zstd: Reduce per decoder allocations significantly (#252)

* zstd: Reduce decoder allocations significantly

* More strict window size check.

Author: klauspost

zstd/blockdec.go

@@ -131,17 +131,25 @@ func (b *blockDec) reset(br byteBuffer, windowSize uint64) error {
 	b.Type = blockType((bh >> 1) & 3)
 	// find size.
 	cSize := int(bh >> 3)
+	maxSize := maxBlockSize
 	switch b.Type {
 	case blockTypeReserved:
 		return ErrReservedBlockType
 	case blockTypeRLE:
 		b.RLESize = uint32(cSize)
+		if b.lowMem {
+			maxSize = cSize
+		}
 		cSize = 1
 	case blockTypeCompressed:
 		if debug {
 			println("Data size on stream:", cSize)
 		}
 		b.RLESize = 0
+		maxSize = maxCompressedBlockSize
+		if windowSize < maxCompressedBlockSize && b.lowMem {
+			maxSize = int(windowSize)
+		}
 		if cSize > maxCompressedBlockSize || uint64(cSize) > b.WindowSize {
 			if debug {
 				printf("compressed block too big: csize:%d block: %+v\n", uint64(cSize), b)
@@ -160,8 +168,8 @@ func (b *blockDec) reset(br byteBuffer, windowSize uint64) error {
 			b.dataStorage = make([]byte, 0, maxBlockSize)
 		}
 	}
-	if cap(b.dst) <= maxBlockSize {
-		b.dst = make([]byte, 0, maxBlockSize+1)
+	if cap(b.dst) <= maxSize {
+		b.dst = make([]byte, 0, maxSize+1)
 	}
 	var err error
 	b.data, err = br.readBig(cSize, b.dataStorage)
@@ -679,8 +687,11 @@ func (b *blockDec) decodeCompressed(hist *history) error {
 		println("initializing sequences:", err)
 		return err
 	}
-
-	err = seqs.decode(nSeqs, br, hist.b)
+	hbytes := hist.b
+	if len(hbytes) > hist.windowSize {
+		hbytes = hbytes[len(hbytes)-hist.windowSize:]
+	}
+	err = seqs.decode(nSeqs, br, hbytes)
 	if err != nil {
 		return err
 	}

zstd/decoder_test.go

@@ -149,6 +149,68 @@ func TestNewDecoder(t *testing.T) {
 	testDecoderDecodeAll(t, "testdata/decoder.zip", dec)
 }
 
+func TestNewDecoderMemory(t *testing.T) {
+	defer timeout(60 * time.Second)()
+	var testdata bytes.Buffer
+	enc, err := NewWriter(&testdata, WithWindowSize(64<<10), WithSingleSegment(false))
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Write 256KB
+	for i := 0; i < 256; i++ {
+		tmp := strings.Repeat(string([]byte{byte(i)}), 1024)
+		_, err := enc.Write([]byte(tmp))
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+	err = enc.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var n = 5000
+	if testing.Short() {
+		n = 200
+	}
+
+	var before, after runtime.MemStats
+	runtime.GC()
+	runtime.ReadMemStats(&before)
+
+	var decs = make([]*Decoder, n)
+	for i := range decs {
+		// Wrap in NopCloser to avoid shortcut.
+		input := ioutil.NopCloser(bytes.NewBuffer(testdata.Bytes()))
+		decs[i], err = NewReader(input, WithDecoderConcurrency(1), WithDecoderLowmem(true))
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	// 32K buffer
+	var tmp [128 << 10]byte
+	for i := range decs {
+		_, err := io.ReadFull(decs[i], tmp[:])
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	runtime.GC()
+	runtime.ReadMemStats(&after)
+	size := (after.HeapInuse - before.HeapInuse) / uint64(n) / 1024
+	t.Log(size, "KiB per decoder")
+	// This is not exact science, but fail if we suddenly get more than 2x what we expect.
+	if size > 221*2 && !testing.Short() {
+		t.Errorf("expected < 221KB per decoder, got %d", size)
+	}
+
+	for _, dec := range decs {
+		dec.Close()
+	}
+}
+
 func TestNewDecoderGood(t *testing.T) {
 	defer timeout(30 * time.Second)()
 	testDecoderFile(t, "testdata/good.zip")

zstd/framedec.go

@@ -233,7 +233,11 @@ func (d *frameDec) reset(br byteBuffer) error {
 		return ErrWindowSizeTooSmall
 	}
 	d.history.windowSize = int(d.WindowSize)
-	d.history.maxSize = d.history.windowSize + maxBlockSize
+	if d.o.lowMem && d.history.windowSize < maxBlockSize {
+		d.history.maxSize = d.history.windowSize * 2
+	} else {
+		d.history.maxSize = d.history.windowSize + maxBlockSize
+	}
 	// history contains input - maybe we do something
 	d.rawInput = br
 	return nil
@@ -320,8 +324,8 @@ func (d *frameDec) checkCRC() error {
 
 func (d *frameDec) initAsync() {
 	if !d.o.lowMem && !d.SingleSegment {
-		// set max extra size history to 20MB.
-		d.history.maxSize = d.history.windowSize + maxBlockSize*10
+		// set max extra size history to 10MB.
+		d.history.maxSize = d.history.windowSize + maxBlockSize*5
 	}
 	// re-alloc if more than one extra block size.
 	if d.o.lowMem && cap(d.history.b) > d.history.maxSize+maxBlockSize {