bufpool: Improve perf by using buckets of different size classes

7 files changed, 182 insertions, 34 deletions

diff --git a/internal/bufpool/buffers.go b/internal/bufpool/buffers.go
index 11ea1d54..24b29fb5 100644
--- a/internal/bufpool/buffers.go
+++ b/internal/bufpool/buffers.go
@@ -22,19 +22,46 @@ const (
 // A Buffer's underlying slice may grow as needed. When finished with a
 // pooled buffer, the caller should invoke Release() to return it to the pool.
 //
-// A zero-value Buffer is not valid for use.
+// Buffers must not be copied after first use; doing so can cause double-returns
+// to the pool and data races. A zero-value Buffer is not valid for use.
+//
+//go:nocopy
 type Buffer struct {
-	buf    []byte
-	pooled bool
+	_    noCopy
+	buf  []byte
+	pool poolIndex
 }
 
-var bufPool = sync.Pool{
-	New: func() any {
-		buf := make([]byte, 0, DefaultBufferCap)
-		return &buf
-	},
+type poolIndex int8
+
+const (
+	unpooled poolIndex = -1
+)
+
+var sizeClasses = [...]int{
+	DefaultBufferCap,
+	64 << 10,
+	128 << 10,
+	256 << 10,
+	512 << 10,
+	1 << 20,
+	2 << 20,
+	4 << 20,
+	maxPooledBuffer,
 }
 
+var bufferPools = func() []sync.Pool {
+	pools := make([]sync.Pool, len(sizeClasses))
+	for i, classCap := range sizeClasses {
+		capCopy := classCap
+		pools[i].New = func() any {
+			buf := make([]byte, 0, capCopy)
+			return &buf
+		}
+	}
+	return pools
+}()
+
 // Borrow retrieves a Buffer suitable for storing up to capHint bytes.
 // The returned Buffer may come from an internal sync.Pool.
 //
@@ -47,21 +74,24 @@ func Borrow(capHint int) Buffer {
 	if capHint < DefaultBufferCap {
 		capHint = DefaultBufferCap
 	}
-	buf := bufPool.Get().(*[]byte)
-	if cap(*buf) < capHint {
-		bufPool.Put(buf)
+	classIdx, classCap, pooled := classFor(capHint)
+	if !pooled {
 		newBuf := make([]byte, 0, capHint)
-		return Buffer{buf: newBuf, pooled: false}
+		return Buffer{buf: newBuf, pool: unpooled}
+	}
+	buf := bufferPools[classIdx].Get().(*[]byte)
+	if cap(*buf) < classCap {
+		*buf = make([]byte, 0, classCap)
 	}
 	slice := (*buf)[:0]
-	return Buffer{buf: slice, pooled: true}
+	return Buffer{buf: slice, pool: poolIndex(classIdx)}
 }
 
 // FromOwned constructs a Buffer from a caller-owned byte slice. The resulting
 // Buffer does not participate in pooling and will never be returned to the
 // internal pool when released.
 func FromOwned(buf []byte) Buffer {
-	return Buffer{buf: buf, pooled: false}
+	return Buffer{buf: buf, pool: unpooled}
 }
 
 // Resize adjusts the length of the buffer to n bytes. If n exceeds the current
@@ -107,12 +137,9 @@ func (buf *Buffer) Release() {
 	if buf.buf == nil {
 		return
 	}
-	if buf.pooled && cap(buf.buf) <= maxPooledBuffer {
-		tmp := buf.buf[:0]
-		bufPool.Put(&tmp)
-	}
+	buf.returnToPool()
 	buf.buf = nil
-	buf.pooled = false
+	buf.pool = unpooled
 }
 
