Optimize delta generation (in speed and size)

daemon/images/image_delta.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"io"
 	"io/ioutil"
+	"math"
 	"os"
 	"time"
 
@@ -64,8 +65,9 @@ func (i *ImageService) DeltaCreate(deltaSrc, deltaDest string, options types.Ima
 	progressReader := progress.NewProgressReader(srcData, progressOutput, srcDataLen, deltaSrc, "Fingerprinting")
 	defer progressReader.Close()
 
+	blockLen := deltaBlockSize(srcDataLen)
 	sigStart := time.Now()
-	srcSig, err := librsync.Signature(bufio.NewReaderSize(progressReader, 65536), ioutil.Discard, 512, 32, librsync.BLAKE2_SIG_MAGIC)
+	srcSig, err := librsync.Signature(bufio.NewReaderSize(progressReader, 65536), ioutil.Discard, blockLen, 32, librsync.BLAKE2_SIG_MAGIC)
 	if err != nil {
 		return err
 	}
@@ -285,3 +287,30 @@ func (lock *imglock) unlock(ls layer.Store) {
 		layer.ReleaseAndLog(ls, l)
 	}
 }
+
+// deltaBlockSize returns the block size to use when generating a delta for a
+// basis file that is basisSize bytes long.
+func deltaBlockSize(basisSize int64) uint32 {
+	// Start with the "ideal" size recommended by the librsync devs. See
+	// https://github.com/librsync/librsync/pull/109/files#diff-7a3cd9075c1eaa0d219f7c0a516a10679dae11922bd1dd0ff54a10cdaa457f6fR147
+	if basisSize <= 256*256 {
+		return 256
+	}
+	x := uint32(math.Sqrt(float64(basisSize)))
+
+	// Avoid overflowing.
+	if x >= 2147483648 {
+		return 2147483648 // the largest power of two that fits into an uint32.
+	}
+
+	// Round to the next power of two (because librsync-go has an optimized code
+	// path for power of two blocks). This algorithm is from Hacker's Delight,
+	// 2nd Edition, p.62.
+	x -= 1
+	x |= x >> 1
+	x |= x >> 2
+	x |= x >> 4
+	x |= x >> 8
+	x |= x >> 16
+	return x + 1
+}

daemon/images/image_delta_test.go

@@ -0,0 +1,39 @@
+package images
+
+import (
+	"fmt"
+	"math"
+	"testing"
+)
+
+func Test_deltaBlockSize(t *testing.T) {
+	tests := []struct {
+		x    int64
+		want uint32
+	}{
+		{0, 256},
+		{1, 256},
+		{100, 256},
+		{1_024, 256},
+		{33_333, 256},
+		{65_536, 256},
+		{88_887, 512},
+		{262_144, 512},
+		{262_145, 512},
+		{777_111, 1024},
+		{22_654_123, 8192},
+		{1_333_555_888, 65536},
+		{35_000_000_000, 262144},
+		{123_456_678_901, 524288},
+		{4_611_686_018_427_387_904, 2147483648},
+		{5_000_000_000_000_000_000, 2147483648},
+		{math.MaxInt64, 2147483648},
+	}
+	for _, tt := range tests {
+		t.Run(fmt.Sprintf("deltaBlockSize(%v)", tt.x), func(t *testing.T) {
+			if got := deltaBlockSize(tt.x); got != tt.want {
+				t.Errorf("got deltaBlockSize(%v) = %v, want %v", tt.x, got, tt.want)
+			}
+		})
+	}
+}