  heap size at GC start, at GC end, and live heap
 		# MB goal   goal heap size
 		# P         number of processors used
-	The phases are stop-the-world (STW) sweep termination, scan,
-	synchronize Ps, mark, and STW mark termination. The CPU times
-	for mark are broken down in to assist time (GC performed in
+	The phases are stop-the-world (STW) sweep termination, concurrent
+	mark and scan, and STW mark termination. The CPU times
+	for mark/scan are broken down in to assist time (GC performed in
 	line with allocation), background GC time, and idle GC time.
 	If the line ends with "(forced)", this GC was forced by a
 	runtime.GC() call and all phases are STW.

src/runtime/mgc.go

@@ -1263,23 +1263,13 @@ func gcMarkTermination() {
 	if debug.gctrace > 0 {
 		util := int(memstats.gc_cpu_fraction * 100)
 
-		// Install WB phase is no longer used.
-		tInstallWB := work.tMark
-		installWBCpu := int64(0)
-
-		// Scan phase is no longer used.
-		tScan := tInstallWB
-		scanCpu := int64(0)
-
-		// TODO: Clean up the gctrace format.
-
 		var sbuf [24]byte
 		printlock()
 		print("gc ", memstats.numgc,
 			" @", string(itoaDiv(sbuf[:], uint64(work.tSweepTerm-runtimeInitTime)/1e6, 3)), "s ",
 			util, "%: ")
 		prev := work.tSweepTerm
-		for i, ns := range []int64{tScan, tInstallWB, work.tMark, work.tMarkTerm, work.tEnd} {
+		for i, ns := range []int64{work.tMark, work.tMarkTerm, work.tEnd} {
 			if i != 0 {
 				print("+")
 			}
@@ -1287,8 +1277,8 @@ func gcMarkTermination() {
 			prev = ns
 		}
 		print(" ms clock, ")
-		for i, ns := range []int64{sweepTermCpu, scanCpu, installWBCpu, gcController.assistTime, gcController.dedicatedMarkTime + gcController.fractionalMarkTime, gcController.idleMarkTime, markTermCpu} {
-			if i == 4 || i == 5 {
+		for i, ns := range []int64{sweepTermCpu, gcController.assistTime, gcController.dedicatedMarkTime + gcController.fractionalMarkTime, gcController.idleMarkTime, markTermCpu} {
+			if i == 2 || i == 3 {
 				// Separate mark time components with /.
 				print("/")
 			} else if i != 0 {