 // ensureCapacity grows the underlying buffer to accommodate the requested
@@ -123,18 +150,45 @@ func (buf *Buffer) ensureCapacity(needed int) {
 	if cap(buf.buf) >= needed {
 		return
 	}
-	old := buf.buf
-	wasPooled := buf.pooled
-	newCap := cap(buf.buf) * 2
-	if newCap < needed {
-		newCap = needed
+	classIdx, classCap, pooled := classFor(needed)
+	var newBuf []byte
+	if pooled {
+		raw := bufferPools[classIdx].Get().(*[]byte)
+		if cap(*raw) < classCap {
+			*raw = make([]byte, 0, classCap)
+		}
+		newBuf = (*raw)[:len(buf.buf)]
+	} else {
+		newBuf = make([]byte, len(buf.buf), classCap)
 	}
-	newBuf := make([]byte, len(buf.buf), newCap)
 	copy(newBuf, buf.buf)
+	buf.returnToPool()
 	buf.buf = newBuf
-	buf.pooled = false
-	if wasPooled && cap(old) <= maxPooledBuffer {
-		tmp := old[:0]
-		bufPool.Put(&tmp)
+	if pooled {
+		buf.pool = poolIndex(classIdx)
+	} else {
+		buf.pool = unpooled
 	}
 }
+
+func classFor(size int) (idx int, classCap int, ok bool) {
+	for i, class := range sizeClasses {
+		if size <= class {
+			return i, class, true
+		}
+	}
+	return -1, size, false
+}
+
+func (buf *Buffer) returnToPool() {
+	if buf.pool == unpooled {
+		return
+	}
+	tmp := buf.buf[:0]
+	bufferPools[int(buf.pool)].Put(&tmp)
+}
+
+type noCopy struct{}
+
+func (*noCopy) Lock()   {}
+func (*noCopy) Unlock() {}
diff --git a/internal/bufpool/buffers_test.go b/internal/bufpool/buffers_test.go
index 87dbdd73..f5c006da 100644
--- a/internal/bufpool/buffers_test.go
+++ b/internal/bufpool/buffers_test.go
@@ -38,3 +38,40 @@ func TestBorrowBufferRelease(t *testing.T) {
 		t.Fatal("expected buffer cleared after release")
 	}
 }
+
+func TestBorrowUsesLargerPools(t *testing.T) {
+	const request = DefaultBufferCap * 4
+
+	classIdx, classCap, pooled := classFor(request)
+	if !pooled {
+		t.Fatalf("expected %d to map to a pooled class", request)
+	}
+
+	b := Borrow(request)
+	if b.pool != poolIndex(classIdx) {
+		t.Fatalf("expected pooled buffer in class %d, got %d", classIdx, b.pool)
+	}
+	if cap(b.buf) != classCap {
+		t.Fatalf("expected capacity %d, got %d", classCap, cap(b.buf))
+	}
+	b.Release()
+
+	b2 := Borrow(request)
+	defer b2.Release()
+	if b2.pool != poolIndex(classIdx) {
+		t.Fatalf("expected pooled buffer in class %d on reuse, got %d", classIdx, b2.pool)
+	}
+	if cap(b2.buf) != classCap {
+		t.Fatalf("expected capacity %d on reuse, got %d", classCap, cap(b2.buf))
+	}
+}
+
+func TestGrowingBufferStaysPooled(t *testing.T) {
+	b := Borrow(DefaultBufferCap)
+	defer b.Release()
+
+	b.Append(make([]byte, DefaultBufferCap*3))
+	if b.pool == unpooled {
+		t.Fatal("buffer should stay pooled after growth within limit")
+	}
+}
diff --git a/internal/flatex/decompress_bytes.go b/internal/flatex/decompress_bytes.go
index 5a660b0a..5e29f82d 100644
--- a/internal/flatex/decompress_bytes.go
+++ b/internal/flatex/decompress_bytes.go
@@ -26,13 +26,20 @@ var bufferDecompressorPool = sync.Pool{
 // Decompress inflates the provided DEFLATE stream and returns the full output
 // in a pooled bufpool.Buffer along with the number of consumed bytes from src.
 func Decompress(src []byte) (bufpool.Buffer, int, error) {
-	return DecompressDict(src, nil)
+	return DecompressDictSized(src, nil, 0)
 }
 
 // DecompressDict inflates the provided DEFLATE stream using dict as the preset
 // dictionary and returns the full output in a pooled bufpool.Buffer. The second
 // returned value reports how many bytes of src were consumed.
 func DecompressDict(src []byte, dict []byte) (bufpool.Buffer, int, error) {
+	return DecompressDictSized(src, dict, 0)
+}
+
+// DecompressDictSized is like DecompressDict but allows providing an expected
+// output size to pre-size the destination buffer and avoid repeated growth.
+// A non-positive sizeHint falls back to the default buffer capacity.
+func DecompressDictSized(src []byte, dict []byte, sizeHint int) (bufpool.Buffer, int, error) {
 	d := bufferDecompressorPool.Get().(*bufferDecompressor)
 	defer bufferDecompressorPool.Put(d)
 
@@ -40,7 +47,7 @@ func DecompressDict(src []byte, dict []byte) (bufpool.Buffer, int, error) {
 		return bufpool.Buffer{}, 0, err
 	}
 
