From 0ad2cd004c94c664dfee3ff16e9d587467f99883 Mon Sep 17 00:00:00 2001
From: Dmitriy Vyukov <dvyukov@google.com>
Date: Thu, 23 Jan 2014 15:13:21 -0500
Subject: [PATCH] bufio: fix benchmarks behavior Currently the benchmarks lie
 to testing package by doing O(N) work under StopTimer. And that hidden O(N)
 actually consitutes the bulk of benchmark work (e.g includes GC per
 iteration). This behavior accounts for windows-amd64-race builder hangs.

Before:
BenchmarkReaderCopyOptimal-4	 1000000	      1861 ns/op
BenchmarkReaderCopyUnoptimal-4	  500000	      3327 ns/op
BenchmarkReaderCopyNoWriteTo-4	   50000	     34549 ns/op
BenchmarkWriterCopyOptimal-4	  100000	     16849 ns/op
BenchmarkWriterCopyUnoptimal-4	  500000	      3126 ns/op
BenchmarkWriterCopyNoReadFrom-4	   50000	     34609 ns/op
ok  	bufio	65.273s

After:
BenchmarkReaderCopyOptimal-4	10000000	       172 ns/op
BenchmarkReaderCopyUnoptimal-4	10000000	       267 ns/op
BenchmarkReaderCopyNoWriteTo-4	  100000	     22905 ns/op
BenchmarkWriterCopyOptimal-4	10000000	       170 ns/op
BenchmarkWriterCopyUnoptimal-4	10000000	       226 ns/op
BenchmarkWriterCopyNoReadFrom-4	  100000	     20575 ns/op
ok  	bufio	14.074s

Note the change in total time.

LGTM=alex.brainman, rsc
R=golang-codereviews, alex.brainman, rsc
CC=golang-codereviews
https://golang.org/cl/51360046
---
 src/pkg/bufio/bufio_test.go | 68 ++++++++++++++++++++++++-------------
 1 file changed, 44 insertions(+), 24 deletions(-)

diff --git a/src/pkg/bufio/bufio_test.go b/src/pkg/bufio/bufio_test.go
index 5cd030714d..06d365b796 100644
--- a/src/pkg/bufio/bufio_test.go
+++ b/src/pkg/bufio/bufio_test.go
@@ -1112,63 +1112,83 @@ func (w onlyWriter) Write(b []byte) (int, error) {
 
 func BenchmarkReaderCopyOptimal(b *testing.B) {
 	// Optimal case is where the underlying reader implements io.WriterTo
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	src := NewReader(srcBuf)
+	dstBuf := new(bytes.Buffer)
+	dst := onlyWriter{dstBuf}
 	for i := 0; i < b.N; i++ {
-		b.StopTimer()
-		src := NewReader(bytes.NewBuffer(make([]byte, 8192)))
-		dst := onlyWriter{new(bytes.Buffer)}
-		b.StartTimer()
+		srcBuf.Reset()
+		src.Reset(srcBuf)
+		dstBuf.Reset()
 		io.Copy(dst, src)
 	}
 }
 
 func BenchmarkReaderCopyUnoptimal(b *testing.B) {
 	// Unoptimal case is where the underlying reader doesn't implement io.WriterTo
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	src := NewReader(onlyReader{srcBuf})
+	dstBuf := new(bytes.Buffer)
+	dst := onlyWriter{dstBuf}
 	for i := 0; i < b.N; i++ {
-		b.StopTimer()
-		src := NewReader(onlyReader{bytes.NewBuffer(make([]byte, 8192))})
-		dst := onlyWriter{new(bytes.Buffer)}
-		b.StartTimer()
+		srcBuf.Reset()
+		src.Reset(onlyReader{srcBuf})
+		dstBuf.Reset()
 		io.Copy(dst, src)
 	}
 }
 
 func BenchmarkReaderCopyNoWriteTo(b *testing.B) {
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	srcReader := NewReader(srcBuf)
+	src := onlyReader{srcReader}
+	dstBuf := new(bytes.Buffer)
+	dst := onlyWriter{dstBuf}
 	for i := 0; i < b.N; i++ {
-		b.StopTimer()
-		src := onlyReader{NewReader(bytes.NewBuffer(make([]byte, 8192)))}
-		dst := onlyWriter{new(bytes.Buffer)}
-		b.StartTimer()
+		srcBuf.Reset()
+		srcReader.Reset(srcBuf)
+		dstBuf.Reset()
 		io.Copy(dst, src)
 	}
 }
 
 func BenchmarkWriterCopyOptimal(b *testing.B) {
 	// Optimal case is where the underlying writer implements io.ReaderFrom
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	src := onlyReader{srcBuf}
+	dstBuf := new(bytes.Buffer)
+	dst := NewWriter(dstBuf)
 	for i := 0; i < b.N; i++ {
-		b.StopTimer()
-		src := onlyReader{bytes.NewBuffer(make([]byte, 8192))}
-		dst := NewWriter(new(bytes.Buffer))
-		b.StartTimer()
+		srcBuf.Reset()
+		dstBuf.Reset()
+		dst.Reset(dstBuf)
 		io.Copy(dst, src)
 	}
 }
 
 func BenchmarkWriterCopyUnoptimal(b *testing.B) {
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	src := onlyReader{srcBuf}
+	dstBuf := new(bytes.Buffer)
+	dst := NewWriter(onlyWriter{dstBuf})
 	for i := 0; i < b.N; i++ {
-		b.StopTimer()
-		src := onlyReader{bytes.NewBuffer(make([]byte, 8192))}
-		dst := NewWriter(onlyWriter{new(bytes.Buffer)})
-		b.StartTimer()
+		srcBuf.Reset()
+		dstBuf.Reset()
+		dst.Reset(onlyWriter{dstBuf})
 		io.Copy(dst, src)
 	}
 }
 
 func BenchmarkWriterCopyNoReadFrom(b *testing.B) {
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	src := onlyReader{srcBuf}
+	dstBuf := new(bytes.Buffer)
+	dstWriter := NewWriter(dstBuf)
+	dst := onlyWriter{dstWriter}
 	for i := 0; i < b.N; i++ {
-		b.StopTimer()
-		src := onlyReader{bytes.NewBuffer(make([]byte, 8192))}
-		dst := onlyWriter{NewWriter(new(bytes.Buffer))}
-		b.StartTimer()
+		srcBuf.Reset()
+		dstBuf.Reset()
+		dstWriter.Reset(dstBuf)
 		io.Copy(dst, src)
 	}
 }
-- 
2.48.1