Set level 15 as default compression level in ZSTD (#1111)

The default level is 5. This PR sets the compression level to 15 which 
gives the best speed vs compression ratio trade-off.

options.go

@@ -72,9 +72,11 @@ type Options struct {
 	ValueLogFileSize   int64
 	ValueLogMaxEntries uint32
 
-	NumCompactors     int
-	CompactL0OnClose  bool
-	LogRotatesToFlush int32
+	NumCompactors        int
+	CompactL0OnClose     bool
+	LogRotatesToFlush    int32
+	ZSTDCompressionLevel int
+
 	// When set, checksum will be validated for each entry read from the value log file.
 	VerifyValueChecksum bool
 
@@ -128,6 +130,14 @@ func DefaultOptions(path string) Options {
 		VerifyValueChecksum:     false,
 		Compression:             defaultCompression,
 		MaxCacheSize:            1 << 30, // 1 GB
+		// Benchmarking compression level against performance showed that level 15 gives
+		// the best speed vs ratio tradeoff.
+		// For a data size of 4KB we get
+		// Level: 3  Ratio: 2.72 Time:  24112 n/s
+		// Level: 10 Ratio: 2.95 Time:  75655 n/s
+		// Level: 15 Ratio: 4.38 Time: 239042 n/s
+		// See https://github.com/dgraph-io/badger/pull/1111#issue-338120757
+		ZSTDCompressionLevel: 15,
 		// Nothing to read/write value log using standard File I/O
 		// MemoryMap to mmap() the value log files
 		// (2^30 - 1)*2 when mmapping < 2^31 - 1, max int32.
@@ -147,11 +157,12 @@ func DefaultOptions(path string) Options {
 
 func buildTableOptions(opt Options) table.Options {
 	return table.Options{
-		BlockSize:          opt.BlockSize,
-		BloomFalsePositive: opt.BloomFalsePositive,
-		LoadingMode:        opt.TableLoadingMode,
-		ChkMode:            opt.ChecksumVerificationMode,
-		Compression:        opt.Compression,
+		BlockSize:            opt.BlockSize,
+		BloomFalsePositive:   opt.BloomFalsePositive,
+		LoadingMode:          opt.TableLoadingMode,
+		ChkMode:              opt.ChecksumVerificationMode,
+		Compression:          opt.Compression,
+		ZSTDCompressionLevel: opt.ZSTDCompressionLevel,
 	}
 }
 
@@ -532,3 +543,15 @@ func (opt Options) WithMaxCacheSize(size int64) Options {
 	opt.MaxCacheSize = size
 	return opt
 }
+
+// WithZSTDCompressionLevel returns a new Options value with ZSTDCompressionLevel set
+// to the given value.
+//
+// The ZSTD compression algorithm supports 20 compression levels. The higher the compression
+// level, the better is the compression ratio but lower is the performance. Lower levels
+// have better performance and higher levels have better compression ratios.
+// The default value of ZSTDCompressionLevel is 15.
+func (opt Options) WithZSTDCompressionLevel(cLevel int) Options {
+	opt.ZSTDCompressionLevel = cLevel
+	return opt
+}

table/builder.go

@@ -345,7 +345,7 @@ func (b *Builder) compressData(data []byte) ([]byte, error) {
 	case options.Snappy:
 		return snappy.Encode(nil, data), nil
 	case options.ZSTD:
-		return y.ZSTDCompress(nil, data)
+		return y.ZSTDCompress(nil, data, b.opt.ZSTDCompressionLevel)
 	}
 	return nil, errors.New("Unsupported compression type")
 }

table/table.go

@@ -69,6 +69,9 @@ type Options struct {
 	Compression options.CompressionType
 
 	Cache *ristretto.Cache
+
+	// ZSTDCompressionLevel is the ZSTD compression level used for compressing blocks.
+	ZSTDCompressionLevel int
 }
 
 // TableInterface is useful for testing.

table/table_test.go

@@ -46,10 +46,11 @@ func key(prefix string, i int) string {
 
 func getTestTableOptions() Options {
 	return Options{
-		Compression:        options.ZSTD,
-		LoadingMode:        options.LoadToRAM,
-		BlockSize:          4 * 1024,
-		BloomFalsePositive: 0.01,
+		Compression:          options.ZSTD,
+		ZSTDCompressionLevel: 15,
+		LoadingMode:          options.LoadToRAM,
+		BlockSize:            4 * 1024,
+		BloomFalsePositive:   0.01,
 	}
 
 }

y/zstd_cgo.go

@@ -31,6 +31,6 @@ func ZSTDDecompress(dst, src []byte) ([]byte, error) {
 }
 
 // ZSTDCompress compresses a block using ZSTD algorithm.
-func ZSTDCompress(dst, src []byte) ([]byte, error) {
-	return zstd.Compress(dst, src)
+func ZSTDCompress(dst, src []byte, compressionLevel int) ([]byte, error) {
+	return zstd.CompressLevel(dst, src, compressionLevel)
 }

y/zstd_nocgo.go

@@ -33,6 +33,6 @@ func ZSTDDecompress(dst, src []byte) ([]byte, error) {
 }
 
 // ZSTDCompress compresses a block using ZSTD algorithm.
-func ZSTDCompress(dst, src []byte) ([]byte, error) {
+func ZSTDCompress(dst, src []byte, compressionLevel int) ([]byte, error) {
 	return nil, errZstdCgo
 }