jit

I have come across a few references regarding the JVM/JIT activity where there appears to be a distinction made between compiling bytecode and interpreting bytecode. The particular comment stated bytecode is interpreted for the first 10000 runs and compiled thereafter. What is the difference between "compiling" and "interpreting" bytecode? Interpreting byte code basically reads the bytecode line by line, doing no optimization or anything, and parsing it and executing it in realtime. This is notably inefficient for a number of reasons, including the issue that Java bytecode isn't designed to be

The title pretty much sums it up but I was wondering why systems like .net compile code every time it is run instead of just compiling it once on the target machine? There are two things to be gained by using an intermediate format like .NET or Java: You can run the program on any platform, exactly because the code is represented in an intermediate format instead of native code. You just need to write an interpreter for the intermediate format. It allows for some run-time optimizations which are not (easily) possible at compile-time: for example, you can take advantage of special features on

OK, I realize that question might seem weird, but I just noticed something that really puzzled me... Have a look at this code : static void TestGC() { object o1 = new Object(); object o2 = new Object(); WeakReference w1 = new WeakReference(o1); WeakReference w2 = new WeakReference(o2); GC.Collect(); Console.WriteLine("o1 is alive: {0}", w1.IsAlive); Console.WriteLine("o2 is alive: {0}", w2.IsAlive); } Since o1 and o2 are still in scope when the garbage collection occurs, I would have expected the following output: o1 is alive: True o2 is alive: True But instead, here's what I got: o1 is alive:

While writing an answer to another question , I noticed a strange border case for JIT optimization. The following program is not a "Microbenchmark" and not intended to reliably measure an execution time (as pointed out in the answers to the other question). It is solely intended as an MCVE to reproduce the issue: class MissedLoopOptimization { public static void main(String args[]) { for (int j=0; j<3; j++) { for (int i=0; i<5; i++) { long before = System.nanoTime(); runWithMaxValue(); long after = System.nanoTime(); System.out.println("With MAX_VALUE : "+(after-before)/1e6); } for (int i=0; i

Is there any software prefetching instructions in Java language or JVM, like __builtin_prefetch which is available in GCC One interesting thing is that Hotspot JVM actually does support prefetch! It treats Unsafe.prefetchRead() and Unsafe.prefetchWrite() methods as intrinsics and compiles them into corresponding CPU instructions. Unfortunately, sun.misc.Unsafe does not declare such methods. But, if you add the following methods to Unsafe.java, recompile it and replace Unsafe.class inside rt.jar (or just add -Xbootclasspath/p JVM argument) you would be able to use prefetch intrinsics in your

HotSpot's tiered compilation uses the interpreter until a threshold of invocations (for methods) or iterations (for loops) triggers a client compilation with self-profiling. The client compilation is used until another threshold of invocations or iterations triggers a server compilation. Printing HotSpot's flags shows the following flag values with -XX:+TieredCompilation. intx CompileThreshold = 10000 {pd product} intx Tier2CompileThreshold = 0 {product} intx Tier3CompileThreshold = 2000 {product} intx Tier4CompileThreshold = 15000 {product} There are too many flags for just a client and

I recently discovered that one can use JIT (just in time) compilation with R using the compiler package (I summarizes my findings on this topic in a recent blog post ). One of the questions I was asked is: Is there any pitfall? it sounds too good to be true, just put one line of code and that's it. After looking around I could find one possible issue having to do with the "start up" time for the JIT. But is there any other issue to be careful about when using JIT? I guess that there will be some limitation having to do with R's environments architecture, but I can not think of a simple

We are making some kNN and SVD implementations in Python. Others picked Java. Our execution times are very different. I used cProfile to see where I make mistakes but everything is quite fine actually. Yes, I use numpy also. But I would like to ask simple question. total = 0.0 for i in range(9999): # xrange is slower according for j in range(1, 9999): #to my test but more memory-friendly. total += (i / j) print total This snippet takes 31.40s on my computer. Java version of this code takes 1 second or less on the same computer. Type checking is a main problem for this code, I suppose. But I