s2: Add concurrent stream decompression (#602)

Add concurrent stream decompression of snappy/s2 streams

Fixes #397

For full stream decompression S2 offers a `DecodeConcurrent` function that will decode a full stream using multiple goroutines.

Example scaling, AMD Ryzen 3950X, 16 cores, decompression using `s2d -bench=3 <input>`, best of 3: 

| Input                                     | `-cpu=1`   | `-cpu=2`   | `-cpu=4`   | `-cpu=8`   | `-cpu=16`   |
|-------------------------------------------|------------|------------|------------|------------|-------------|
| enwik10.snappy                            | 1098.6MB/s | 1819.8MB/s | 3625.6MB/s | 6910.6MB/s | 10818.2MB/s |
| enwik10.s2                                | 1303.5MB/s | 2606.1MB/s | 4847.9MB/s | 8878.4MB/s | 9592.1MB/s  |
| sofia-air-quality-dataset.tar.snappy      | 1302.0MB/s | 2165.0MB/s | 4244.5MB/s | 8241.0MB/s | 12920.5MB/s |
| sofia-air-quality-dataset.tar.s2          | 1399.2MB/s | 2463.2MB/s | 5196.5MB/s | 9639.8MB/s | 11439.5MB/s |
| sofia-air-quality-dataset.tar.s2 (no asm) | 837.5MB/s  | 1652.6MB/s | 3183.6MB/s | 5945.0MB/s | 9620.7MB/s  |

Scaling can be expected to be pretty linear until memory bandwidth is saturated. 

For now the `DecodeConcurrent` can only be used for full streams without seeking or combining with regular reads.

s2/README.md

@@ -19,6 +19,7 @@ This is important, so you don't have to worry about spending CPU cycles on alrea
 * Adjustable compression (3 levels) 
 * Concurrent stream compression
 * Faster decompression, even for Snappy compatible content
+* Concurrent Snappy/S2 stream decompression
 * Ability to quickly skip forward in compressed stream
 * Random seeking with indexes
 * Compatible with reading Snappy compressed content
@@ -415,6 +416,25 @@ Without assembly decompression is also very fast; single goroutine decompression
 
 Even though S2 typically compresses better than Snappy, decompression speed is always better. 
 
+### Concurrent Stream Decompression
+
+For full stream decompression S2 offers a [DecodeConcurrent](https://pkg.go.dev/github.com/klauspost/compress/s2#Reader.DecodeConcurrent) 
+that will decode a full stream using multiple goroutines.
+
+Example scaling, AMD Ryzen 3950X, 16 cores, decompression using `s2d -bench=3 <input>`, best of 3: 
+
+| Input                                     | `-cpu=1`   | `-cpu=2`   | `-cpu=4`   | `-cpu=8`   | `-cpu=16`   |
+|-------------------------------------------|------------|------------|------------|------------|-------------|
+| enwik10.snappy                            | 1098.6MB/s | 1819.8MB/s | 3625.6MB/s | 6910.6MB/s | 10818.2MB/s |
+| enwik10.s2                                | 1303.5MB/s | 2606.1MB/s | 4847.9MB/s | 8878.4MB/s | 9592.1MB/s  |
+| sofia-air-quality-dataset.tar.snappy      | 1302.0MB/s | 2165.0MB/s | 4244.5MB/s | 8241.0MB/s | 12920.5MB/s |
+| sofia-air-quality-dataset.tar.s2          | 1399.2MB/s | 2463.2MB/s | 5196.5MB/s | 9639.8MB/s | 11439.5MB/s |
+| sofia-air-quality-dataset.tar.s2 (no asm) | 837.5MB/s  | 1652.6MB/s | 3183.6MB/s | 5945.0MB/s | 9620.7MB/s  |
+
+Scaling can be expected to be pretty linear until memory bandwidth is saturated. 
+
+For now the DecodeConcurrent can only be used for full streams without seeking or combining with regular reads.
+
 ## Block compression
 
