runtime: print start of GC cycle in gctrace, rather than end

Currently the GODEBUG=gctrace=1 trace line includes "@n.nnns" to
indicate the time that the GC cycle ended relative to the time the
program started. This was meant to be consistent with the utilization
as of the end of the cycle, which is printed next on the trace line,
but it winds up just being confusing and unexpected.

Change the trace line to include the time that the GC cycle started
relative to the time the program started.

Change-Id: I7d64580cd696eb17540716d3e8a74a9d6ae50650
Reviewed-on: https://go-review.googlesource.com/10634
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index f5877e6847bbe94445752575f1f9f464f578047d..71b083b29cca2bd6c22e7f0dee4cc594eadb6668 100644 (file)
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -1006,7 +1006,7 @@ func gc(mode int) {
                var sbuf [24]byte
                printlock()
                print("gc #", memstats.numgc,
-                       " @", string(itoaDiv(sbuf[:], uint64(tEnd-runtimeInitTime)/1e6, 3)), "s ",
+                       " @", string(itoaDiv(sbuf[:], uint64(tSweepTerm-runtimeInitTime)/1e6, 3)), "s ",
                        util, "%: ")
                prev := tSweepTerm
                for i, ns := range []int64{tScan, tInstallWB, tMark, tMarkTerm, tEnd} {