I know Microsoft .NET uses the CLR as a JIT compiler while Java has the Hotspot. What Are the differences between them?
我知道Microsoft .NET使用CLR作为JIT编译器,而Java具有Hotspot。它们之间有什么区别?
1 个解决方案
#1
23
They are very different beasts. As people pointed out, the CLR compiles to machine code before it executes a piece of MSIL. This allows it in addition to the typical dead-code elimination and inlining off privates optimizations to take advantage of the particular CPU architecture of the target machine (though I'm not sure whether it does it). This also incurs a hit for each class (though the compiler is fairly fast and many platform libraries are just a thin layer over the Win32 API).
他们是非常不同的野兽。正如人们所指出的那样,CLR在执行一段MSIL之前编译为机器代码。这允许它除了典型的死代码消除和内联关闭私有优化以利用目标机器的特定CPU架构(虽然我不确定它是否这样做)。这也引起了每个类的冲击(尽管编译器相当快,并且许多平台库只是Win32 API上的一个薄层)。
The HotSpot VM is taking a different approach. It stipulates that most of the code is executed rarely, hence it's not worth to spend time compiling it. All bytecode starts in interpreted mode. The VM keeps statistics at call-sites and tries to identify methods which are called more than a predefined number of times. Then it compiles only these methods with a fast JIT compiler (C1) and swaps the method while it is running (that's the special sauce of HS). After the C1-compiled method has been invoked some more times, the same method is compiled with slow, but sophisticated compiler and the code is swapped again on the fly.
HotSpot VM采用了不同的方法。它规定大多数代码很少执行,因此花时间编译它是不值得的。所有字节码都以解释模式启动。 VM在呼叫站点保留统计信息,并尝试识别被调用超过预定义次数的方法。然后它只使用快速JIT编译器(C1)编译这些方法,并在运行时交换方法(这是HS的特殊情况)。在多次调用C1编译的方法之后,使用缓慢但复杂的编译器编译相同的方法,并且动态地再次交换代码。
Since HotSpot can swap methods while they are running, the VM compilers can perform some speculative optimizations that are unsafe in statically compiled code. A canonical example is static dispatch / inlining of monomorphic calls (polymorphic method with only one implementation). This is done if the VM sees that this method always resolves to the same target. What used to be complex invocation is reduced to a few CPU instructions guard, which are predicted and pipelined by modern CPUs. When the guard condition stops being true, the VM can take a different code path or even drop back to interpreting mode. Based on statistics and program workload, the generated machine code can be different at different time. Many of these optimizations rely on the information gathered during the program execution and are not possible if you compile once whan you load the class.
由于HotSpot可以在运行时交换方法,因此VM编译器可以执行一些在静态编译代码中不安全的推测性优化。一个典型的例子是单态调用的静态调度/内联(只有一个实现的多态方法)。如果VM发现此方法始终解析为同一目标,则会执行此操作。以前的复杂调用简化为少数CPU指令保护,由现代CPU预测和流水线化。当保护条件停止为真时,VM可以采用不同的代码路径,甚至回退到解释模式。根据统计和程序工作量,生成的机器代码可以在不同的时间不同。其中许多优化依赖于在程序执行期间收集的信息,如果您在加载类时编译一次,则无法实现。
This is why you need to warm-up the JVM and emulate realistic workload when you benchmark algorithms (skewed data can lead to unrealistic assesment of the optimizations). Other optimizations are lock elision, adaptive spin-locking, escape analysis and stack allocation, etc.
这就是为什么在对基准算法进行基准测试时需要预热JVM并模拟实际工作负载的原因(偏差数据会导致对优化的不切实际的评估)。其他优化包括锁定省略,自适应自旋锁定,逃逸分析和堆栈分配等。
That said, HotSpot is only one of the VMs. JRockit, Azul, IBM's J9 and the Resettable RVM, - all have different performance profiles.
