cmd/trace: force GC occassionally

to return memory to the OS after completing potentially
large operations.

Update #21870

Sys went down to 3.7G

$ DEBUG_MEMORY_USAGE=1 go tool trace trace.out

2018/03/07 09:35:52 Parsing trace...
after parsing trace
 Alloc:	3385754360 Bytes
 Sys:	3662047864 Bytes
 HeapReleased:	0 Bytes
 HeapSys:	3488907264 Bytes
 HeapInUse:	3426549760 Bytes
 HeapAlloc:	3385754360 Bytes
Enter to continue...
2018/03/07 09:36:09 Splitting trace...
after spliting trace
 Alloc:	3238309424 Bytes
 Sys:	3684410168 Bytes
 HeapReleased:	0 Bytes
 HeapSys:	3488874496 Bytes
 HeapInUse:	3266461696 Bytes
 HeapAlloc:	3238309424 Bytes
Enter to continue...
2018/03/07 09:36:39 Opening browser. Trace viewer is listening on http://100.101.224.241:12345

after httpJsonTrace
 Alloc:	3000633872 Bytes
 Sys:	3693978424 Bytes
 HeapReleased:	0 Bytes
 HeapSys:	3488743424 Bytes
 HeapInUse:	3030966272 Bytes
 HeapAlloc:	3000633872 Bytes
Enter to continue...

Change-Id: I56f64cae66c809cbfbad03fba7bd0d35494c1d04
Reviewed-on: https://go-review.googlesource.com/92376Reviewed-by: Peter Weinberger <pjw@google.com>

src/cmd/trace/main.go

@@ -17,6 +17,7 @@ import (
 	"net/http"
 	"os"
 	"runtime"
+	"runtime/debug"
 	"sync"
 
 	_ "net/http/pprof" // Required to use pprof
@@ -119,10 +120,12 @@ func main() {
 		os.Exit(0)
 	}
 	reportMemoryUsage("after parsing trace")
+	debug.FreeOSMemory()
 
 	log.Print("Splitting trace...")
 	ranges = splitTrace(res)
 	reportMemoryUsage("after spliting trace")
+	debug.FreeOSMemory()
 
 	addr := "http://" + ln.Addr().String()
 	log.Printf("Opening browser. Trace viewer is listening on %s", addr)

src/cmd/trace/trace.go

@@ -14,6 +14,7 @@ import (
 	"net/http"
 	"path/filepath"
 	"runtime"
+	"runtime/debug"
 	"strconv"
 	"strings"
 	"time"
@@ -165,6 +166,7 @@ func httpTraceViewerHTML(w http.ResponseWriter, r *http.Request) {
 
 // httpJsonTrace serves json trace, requested from within templTrace HTML.
 func httpJsonTrace(w http.ResponseWriter, r *http.Request) {
+	defer debug.FreeOSMemory()
 	defer reportMemoryUsage("after httpJsonTrace")
 	// This is an AJAX handler, so instead of http.Error we use log.Printf to log errors.
 	res, err := parseTrace()
@@ -293,6 +295,7 @@ func splittingTraceConsumer(max int) (*splitter, traceConsumer) {
 				// so flush can include them in the required
 				// part of the trace.
 				data.Events = append(data.Events, v)
+				return
 			}
 			enc := json.NewEncoder(&cw)
 			enc.Encode(v)
@@ -1025,6 +1028,8 @@ func viewerDataTraceConsumer(w io.Writer, start, end int64) traceConsumer {
 				io.WriteString(w, ",")
 			}
 			enc.Encode(v)
+			// TODO: get rid of the extra \n inserted by enc.Encode.
+			// Same should be applied to splittingTraceConsumer.
 			written++
 		},
 		consumeViewerFrame: func(k string, v ViewerFrame) {