godoc: added systematic throttling to indexing goroutine

- implemented stand-alone Throttle mechanism
- added new flag -index_throttle to godoc
- index throttling enables index creation when running
  godoc on app engine as it keeps godoc responsive

R=rsc, dsymonds, adg
CC=golang-dev
https://golang.org/cl/4963043

src/cmd/godoc/Makefile

@@ -18,6 +18,7 @@ GOFILES=\
 	parser.go\
 	snippet.go\
 	spec.go\
+	throttle.go\
 	utils.go\
 	zip.go\
 

src/cmd/godoc/doc.go

@@ -50,6 +50,11 @@ The flags are:
 	-index
 		enable identifier and full text search index
 		(no search box is shown if -index is not set)
+	-index_throttle=0.75
+		index throttle value; a value of 0 means no time is allocated
+		to the indexer (the indexer will never finish), a value of 1.0
+		means that index creation is running at full throttle (other
+		goroutines may get no time while the index is built)
 	-maxresults=10000
 		maximum number of full text search results shown
 		(no full text index is built if maxresults <= 0)

src/cmd/godoc/godoc.go

@@ -65,6 +65,7 @@ var (
 	// search index
 	indexEnabled  = flag.Bool("index", false, "enable search index")
 	maxResults    = flag.Int("maxresults", 10000, "maximum number of full text search results shown")
+	indexThrottle = flag.Float64("index_throttle", 0.75, "index throttle value; 0.0 = no time allocated, 1.0 = full throttle")
 
 	// file system mapping
 	fs         FileSystem      // the underlying file system for godoc
@@ -1148,7 +1149,7 @@ func indexer() {
 				log.Printf("updating index...")
 			}
 			start := time.Nanoseconds()
-			index := NewIndex(fsDirnames(), *maxResults > 0)
+			index := NewIndex(fsDirnames(), *maxResults > 0, *indexThrottle)
 			stop := time.Nanoseconds()
 			searchIndex.set(index)
 			if *verbose {

src/cmd/godoc/index.go

@@ -736,8 +736,9 @@ func canonical(w string) string { return strings.ToLower(w) }
 // NewIndex creates a new index for the .go files
 // in the directories given by dirnames.
 //
-func NewIndex(dirnames <-chan string, fulltextIndex bool) *Index {
+func NewIndex(dirnames <-chan string, fulltextIndex bool, throttle float64) *Index {
 	var x Indexer
+	th := NewThrottle(throttle, 0.1e9) // run at least 0.1s at a time
 
 	// initialize Indexer
 	x.fset = token.NewFileSet()
@@ -753,6 +754,7 @@ func NewIndex(dirnames <-chan string, fulltextIndex bool) *Index {
 			if !f.IsDirectory() {
 				x.visitFile(dirname, f, fulltextIndex)
 			}
+			th.Throttle()
 		}
 	}
 
@@ -778,6 +780,7 @@ func NewIndex(dirnames <-chan string, fulltextIndex bool) *Index {
 			Others: others,
 		}
 		wlist = append(wlist, &wordPair{canonical(w), w})
+		th.Throttle()
 	}
 	x.stats.Words = len(words)
 

src/cmd/godoc/throttle.go

@@ -0,0 +1,88 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "time"
+
+// A Throttle permits throttling of a goroutine by
+// calling the Throttle method repeatedly.
+//
+type Throttle struct {
+	f  float64 // f = (1-r)/r for 0 < r < 1
+	tm int64   // minimum run time slice; >= 0
+	tr int64   // accumulated time running
+	ts int64   // accumulated time stopped
+	tt int64   // earliest throttle time (= time Throttle returned + tm)
+}
+
+// NewThrottle creates a new Throttle with a throttle value r and
+// a minimum allocated run time slice of tm nanoseconds:
+//
+//	r == 0: "empty" throttle; the goroutine is always sleeping
+//	r == 1: full throttle; the goroutine is never sleeping
+//
+// A value of r == 0.6 throttles a goroutine such that it runs
+// approx. 60% of the time, and sleeps approx. 40% of the time.
+// Values of r < 0 or r > 1 are clamped down to values between 0 and 1.
+// Values of tm < 0 are set to 0.
+//
+func NewThrottle(r float64, tm int64) *Throttle {
+	var f float64
+	switch {
+	case r <= 0:
+		f = -1 // indicates always sleep
+	case r >= 1:
+		f = 0 // assume r == 1 (never sleep)
+	default:
+		// 0 < r < 1
+		f = (1 - r) / r
+	}
+	if tm < 0 {
+		tm = 0
+	}
+	return &Throttle{f: f, tm: tm, tt: time.Nanoseconds() + tm}
+}
+
+// Throttle calls time.Sleep such that over time the ratio tr/ts between
+// accumulated run (tr) and sleep times (ts) approximates the value 1/(1-r)
+// where r is the throttle value. Throttle returns immediately (w/o sleeping)
+// if less than tm ns have passed since the last call to Throttle.
+//
+func (p *Throttle) Throttle() {
+	if p.f < 0 {
+		select {} // always sleep
+	}
+
+	t0 := time.Nanoseconds()
+	if t0 < p.tt {
+		return // keep running (minimum time slice not exhausted yet)
+	}
+
+	// accumulate running time
+	p.tr += t0 - (p.tt - p.tm)
+
+	// compute sleep time
+	// Over time we want:
+	//
+	//	tr/ts = r/(1-r)
+	//
+	// Thus:
+	//
+	//	ts = tr*f with f = (1-r)/r
+	//
+	// After some incremental run time δr added to the total run time
+	// tr, the incremental sleep-time δs to get to the same ratio again
+	// after waking up from time.Sleep is:
+	if δs := int64(float64(p.tr)*p.f) - p.ts; δs > 0 {
+		time.Sleep(δs)
+	}
+
+	// accumulate (actual) sleep time
+	t1 := time.Nanoseconds()
+	p.ts += t1 - t0
+
+	// set earliest next throttle time
+	p.tt = t1 + p.tm
+}