Add LimitedBytePool for wal buffers

This pool was previously a pool.Bytes to avoid repetitive allocations.
It was recently switchted to a sync.Pool because pool.Bytes held onto
very larger buffers at times which were never released.  sync.Pool is
showing up in allocation profiles quite frequently.

This switches the pool to a new pool that limits how many buffers are
in the pool as well as the max size of each buffer in the pool.  This
provides better bounds on allocations.

pkg/pool/bytes.go

@@ -41,3 +41,60 @@ func (p *Bytes) Put(c []byte) {
 	default:
 	}
 }
+
+// LimitedBytes is a pool of byte slices that can be re-used.  Slices in
+// this pool will not be garbage collected when not in use.  The pool will
+// hold onto a fixed number of byte slices of a maximum size.  If the pool
+// is empty and max pool size has not been allocated yet, it will return a
+// new byte slice.  Byte slices added to the pool that are over the max size
+// are dropped.
+type LimitedBytes struct {
+	allocated int64
+	maxSize   int
+	pool      chan []byte
+}
+
+// NewBytes returns a Bytes pool with capacity for max byte slices
+// to be pool.
+func NewLimitedBytes(capacity int, maxSize int) *LimitedBytes {
+	return &LimitedBytes{
+		pool:    make(chan []byte, capacity),
+		maxSize: maxSize,
+	}
+}
+
+// Get returns a byte slice size with at least sz capacity. Items
+// returned may not be in the zero state and should be reset by the
+// caller.
+func (p *LimitedBytes) Get(sz int) []byte {
+	var c []byte
+
+	// If we have not allocated our capacity, return a new allocation,
+	// otherwise block until one frees up.
+	select {
+	case c = <-p.pool:
+	default:
+		return make([]byte, sz)
+	}
+
+	if cap(c) < sz {
+		return make([]byte, sz)
+	}
+
+	return c[:sz]
+}
+
+// Put returns a slice back to the pool.  If the pool is full, the byte
+// slice is discarded.  If the byte slice is over the configured max size
+// of any byte slice in the pool, it is discared.
+func (p *LimitedBytes) Put(c []byte) {
+	// Drop buffers that are larger than the max size
+	if cap(c) >= p.maxSize {
+		return
+	}
+
+	select {
+	case p.pool <- c:
+	default:
+	}
+}

pkg/pool/bytes_test.go

@@ -0,0 +1,16 @@
+package pool_test
+
+import (
+	"testing"
+
+	"github.com/influxdata/influxdb/pkg/pool"
+)
+
+func TestLimitedBytePool_Put_MaxSize(t *testing.T) {
+	bp := pool.NewLimitedBytes(1, 10)
+	bp.Put(make([]byte, 1024)) // should be dropped
+
+	if got, exp := cap(bp.Get(10)), 10; got != exp {
+		t.Fatalf("max cap size exceeded: got %v, exp %v", got, exp)
+	}
+}

tsdb/engine/tsm1/wal.go

@@ -20,6 +20,7 @@ import (
 	"github.com/golang/snappy"
 	"github.com/influxdata/influxdb/models"
 	"github.com/influxdata/influxdb/pkg/limiter"
+	"github.com/influxdata/influxdb/pkg/pool"
 	"github.com/uber-go/zap"
 )
 
@@ -66,6 +67,9 @@ var (
 	ErrWALCorrupt = fmt.Errorf("corrupted WAL entry")
 
 	defaultWaitingWALWrites = runtime.GOMAXPROCS(0) * 2
+
+	// bytePool is a shared bytes pool buffer re-cycle []byte slices to reduce allocations.
+	bytesPool = pool.NewLimitedBytes(256, walEncodeBufSize*2)
 )
 
 // Statistics gathered by the WAL.
@@ -383,20 +387,17 @@ func (l *WAL) writeToLog(entry WALEntry) (int, error) {
 	// limit how many concurrent encodings can be in flight.  Since we can only
 	// write one at a time to disk, a slow disk can cause the allocations below
 	// to increase quickly.  If we're backed up, wait until others have completed.
-	//l.limiter.Take()
-	//defer l.limiter.Release()
-
-	// encode and compress the entry while we're not locked
-	bytes := *(getBuf(walEncodeBufSize))
-	defer putBuf(&bytes)
+	bytes := bytesPool.Get(walEncodeBufSize)
+	defer bytesPool.Put(bytes)
 
 	b, err := entry.Encode(bytes)
 	if err != nil {
 		return -1, err
 	}
 
-	encBuf := *(getBuf(snappy.MaxEncodedLen(len(b))))
-	defer putBuf(&encBuf)
+	encBuf := bytesPool.Get(snappy.MaxEncodedLen(len(b)))
+	defer bytesPool.Put(encBuf)
+
 	compressed := snappy.Encode(encBuf, b)
 	syncErr := make(chan error)