问题
I would like to know how to check if the JIT compiler is turned off. I have the following code which is meant to turn the JIT compiler off.The problem is, I am not sure if it is actually doing that. So I was wondering if there is a way of checking if the JIT is off.
I looked at the Compiler class but there isn't any method like isDisabled/enabled().
Code:
Compiler.disable();
Any help or direction will be highly appreciated.
回答1:
I don't believe you can turn the JIT off at runtime.
If you want to seriously benchmark a Java program, you should definitely be ignoring the first few runs. Getting reliable benchmarks in Java is an extremely tricky business, best left to people much smarter than you or I.
I recommend using Caliper, which is used internally at Google for microbenchmarking and is plenty smart about warming up the JIT and stuff. In particular, look at the example here, which shows how to measure the efficiency of an algorithm for different input sizes.
回答2:
(Not a direct answer to your question since it seems your were trying to turn off the JIT compiler programmatically, but based on your comment, this might be of interest.)
If you want to turn off the JIT compiler on a Sun/Oracle JVM, you should try the -Xint option:
-Xint
Operate in interpreted-only mode. Compilation to native code is disabled, and all bytecodes are executed by the interpreter. The performance benefits offered by the Java HotSpot Client VM's adaptive compiler will not be present in this mode.
回答3:
IBM JVMs definitely support the Java interface java/lang/Compiler.disable() and .enable() which was introduced in Java 5, I believe. That includes WebSphere Real Time (which is a JVM designed to provide more predictable performance) as well as our "standard" JVMs. If you call disable(), it will prevent JIT compilations until you call enable().
I work for IBM on the JIT compiler team. We don't usually recommend people to use this interface, because interfering with JIT compilation heuristics is generally not a good idea, but there are reasonable real-time scenarios where you would use it.
回答4:
In the article Performance Features and Tools.
The JIT compiler was first made available as a performance update in the Java Development Kit (JDK) 1.1.6 software release and is now a standard tool invoked whenever you use the java interpreter command in the Java 2 platform release.
You can disable the JIT compiler using the
-Djava.compiler=NONEoption to the Java VM.
So, you can deduce that when the variable is not set, or set to something other than NONE, then the JIT is enabled.
回答5:
You can printout methods when they get compiled, with `-XX:+PrintCompilation if your method isn't printed out or suddenly gets faster after it is print out, you can see the likely cause.
来源：https://stackoverflow.com/questions/9457405/how-to-check-if-the-jit-compiler-is-off-in-java