optimize memory allocation during erasure-read by using temporary buffer pool. (#2259)

* XL/erasure-read: optimize memory allocation during erasure-read by using temporary buffer pool.

With the change the buffer needed during GetObject by erasureReadFile is allocated only once.

pkg/bpool/bpool.go

@@ -0,0 +1,69 @@
+/*
+ * Minio Cloud Storage, (C) 2016 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// Package bpool implements a fixed size pool of byte slices.
+package bpool
+
+import (
+	"errors"
+	"sync"
+)
+
+// ErrBpoolNoFree - Normally this error should never be returned, this error
+// indicates a bug in the package consumer.
+var ErrBpoolNoFree = errors.New("no free byte slice in pool")
+
+// BytePool - temporary pool of byte slices.
+type BytePool struct {
+	buf  [][]byte // array of byte slices
+	used []bool   // indicates if a buf[i] is in use
+	size int64    // size of buf[i]
+	mu   sync.Mutex
+}
+
+// Get - Returns an unused byte slice.
+func (b *BytePool) Get() (buf []byte, err error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	for i := 0; i < len(b.used); i++ {
+		if !b.used[i] {
+			b.used[i] = true
+			if b.buf[i] == nil {
+				b.buf[i] = make([]byte, b.size)
+			}
+			return b.buf[i], nil
+		}
+	}
+	return nil, ErrBpoolNoFree
+}
+
+// Reset - Marks all slices as unused.
+func (b *BytePool) Reset() {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	for i := 0; i < len(b.used); i++ {
+		b.used[i] = false
+	}
+}
+
+// NewBytePool - Returns new pool.
+// size - length of each slice.
+// n - number of slices in the pool.
+func NewBytePool(size int64, n int) *BytePool {
+	used := make([]bool, n)
+	buf := make([][]byte, n)
+	return &BytePool{buf, used, size, sync.Mutex{}}
+}

pkg/bpool/bpool_test.go

@@ -0,0 +1,53 @@
+/*
+ * Minio Cloud Storage, (C) 2016 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package bpool
+
+import "testing"
+
+func TestBytePool(t *testing.T) {
+	size := int64(10)
+	n := 16
+	pool := NewBytePool(size, n)
+	enBlocks := make([][]byte, n)
+
+	// Allocates all the 16 byte slices in the pool.
+	alloc := func() {
+		for i := range enBlocks {
+			var err error
+			enBlocks[i], err = pool.Get()
+			if err != nil {
+				t.Fatal("expected nil, got", err)
+			}
+			// Make sure the slice length is as expected.
+			if len(enBlocks[i]) != int(size) {
+				t.Fatalf("expected size %d, got %d", len(enBlocks[i]), size)
+			}
+		}
+	}
+
+	// Allocate everything in the pool.
+	alloc()
+	// Any Get() will fail when the pool does not have any free buffer.
+	_, err := pool.Get()
+	if err == nil {
+		t.Fatalf("expected %s, got nil", err)
+	}
+	// Reset - so that all the buffers are marked as unused.
+	pool.Reset()
+	// Allocation of all the buffers in the pool should succeed now.
+	alloc()
+}