-	out := bufpool.Borrow(bufpool.DefaultBufferCap)
+	out := bufpool.Borrow(sizeHint)
 	out.Resize(0)
 
 	for {
diff --git a/internal/flatex/decompress_test.go b/internal/flatex/decompress_test.go
index afb84292..7c290555 100644
--- a/internal/flatex/decompress_test.go
+++ b/internal/flatex/decompress_test.go
@@ -74,3 +74,22 @@ func TestDecompressDictMissing(t *testing.T) {
 		t.Fatalf("expected error when dictionary missing")
 	}
 }
+
+func TestDecompressDictSizedUsesHint(t *testing.T) {
+	payload := []byte("short")
+	compressed := compressDeflate(t, payload, nil)
+
+	const hint = 1 << 19
+	out, _, err := DecompressDictSized(compressed, nil, hint)
+	if err != nil {
+		t.Fatalf("DecompressDictSized: %v", err)
+	}
+	defer out.Release()
+
+	if !bytes.Equal(out.Bytes(), payload) {
+		t.Fatalf("unexpected payload: got %q", out.Bytes())
+	}
+	if cap(out.Bytes()) < hint {
+		t.Fatalf("expected capacity >= %d, got %d", hint, cap(out.Bytes()))
+	}
+}
diff --git a/internal/zlibx/decompress.go b/internal/zlibx/decompress.go
index 34e62c6f..c6eb65e5 100644
--- a/internal/zlibx/decompress.go
+++ b/internal/zlibx/decompress.go
@@ -12,13 +12,25 @@ import (
 // Decompress inflates the provided zlib wrapped stream and returns the
 // uncompressed data inside a pooled bufpool.Buffer.
 func Decompress(src []byte) (bufpool.Buffer, error) {
-	return DecompressDict(src, nil)
+	return DecompressSized(src, 0)
+}
+
+// DecompressSized inflates the provided zlib stream, using sizeHint to
+// preallocate the output buffer when known (e.g. packfile entries).
+func DecompressSized(src []byte, sizeHint int) (bufpool.Buffer, error) {
+	return DecompressDictSized(src, nil, sizeHint)
 }
 
 // DecompressDict is like Decompress but accepts a preset dictionary. The
 // dictionary must match the checksum embedded in the stream if the dictionary
 // flag is present.
 func DecompressDict(src []byte, dict []byte) (bufpool.Buffer, error) {
+	return DecompressDictSized(src, dict, 0)
+}
+
+// DecompressDictSized is like DecompressDict but allows providing an expected
+// uncompressed size to avoid buffer growth copies.
+func DecompressDictSized(src []byte, dict []byte, sizeHint int) (bufpool.Buffer, error) {
 	if len(src) < 6 {
 		return bufpool.Buffer{}, io.ErrUnexpectedEOF
 	}
@@ -50,7 +62,7 @@ func DecompressDict(src []byte, dict []byte) (bufpool.Buffer, error) {
 	}
 
 	deflateData := src[offset:]
-	out, consumed, err := flatex.DecompressDict(deflateData, dict)
+	out, consumed, err := flatex.DecompressDictSized(deflateData, dict, sizeHint)
 	if err != nil {
 		return bufpool.Buffer{}, err
 	}
diff --git a/internal/zlibx/decompress_test.go b/internal/zlibx/decompress_test.go
index a4e9c608..3dfc07a5 100644
--- a/internal/zlibx/decompress_test.go
+++ b/internal/zlibx/decompress_test.go
@@ -82,3 +82,22 @@ func TestDecompressChecksumError(t *testing.T) {
 		t.Fatalf("expected ErrChecksum, got %v", err)
 	}
 }
+
+func TestDecompressSizedUsesHint(t *testing.T) {
+	payload := []byte("tiny payload")
+	compressed := compressZlib(t, payload, nil)
+
+	const hint = 1 << 20
+	out, err := DecompressSized(compressed, hint)
+	if err != nil {
+		t.Fatalf("DecompressSized: %v", err)
+	}
+	defer out.Release()
+
+	if !bytes.Equal(out.Bytes(), payload) {
+		t.Fatalf("unexpected payload %q", out.Bytes())
+	}
+	if cap(out.Bytes()) < hint {
+		t.Fatalf("expected capacity >= %d, got %d", hint, cap(out.Bytes()))
+	}
+}
diff --git a/pack_pack.go b/pack_pack.go
index 1cd6b35b..6dcfb5d9 100644
--- a/pack_pack.go
+++ b/pack_pack.go
@@ -104,7 +104,7 @@ func packSectionInflate(pf *packFile, start uint64, sizeHint int) (bufpool.Buffe
 	if start > uint64(len(pf.data)) {
 		return bufpool.Buffer{}, ErrInvalidObject
 	}
-	body, err := zlibx.Decompress(pf.data[start:])
+	body, err := zlibx.DecompressSized(pf.data[start:], sizeHint)
 	if err != nil {
 		return bufpool.Buffer{}, err
 	}