Codegen,动态字节码技术,那么什么是动态字节码技术呢?先看来一段代码,假设SparkPlan为Sort
case class Sort(
sortOrder: Seq[SortOrder],
global: Boolean,
child: SparkPlan)
extends UnaryNode {
override def requiredChildDistribution: Seq[Distribution] =
if (global) OrderedDistribution(sortOrder) :: Nil else UnspecifiedDistribution :: Nil
protected override def doExecute(): RDD[Row] = attachTree(this, "sort") {
child.execute().mapPartitions( { iterator =>
val ordering = newOrdering(sortOrder, child.output)
iterator.map(_.copy()).toArray.sorted(ordering).iterator
}, preservesPartitioning = true)
}
override def output: Seq[Attribute] = child.output
override def outputOrdering: Seq[SortOrder] = sortOrder
}
abstract class SparkPlan extends QueryPlan[SparkPlan] with Logging with Serializable {
protected def newOrdering(order: Seq[SortOrder], inputSchema: Seq[Attribute]): Ordering[Row] = {
if (codegenEnabled) {//开启动态字节码技术
GenerateOrdering.generate(order, inputSchema)
} else {//否则关闭
new RowOrdering(order, inputSchema)
}
}
}
可见针对Sort的SparkPlan,针对是否开启动态字节码技术的情况下会发生两种情况:当关闭的时候,其Compare函数如下:
class RowOrdering(ordering: Seq[SortOrder]) extends Ordering[Row] {
def this(ordering: Seq[SortOrder], inputSchema: Seq[Attribute]) =
this(ordering.map(BindReferences.bindReference(_, inputSchema)))
def compare(a: Row, b: Row): Int = {
var i = 0
while (i < ordering.size) {
val order = ordering(i)
val left = order.child.eval(a)//虚函数调用,然后装箱
val right = order.child.eval(b)//虚函数调用,然后装箱
if (left == null && right == null) {
// Both null, continue looking.
} else if (left == null) {
return if (order.direction == Ascending) -1 else 1
} else if (right == null) {
return if (order.direction == Ascending) 1 else -1
} else {
val comparison = order.dataType match {
case n: AtomicType if order.direction == Ascending =>
n.ordering.asInstanceOf[Ordering[Any]].compare(left, right)//调用具体对象的compare函数
case n: AtomicType if order.direction == Descending =>
n.ordering.asInstanceOf[Ordering[Any]].reverse.compare(left, right)//调用具体对象的compare函数
case other => sys.error(s"Type $other does not support ordered operations")
}
if (comparison != 0) return comparison
}
i += 1
}
return 0
}
}
其涉及到虚函数调用及装箱,虚函数的调用相对普通函数而言比较耗时。
当开启动态字节码技术的时候,其Compare函数如下:
object GenerateOrdering extends CodeGenerator[Seq[SortOrder], Ordering[Row]] with Logging {
import scala.reflect.runtime.{universe => ru}
import scala.reflect.runtime.universe._
protected def canonicalize(in: Seq[SortOrder]): Seq[SortOrder] =
in.map(ExpressionCanonicalizer.execute(_).asInstanceOf[SortOrder])
protected def bind(in: Seq[SortOrder], inputSchema: Seq[Attribute]): Seq[SortOrder] =
in.map(BindReferences.bindReference(_, inputSchema))
protected def create(ordering: Seq[SortOrder]): Ordering[Row] = {
val a = newTermName("a")
val b = newTermName("b")
val comparisons = ordering.zipWithIndex.map { case (order, i) =>
val evalA = expressionEvaluator(order.child)
val evalB = expressionEvaluator(order.child)
val compare = order.child.dataType match {
case BinaryType =>
q"""
val x = ${if (order.direction == Ascending) evalA.primitiveTerm else evalB.primitiveTerm}//直接指定类型,不涉及虚函数调用
val y = ${if (order.direction != Ascending) evalB.primitiveTerm else evalA.primitiveTerm}//直接指定类型,不涉及虚函数调用
var i = 0
while (i < x.length && i < y.length) {
val res = x(i).compareTo(y(i))
if (res != 0) return res
i = i+1
}
return x.length - y.length
"""
case _: NumericType =>
q"""
val comp = ${evalA.primitiveTerm} - ${evalB.primitiveTerm}//直接指定类型
if(comp != 0) {
return ${if (order.direction == Ascending) q"comp.toInt" else q"-comp.toInt"}
}
"""
case StringType =>
if (order.direction == Ascending) {
q"""return ${evalA.primitiveTerm}.compare(${evalB.primitiveTerm})"""//直接指定类型,不涉及虚函数调用
} else {
q"""return ${evalB.primitiveTerm}.compare(${evalA.primitiveTerm})"""
}
}
q"""
i = $a
..${evalA.code}
i = $b
..${evalB.code}
if (${evalA.nullTerm} && ${evalB.nullTerm}) {
// Nothing
} else if (${evalA.nullTerm}) {
return ${if (order.direction == Ascending) q"-1" else q"1"}
} else if (${evalB.nullTerm}) {
return ${if (order.direction == Ascending) q"1" else q"-1"}
} else {
$compare
}
"""
}
val q"class $orderingName extends $orderingType { ..$body }" = reify {
class SpecificOrdering extends Ordering[Row] {
val o = ordering
}
}.tree.children.head
val code = q"""
class $orderingName extends $orderingType {
..$body
def compare(a: $rowType, b: $rowType): Int = {
var i: $rowType = null // Holds current row being evaluated.
..$comparisons
return 0
}
}
new $orderingName()
"""
logDebug(s"Generated Ordering: $code")
toolBox.eval(code).asInstanceOf[Ordering[Row]]
}
}
可见动态字节码技术中不涉及虚函数的调用,其本质就是scala的反射机制。关于虚调用为什么耗时的原因如下:
以具体的SQL语句 select a+b fromtable 为例进行说明,下面是它的解析过程:
1.调用虚函数Add.eval(),需确认Add两边数据类型
2.调用虚函数a.eval(),需要确认a的数据类型
3.确认a的数据类型是int,装箱
4.调用虚函数b.eval(),需确认b的数据类型
5.确认b的数据类型是int,装箱
6.调用int类型的add
7.返回装箱后的计算结果
从上面的步骤可以看出,一条SQL语句的解析需要进行多次虚函数的调用。我们知道,虚函数的调用会极大的降低效率。那么,虚函数的调用为什么会影响效率呢?
有人答案是:虚函数调用会进行一次间接寻址过程。事实上这一步间接寻址真的会显著降低运行效率?显然不是。
流水线的打断才是真正降低效率的原因。
我们知道,虚函数的调用时是运行时多态,意思就是在编译期你是无法知道虚函数的具体调用。设想一下,如果说不是虚函数,那么在编译时期,其相对地址是确定的,编译器可以直接生成jmp/invoke指令; 如果是虚函数,多出来的一次查找vtable所带来的开销,倒是次要的,关键在于,这个函数地址是动态的,譬如 取到的地址在eax里,则在call eax之后的那些已经被预取进入流水线的所有指令都将失效。流水线越长,一次分支预测失败的代价也就越大,如下所示:
pf->test
001E146D mov eax,dword ptr[pf]
011E1470 mov edx,dword,ptr[eax]
011E1472 mov esi,esp
011E1474 mov ecx,dword ptr[pf]
011E1477 mov eax,dword ptr[edx]
011E1479 eax <-----------------------分支预测失败
011E147B cmp esi esp
011E147D @ILT+355(__RTC_CheckEsp)(11E1168h)