Scala中类型类的性能

Question

我正在尝试分析在Scala中使用类型类的性能成本,因为我注意到当它们被广泛使用时,性能往往会下降.我们来看一个ByteCodec类型类:

trait ByteCodec[T] {
  def put(index: Int, byteBuffer: ByteBuffer, t: T): Unit
  def get(index: Int, byteBuffer: ByteBuffer): T
}

让我们做一个Long实例:

object ByteCodec {
  def apply[T](implicit bc: ByteCodec[T]): ByteCodec[T] = bc

  implicit val longBC = new ByteCodec[Long] {
    @inline override def put(index: Int, byteBuffer: ByteBuffer, long: Long): Unit = {
      val _ = byteBuffer.putLong(index, long)
    }

    @inline override def get(index: Int, byteBuffer: ByteBuffer): Long =
      byteBuffer.getLong(index)
  }
}

如果我运行1亿次获取和放置,则类型类测试需要大约1200ms,否则大约需要800ms.开销在哪里,我可以摆脱它吗？

主要代码:

object Main extends App {
  val cycles = 100000000
  val byteBuffer = ByteBuffer.allocate(java.lang.Long.BYTES)
  var start = System.currentTimeMillis()
  var currCycle = cycles
  while (currCycle > 0) {
    byteBuffer.putLong(0, 10L)
    val k = byteBuffer.getLong(0)
    currCycle -= 1
  }
  var end = System.currentTimeMillis()
  println(s"time elapsed byteBuffer ${ end - start }")

  val codec = ByteCodec[Long]
  start = System.currentTimeMillis()
  currCycle = cycles
  while (currCycle > 0) {
    codec.put(0, byteBuffer, 10L)
    val k = codec.get(0, byteBuffer)
    currCycle -= 1
  }
  end = System.currentTimeMillis()
  println(s"time elapsed ByteCodec ${ end - start }")
}

Answer 1

Aleksey在评论中已经提到了您的测试不准确的一个原因.

除此之外,你的类型类比较慢的主要原因与方法本身无关:它是Longs 的装箱/拆箱使它变慢.您可以使用@specialized注释专门化值类的类型类 :

trait ByteCodec[@specialized(Long) T]

如果您查看签名ByteBuffer,则使用值类型,不涉及装箱/取消装箱:

 public abstract ByteBuffer putLong(int index, long value);
 public abstract long getLong(int index);