也就是说,HotSpot只是其中一个虚拟机。 JRockit,Azul,IBM的J9和Resettable RVM,都有不同的性能配置文件。
#1
23
They are very different beasts. As people pointed out, the CLR compiles to machine code before it executes a piece of MSIL. This allows it in addition to the typical dead-code elimination and inlining off privates optimizations to take advantage of the particular CPU architecture of the target machine (though I'm not sure whether it does it). This also incurs a hit for each class (though the compiler is fairly fast and many platform libraries are just a thin layer over the Win32 API).
他们是非常不同的野兽。正如人们所指出的那样,CLR在执行一段MSIL之前编译为机器代码。这允许它除了典型的死代码消除和内联关闭私有优化以利用目标机器的特定CPU架构(虽然我不确定它是否这样做)。这也引起了每个类的冲击(尽管编译器相当快,并且许多平台库只是Win32 API上的一个薄层)。
The HotSpot VM is taking a different approach. It stipulates that most of the code is executed rarely, hence it's not worth to spend time compiling it. All bytecode starts in interpreted mode. The VM keeps statistics at call-sites and tries to identify methods which are called more than a predefined number of times. Then it compiles only these methods with a fast JIT compiler (C1) and swaps the method while it is running (that's the special sauce of HS). After the C1-compiled method has been invoked some more times, the same method is compiled with slow, but sophisticated compiler and the code is swapped again on the fly.
HotSpot VM采用了不同的方法。它规定大多数代码很少执行,因此花时间编译它是不值得的。所有字节码都以解释模式启动。 VM在呼叫站点保留统计信息,并尝试识别被调用超过预定义次数的方法。然后它只使用快速JIT编译器(C1)编译这些方法,并在运行时交换方法(这是HS的特殊情况)。在多次调用C1编译的方法之后,使用缓慢但复杂的编译器编译相同的方法,并且动态地再次交换代码。
Since HotSpot can swap methods while they are running, the VM compilers can perform some speculative optimizations that are unsafe in statically compiled code. A canonical example is static dispatch / inlining of monomorphic calls (polymorphic method with only one implementation). This is done if the VM sees that this method always resolves to the same target. What used to be complex invocation is reduced to a few CPU instructions guard, which are predicted and pipelined by modern CPUs. When the guard condition stops being true, the VM can take a different code path or even drop back to interpreting mode. Based on statistics and program workload, the generated machine code can be different at different time. Many of these optimizations rely on the information gathered during the program execution and are not possible if you compile once whan you load the class.
由于HotSpot可以在运行时交换方法,因此VM编译器可以执行一些在静态编译代码中不安全的推测性优化。一个典型的例子是单态调用的静态调度/内联(只有一个实现的多态方法)。如果VM发现此方法始终解析为同一目标,则会执行此操作。以前的复杂调用简化为少数CPU指令保护,由现代CPU预测和流水线化。当保护条件停止为真时,VM可以采用不同的代码路径,甚至回退到解释模式。根据统计和程序工作量,生成的机器代码可以在不同的时间不同。其中许多优化依赖于在程序执行期间收集的信息,如果您在加载类时编译一次,则无法实现。
This is why you need to warm-up the JVM and emulate realistic workload when you benchmark algorithms (skewed data can lead to unrealistic assesment of the optimizations). Other optimizations are lock elision, adaptive spin-locking, escape analysis and stack allocation, etc.
这就是为什么在对基准算法进行基准测试时需要预热JVM并模拟实际工作负载的原因(偏差数据会导致对优化的不切实际的评估)。其他优化包括锁定省略,自适应自旋锁定,逃逸分析和堆栈分配等。
That said, HotSpot is only one of the VMs. JRockit, Azul, IBM's J9 and the Resettable RVM, - all have different performance profiles.
也就是说,HotSpot只是其中一个虚拟机。 JRockit,Azul,IBM的J9和Resettable RVM,都有不同的性能配置文件。