archive/zip: parallelize benchmarks

Add subbenchmarks for BenchmarkZip64Test with different sizes to tease
apart construction costs vs. steady-state throughput.

Results remain comparable with the non-parallel version with -cpu=1:

benchmark                           old ns/op     new ns/op     delta
BenchmarkCompressedZipGarbage       26832835      27506953      +2.51%
BenchmarkCompressedZipGarbage-6     27172377      4321534       -84.10%
BenchmarkZip64Test                  196758732     197765510     +0.51%
BenchmarkZip64Test-6                193850605     192625458     -0.63%

benchmark                           old allocs     new allocs     delta
BenchmarkCompressedZipGarbage       44             44             +0.00%
BenchmarkCompressedZipGarbage-6     44             44             +0.00%

benchmark                           old bytes     new bytes     delta
BenchmarkCompressedZipGarbage       5592          5664          +1.29%
BenchmarkCompressedZipGarbage-6     5592          21946         +292.45%

updates #18177

Change-Id: Icfa359d9b1a8df5e085dacc07d2b9221b284764c
Reviewed-on: https://go-review.googlesource.com/36719Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/archive/zip/writer_test.go

@@ -181,12 +181,11 @@ func testReadFile(t *testing.T, f *File, wt *WriteTest) {
 }
 
 func BenchmarkCompressedZipGarbage(b *testing.B) {
-	b.ReportAllocs()
-	var buf bytes.Buffer
 	bigBuf := bytes.Repeat([]byte("a"), 1<<20)
-	for i := 0; i <= b.N; i++ {
+
+	runOnce := func(buf *bytes.Buffer) {
 		buf.Reset()
-		zw := NewWriter(&buf)
+		zw := NewWriter(buf)
 		for j := 0; j < 3; j++ {
 			w, _ := zw.CreateHeader(&FileHeader{
 				Name:   "foo",
@@ -195,11 +194,19 @@ func BenchmarkCompressedZipGarbage(b *testing.B) {
 			w.Write(bigBuf)
 		}
 		zw.Close()
-		if i == 0 {
-			// Reset the timer after the first time through.
+	}
+
+	b.ReportAllocs()
+	// Run once and then reset the timer.
 	// This effectively discards the very large initial flate setup cost,
 	// as well as the initialization of bigBuf.
+	runOnce(&bytes.Buffer{})
 	b.ResetTimer()
+
+	b.RunParallel(func(pb *testing.PB) {
+		var buf bytes.Buffer
+		for pb.Next() {
+			runOnce(&buf)
 		}
-	}
+	})
 }

src/archive/zip/zip_test.go

@@ -681,6 +681,18 @@ func BenchmarkZip64Test(b *testing.B) {
 	}
 }
 
+func BenchmarkZip64TestSizes(b *testing.B) {
+	for _, size := range []int64{1 << 12, 1 << 20, 1 << 26} {
+		b.Run(fmt.Sprint(size), func(b *testing.B) {
+			b.RunParallel(func(pb *testing.PB) {
+				for pb.Next() {
+					testZip64(b, size)
+				}
+			})
+		})
+	}
+}
+
 func TestSuffixSaver(t *testing.T) {
 	const keep = 10
 	ss := &suffixSaver{keep: keep}