问题
Java is slow.
That's more than an "urban legend", it seems to be a fact. You don't use it for live-coding because of latency and you don't use it for clusters/parallel computing. There are thousands of benchmarks out there, specially "Java vs C# vs C++".
http://benchmarksgame.alioth.debian.org/
According to the above site, not only is Java performance almost as good as C (far from the rest), but Scala and Clojure (both functional languages which runs on the JVM) both have a better performance that OCaml, Erlang.
And there are a lot of "Java is faster then X" out there, also (for instance, a question here on SO: Java Runtime Performance Vs Native C / C++ Code?).
So Java seems to be fast, for certain cases. Can someone explain why?
Why is it that bytecode might run faster then native code, in some cases, given dynamic code (Scala, Clojure) and garbage collection? How come if it is faster, there is still latency?
It seems to be a contradiction here, can someone shed light?
回答1:
In the book masterminds for programming, James Gosling explained:
James: Exactly. These days we’re beating the really good C and C++ compilers pretty much always. When you go to the dynamic compiler, you get two advantages when the compiler’s running right at the last moment. One is you know exactly what chipset you’re running on. So many times when people are compiling a piece of C code, they have to compile it to run on kind of the generic x86 architecture. Almost none of the binaries you get are particularly well tuned for any of them. You download the latest copy of Mozilla,and it’ll run on pretty much any Intel architecture CPU. There’s pretty much one Linux binary. It’s pretty generic, and it’s compiled with GCC, which is not a very good C compiler.
When HotSpot runs, it knows exactly what chipset you’re running on. It knows exactly how the cache works. It knows exactly how the memory hierarchy works. It knows exactly how all the pipeline interlocks work in the CPU. It knows what instruction set extensions this chip has got. It optimizes for precisely what machine you’re on. Then the other half of it is that it actually sees the application as it’s running. It’s able to have statistics that know which things are important. It’s able to inline things that a C compiler could never do. The kind of stuff that gets inlined in the Java world is pretty amazing. Then you tack onto that the way the storage management works with the modern garbage collectors. With a modern garbage collector, storage allocation is extremely fast.
回答2:
Fast JVMs use Just-In-Time (JIT) compilation. The bytecode gets translated into native code on the fly at run time. JIT provides many opportunities for optimization. See this Wikipedia article for more info on JIT.
回答3:
JVM's come in many flavours!
The fastest ones compile byte code to native code on the fly, based on performance characteristics being collected. All this require extra memory, so they buy speed at the cost of memory. Also, top speed takes a while to reach, so the benefit does not show for short-lived programs.
Even so, the JamVM interpreter (which is tiny compared to the Oracle JVM) still reaches a top speed of a reasonable fraction of the compiled JVM's.
Regarding garbage collection, the speed again comes from having plenty of memory available. Also the true benefit come from the removal of responsibility from the code to keep track of when an object is not in use anymore. This result in simpler, less error prone programs.
回答4:
Well this is an old argument. Almost all prevalent as Emacs and VI.(but definitely not as old). I have seen lot of c++ developers providing lots of arguments on why most of the performance benchmarks (especially mentioning how java is as fast as c++__) is skewed and to be honest they have a point.
I do not have enough knowledge or time to go deeper in to how Java could be as fast as C++, but the following are the reason why it could be...
1- When you ask two very capable developers to write code in Java and C++ for a real world problem (same problem), then I would be surprised if java performs faster than C++. C++ when well written uses a fraction of memory Java will use. ( Java objects are slightly more bloated).
2- Java is inherently meant to be a simpler language and it was written to make sure that sub- optimal code is hard to write. By abstracting memory management and also by handling low level optimization, its easy to write good code in Java than c++. (this I believe is the most important thing.... Its difficult to write bad code in Java).On the flip side a good C++ developer could handle memory management much better than automatic GC in java. ( Java stores everything in heap, so uses more memory... )
3- Java compiler has been improved consistently and ideas like hotspot has proved to be better than marketing term. (when JIT was introduced, it was just a marketing term.. according to me.. :))
4- Ergonomics or tailoring the settings based on the underlying operating system parameters makes java handle variation better. So in some environments, its not hard to imaging Java performing as good as C++.
5- Opening up high level concurrency and parallelism api's for java developers also is a reason. Java concurrency package is arguably the easiest way to write high performance code that could leverage today's multi processor environments.
6- As hardware as become more and more cheaper, developer competency has become a bigger factor and that's why I believe that lots of c++ code in the wild is probably slower than Java.
回答5:
Java Byte code is much easier to optimize than most native opcode. Because the byte code is restricted, and you can't do some dangerous things, you can optimize more fully. Take pointer aliasing, for instance. http://en.wikipedia.org/wiki/Pointer_aliasing
In c/c++ you can do
char[] somebuffer = getABuffer();
char* ptr = &someBuffer[2];
memcpy(ptr, somebuffer, length);
this makes it difficult to optimize in some cases, because you can't be sure what is referring to what. In Java this kind of thing is not allowed.
in general the code mutations that you can perform on a higher level of abstraction are much more powerful than can be had on object code.
来源：https://stackoverflow.com/questions/5641356/why-is-it-that-bytecode-might-run-faster-than-native-code