C++ performance vs. Java/C#

Question

My understanding is that C/C++ produces native code to run on a particular machine architecture. Conversely, languages like Java and C# run on top of a virtual machine which abstracts away the native architecture. Logically it would seem impossible for Java or C# to match the speed of C++ because of this intermediate step, however I've been told that the latest compilers ("hot spot") can attain this speed or even exceed it.

Perhaps this is more of a compiler question than a language question, but can anyone explain in plain English how it is possible for one of these virtual machine languages to perform better than a native language?

Answer 1

Here's an interesting benchmark http://zi.fi/shootout/

Answer 2

The executable code produced from a Java or C# compiler is not interpretted -- it is compiled to native code "just in time" (JIT). So, the first time code in a Java/C# program is encountered during execution, there is some overhead as the "runtime compiler" (aka JIT compiler) turns the byte code (Java) or IL code (C#) into native machine instructions. However, the next time that code is encountered while the application is still running, the native code is executed immediately. This explains how some Java/C# programs appear to be slow initially, but then perform better the longer they run. A good example is an ASP.Net web site. The very first time the web site is accessed, it may be a bit slower as the C# code is compiled to native code by the JIT compiler. Subsequent accesses result in a much faster web site -- server and client side caching aside.

Answer 3

You should define "perform better than..". Well, I know, you asked about speed, but its not everything that counts.

• Do virtual machines perform more runtime overhead? Yes!
• Do they eat more working memory? Yes!
• Do they have higher startup costs (runtime initialization and JIT compiler) ? Yes!
• Do they require a huge library installed? Yes!

And so on, its biased, yes ;)

With C# and Java you pay a price for what you get (faster coding, automatic memory management, big library and so on). But you have not much room to haggle about the details: take the complete package or nothing.

Even if those languages can optimize some code to execute faster than compiled code, the whole approach is (IMHO) inefficient. Imagine driving every day 5 miles to your workplace, with a truck! Its comfortable, it feels good, you are safe (extreme crumple zone) and after you step on the gas for some time, it will even be as fast as a standard car! Why don't we all have a truck to drive to work? ;)

In C++ you get what you pay for, not more, not less.

Quoting Bjarne Stroustrup: "C++ is my favorite garbage collected language because it generates so little garbage" link text

Answer 4

If you're a Java/C# programmer learning C++, you'll be tempted to keep thinking in terms of Java/C# and translate verbatim to C++ syntax. In that case, you only get the earlier mentioned benefits of native code vs. interpreted/JIT. To get the biggest performance gain in C++ vs. Java/C#, you have to learn to think in C++ and design code specifically to exploit the strengths of C++.

To paraphrase Edsger Dijkstra: [your first language] mutilates the mind beyond recovery.
To paraphrase Jeff Atwood: you can write [your first language] in any new language.

Answer 5

I look at it from a few different points.

1. Given infinite time and resources, will managed or unmanaged code be faster? Clearly, the answer is that unmanaged code can always at least tie managed code in this aspect - as in the worst case, you'd just hard-code the managed code solution.
2. If you take a program in one language, and directly translate it to another, how much worse will it perform? Probably a lot, for any two languages. Most languages require different optimizations and have different gotchas. Micro-performance is often a lot about knowing these details.
3. Given finite time and resources, which of two languages will produce a better result? This is the most interesting question, as while a managed language may produce slightly slower code (given a program reasonably written for that language), that version will likely be done sooner, allowing for more time spent on optimization.

Answer 6

A very short answer: Given a fixed budget you will achieve better performing java application than a C++ application (ROI considerations) In addition Java platform has more decent profilers, that will help you pinpoint your hotspots more quickly