 

s2/cmd/s2d/main.go

@@ -10,6 +10,7 @@ import (
 	"io/ioutil"
 	"net/http"
 	"os"
+	"runtime"
 	"runtime/debug"
 	"strconv"
 	"strings"
@@ -34,6 +35,7 @@ var (
 	help   = flag.Bool("help", false, "Display help")
 	out    = flag.String("o", "", "Write output to another file. Single input file only")
 	block  = flag.Bool("block", false, "Decompress as a single block. Will load content into memory.")
+	cpu    = flag.Int("cpu", runtime.NumCPU(), "Decompress streams using this amount of threads")
 
 	version = "(dev)"
 	date    = "(unknown)"
@@ -160,7 +162,11 @@ Options:`)
 						output = int64(len(dec))
 					} else {
 						r.Reset(bytes.NewBuffer(b))
-						output, err = io.Copy(ioutil.Discard, r)
+						if *cpu > 1 {
+							output, err = r.DecodeConcurrent(ioutil.Discard, *cpu)
+						} else {
+							output, err = io.Copy(ioutil.Discard, r)
+						}
 						exitErr(err)
 					}
 					if !*quiet {
@@ -286,7 +292,13 @@ Options:`)
 				}
 				decoded = r
 			}
-			output, err := io.Copy(out, decoded)
+			var err error
+			var output int64
+			if dec, ok := decoded.(*s2.Reader); ok && tailBytes == 0 && offset == 0 {
+				output, err = dec.DecodeConcurrent(out, *cpu)
+			} else {
+				output, err = io.Copy(out, decoded)
+			}
 			exitErr(err)
 			if !*quiet {
 				elapsed := time.Since(start)

s2/decode.go

@@ -11,6 +11,8 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"runtime"
+	"sync"
 )
 
 var (
@@ -196,13 +198,13 @@ type Reader struct {
 // ensureBufferSize will ensure that the buffer can take at least n bytes.
 // If false is returned the buffer exceeds maximum allowed size.
 func (r *Reader) ensureBufferSize(n int) bool {
-	if len(r.buf) >= n {
-		return true
-	}
 	if n > r.maxBufSize {
 		r.err = ErrCorrupt
 		return false
 	}
+	if cap(r.buf) >= n {
+		return true
+	}
 	// Realloc buffer.
 	r.buf = make([]byte, n)
 	return true
@@ -220,6 +222,7 @@ func (r *Reader) Reset(reader io.Reader) {
 	r.err = nil
 	r.i = 0
 	r.j = 0
+	r.blockStart = 0
 	r.readHeader = r.ignoreStreamID
 }
 
@@ -435,6 +438,259 @@ func (r *Reader) Read(p []byte) (int, error) {
 	}
 }
 
+// DecodeConcurrent will decode the full stream to w.
+// This function should not be combined with reading, seeking or other operations.
+// Up to 'concurrent' goroutines will be used.
+// If <= 0, runtime.NumCPU will be used.
+// On success the number of bytes decompressed nil and is returned.
+// This is mainly intended for bigger streams.
+func (r *Reader) DecodeConcurrent(w io.Writer, concurrent int) (written int64, err error) {
+	if r.i > 0 || r.j > 0 || r.blockStart > 0 {
+		return 0, errors.New("DecodeConcurrent called after ")
+	}
+	if concurrent <= 0 {
+		concurrent = runtime.NumCPU()
+	}
+
+	// Write to output
+	var errMu sync.Mutex
+	var aErr error
+	setErr := func(e error) (ok bool) {
+		errMu.Lock()
+		defer errMu.Unlock()
+		if e == nil {
+			return aErr == nil
+		}
+		if aErr == nil {
+			aErr = e
+		}
+		return false
+	}
+	hasErr := func() (ok bool) {
+		errMu.Lock()
+		v := aErr != nil
+		errMu.Unlock()
+		return v
+	}
+
+	var aWritten int64
+	toRead := make(chan []byte, concurrent)
+	writtenBlocks := make(chan []byte, concurrent)
+	queue := make(chan chan []byte, concurrent)
+	reUse := make(chan chan []byte, concurrent)
+	for i := 0; i < concurrent; i++ {
+		toRead <- make([]byte, 0, r.maxBufSize)
+		writtenBlocks <- make([]byte, 0, r.maxBufSize)
+		reUse <- make(chan []byte, 1)
+	}
+	// Writer
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for toWrite := range queue {
+			entry := <-toWrite
+			reUse <- toWrite
+			if hasErr() {
+				writtenBlocks <- entry
+				continue
+			}
+			n, err := w.Write(entry)
+			want := len(entry)
+			writtenBlocks <- entry
+			if err != nil {
+				setErr(err)
+				continue
+			}
+			if n != want {
+				setErr(io.ErrShortWrite)
+				continue
+			}
+			aWritten += int64(n)
+		}
+	}()
+
+	// Reader
+	defer func() {
+		close(queue)
+		if r.err != nil {
+			err = r.err
+			setErr(r.err)
+		}
+		wg.Wait()
+		if err == nil {
+			err = aErr
+		}
+		written = aWritten
+	}()
+
+	for !hasErr() {
+		if !r.readFull(r.buf[:4], true) {
+			if r.err == io.EOF {
+				r.err = nil
+			}
+			return 0, r.err
+		}
+		chunkType := r.buf[0]
+		if !r.readHeader {
+			if chunkType != chunkTypeStreamIdentifier {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			r.readHeader = true
+		}
+		chunkLen := int(r.buf[1]) | int(r.buf[2])<<8 | int(r.buf[3])<<16
+
+		// The chunk types are specified at
+		// https://github.com/google/snappy/blob/master/framing_format.txt
+		switch chunkType {
+		case chunkTypeCompressedData:
+			r.blockStart += int64(r.j)
+			// Section 4.2. Compressed data (chunk type 0x00).
+			if chunkLen < checksumSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if chunkLen > r.maxBufSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			orgBuf := <-toRead
+			buf := orgBuf[:chunkLen]
+
+			if !r.readFull(buf, false) {
+				return 0, r.err
+			}
+
+			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+			buf = buf[checksumSize:]
+
+			n, err := DecodedLen(buf)
+			if err != nil {
+				r.err = err
+				return 0, r.err
+			}
+			if r.snappyFrame && n > maxSnappyBlockSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+
+			if n > r.maxBlock {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			wg.Add(1)
+
+			decoded := <-writtenBlocks
+			entry := <-reUse
+			queue <- entry
+			go func() {
+				defer wg.Done()
+				decoded = decoded[:n]
+				_, err := Decode(decoded, buf)
+				toRead <- orgBuf
+				if err != nil {
+					writtenBlocks <- decoded
+					setErr(err)
+					return
+				}
+				if crc(decoded) != checksum {
+					writtenBlocks <- decoded
+					setErr(ErrCRC)
+					return
+				}
+				entry <- decoded
+			}()
+			continue
+
+		case chunkTypeUncompressedData:
+
+			// Section 4.3. Uncompressed data (chunk type 0x01).
+			if chunkLen < checksumSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if chunkLen > r.maxBufSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			// Grab write buffer
+			orgBuf := <-writtenBlocks
+			buf := orgBuf[:checksumSize]
+			if !r.readFull(buf, false) {
+				return 0, r.err
+			}
+			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+			// Read content.
+			n := chunkLen - checksumSize
+
+			if r.snappyFrame && n > maxSnappyBlockSize {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if n > r.maxBlock {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			// Read uncompressed
+			buf = orgBuf[:n]
+			if !r.readFull(buf, false) {
+				return 0, r.err
+			}
+
+			if crc(buf) != checksum {
+				r.err = ErrCRC
+				return 0, r.err
+			}
+			entry := <-reUse
+			queue <- entry
+			entry <- buf
+			continue
+
+		case chunkTypeStreamIdentifier:
+			// Section 4.1. Stream identifier (chunk type 0xff).
+			if chunkLen != len(magicBody) {
+				r.err = ErrCorrupt
+				return 0, r.err
+			}
+			if !r.readFull(r.buf[:len(magicBody)], false) {
+				return 0, r.err
+			}
+			if string(r.buf[:len(magicBody)]) != magicBody {
+				if string(r.buf[:len(magicBody)]) != magicBodySnappy {
+					r.err = ErrCorrupt
+					return 0, r.err
+				} else {
+					r.snappyFrame = true
+				}
+			} else {
+				r.snappyFrame = false
+			}
+			continue
+		}
+
+		if chunkType <= 0x7f {
+			// Section 4.5. Reserved unskippable chunks (chunk types 0x02-0x7f).
+			// fmt.Printf("ERR chunktype: 0x%x\n", chunkType)
+			r.err = ErrUnsupported
+			return 0, r.err
+		}
+		// Section 4.4 Padding (chunk type 0xfe).
+		// Section 4.6. Reserved skippable chunks (chunk types 0x80-0xfd).
+		if chunkLen > maxChunkSize {
+			// fmt.Printf("ERR chunkLen: 0x%x\n", chunkLen)
+			r.err = ErrUnsupported
+			return 0, r.err
+		}
+
+		// fmt.Printf("skippable: ID: 0x%x, len: 0x%x\n", chunkType, chunkLen)
+		if !r.skippable(r.buf, chunkLen, false, chunkType) {
+			return 0, r.err
+		}
+	}
+	return 0, r.err
+}
+
 // Skip will skip n bytes forward in the decompressed output.
 // For larger skips this consumes less CPU and is faster than reading output and discarding it.
 // CRC is not checked on skipped blocks.