Is C# Really Slower Than Say C++

Is C# really slower than say C++?

Warning: The question you've asked is really pretty complex -- probably much more so than you realize. As a result, this is a really long answer.

From a purely theoretical viewpoint, there's probably a simple answer to this: there's (probably) nothing about C# that truly prevents it from being as fast as C++. Despite the theory, however, there are some practical reasons that it is slower at some things under some circumstances.

I'll consider three basic areas of differences: language features, virtual machine execution, and garbage collection. The latter two often go together, but can be independent, so I'll look at them separately.

Language Features

C++ places a great deal of emphasis on templates, and features in the template system that are largely intended to allow as much as possible to be done at compile time, so from the viewpoint of the program, they're "static." Template meta-programming allows completely arbitrary computations to be carried out at compile time (I.e., the template system is Turing complete). As such, essentially anything that doesn't depend on input from the user can be computed at compile time, so at runtime it's simply a constant. Input to this can, however, include things like type information, so a great deal of what you'd do via reflection at runtime in C# is normally done at compile time via template metaprogramming in C++. There is definitely a trade-off between runtime speed and versatility though -- what templates can do, they do statically, but they simply can't do everything reflection can.

The differences in language features mean that almost any attempt at comparing the two languages simply by transliterating some C# into C++ (or vice versa) is likely to produce results somewhere between meaningless and misleading (and the same would be true for most other pairs of languages as well). The simple fact is that for anything larger than a couple lines of code or so, almost nobody is at all likely to use the languages the same way (or close enough to the same way) that such a comparison tells you anything about how those languages work in real life.

Virtual Machine

Like almost any reasonably modern VM, Microsoft's for .NET can and will do JIT (aka "dynamic") compilation. This represents a number of trade-offs though.

Primarily, optimizing code (like most other optimization problems) is largely an NP-complete problem. For anything but a truly trivial/toy program, you're pretty nearly guaranteed you won't truly "optimize" the result (i.e., you won't find the true optimum) -- the optimizer will simply make the code better than it was previously. Quite a few optimizations that are well known, however, take a substantial amount of time (and, often, memory) to execute. With a JIT compiler, the user is waiting while the compiler runs. Most of the more expensive optimization techniques are ruled out. Static compilation has two advantages: first of all, if it's slow (e.g., building a large system) it's typically carried out on a server, and nobody spends time waiting for it. Second, an executable can be generated once, and used many times by many people. The first minimizes the cost of optimization; the second amortizes the much smaller cost over a much larger number of executions.

As mentioned in the original question (and many other web sites) JIT compilation does have the possibility of greater awareness of the target environment, which should (at least theoretically) offset this advantage. There's no question that this factor can offset at least part of the disadvantage of static compilation. For a few rather specific types of code and target environments, it can even outweigh the advantages of static compilation, sometimes fairly dramatically. At least in my testing and experience, however, this is fairly unusual. Target dependent optimizations mostly seem to either make fairly small differences, or can only be applied (automatically, anyway) to fairly specific types of problems. Obvious times this would happen would be if you were running a relatively old program on a modern machine. An old program written in C++ would probably have been compiled to 32-bit code, and would continue to use 32-bit code even on a modern 64-bit processor. A program written in C# would have been compiled to byte code, which the VM would then compile to 64-bit machine code. If this program derived a substantial benefit from running as 64-bit code, that could give a substantial advantage. For a short time when 64-bit processors were fairly new, this happened a fair amount. Recent code that's likely to benefit from a 64-bit processor will usually be available compiled statically into 64-bit code though.

Using a VM also has a possibility of improving cache usage. Instructions for a VM are often more compact than native machine instructions. More of them can fit into a given amount of cache memory, so you stand a better chance of any given code being in cache when needed. This can help keep interpreted execution of VM code more competitive (in terms of speed) than most people would initially expect -- you can execute a lot of instructions on a modern CPU in the time taken by one cache miss.

It's also worth mentioning that this factor isn't necessarily different between the two at all. There's nothing preventing (for example) a C++ compiler from producing output intended to run on a virtual machine (with or without JIT). In fact, Microsoft's C++/CLI is nearly that -- an (almost) conforming C++ compiler (albeit, with a lot of extensions) that produces output intended to run on a virtual machine.

The reverse is also true: Microsoft now has .NET Native, which compiles C# (or VB.NET) code to a native executable. This gives performance that's generally much more like C++, but retains the features of C#/VB (e.g., C# compiled to native code still supports reflection). If you have performance intensive C# code, this may be helpful.

Garbage Collection

From what I've seen, I'd say garbage collection is the poorest-understood of these three factors. Just for an obvious example, the question here mentions: "GC doesn't add a lot of overhead either, unless you create and destroy thousands of objects [...]". In reality, if you create and destroy thousands of objects, the overhead from garbage collection will generally be fairly low. .NET uses a generational scavenger, which is a variety of copying collector. The garbage collector works by starting from "places" (e.g., registers and execution stack) that pointers/references are known to be accessible. It then "chases" those pointers to objects that have been allocated on the heap. It examines those objects for further pointers/references, until it has followed all of them to the ends of any chains, and found all the objects that are (at least potentially) accessible. In the next step, it takes all of the objects that are (or at least might be) in use, and compacts the heap by copying all of them into a contiguous chunk at one end of the memory being managed in the heap. The rest of the memory is then free (modulo finalizers having to be run, but at least in well-written code, they're rare enough that I'll ignore them for the moment).

What this means is that if you create and destroy lots of objects, garbage collection adds very little overhead. The time taken by a garbage collection cycle depends almost entirely on the number of objects that have been created but not destroyed. The primary consequence of creating and destroying objects in a hurry is simply that the GC has to run more often, but each cycle will still be fast. If you create objects and don't destroy them, the GC will run more often and each cycle will be substantially slower as it spends more time chasing pointers to potentially-live objects, and it spends more time copying objects that are still in use.

To combat this, generational scavenging works on the assumption that objects that have remained "alive" for quite a while are likely to continue remaining alive for quite a while longer. Based on this, it has a system where objects that survive some number of garbage collection cycles get "tenured", and the garbage collector starts to simply assume they're still in use, so instead of copying them at every cycle, it simply leaves them alone. This is a valid assumption often enough that generational scavenging typically has considerably lower overhead than most other forms of GC.

"Manual" memory management is often just as poorly understood. Just for one example, many attempts at comparison assume that all manual memory management follows one specific model as well (e.g., best-fit allocation). This is often little (if any) closer to reality than many peoples' beliefs about garbage collection (e.g., the widespread assumption that it's normally done using reference counting).

Given the variety of strategies for both garbage collection and manual memory management, it's quite difficult to compare the two in terms of overall speed. Attempting to compare the speed of allocating and/or freeing memory (by itself) is pretty nearly guaranteed to produce results that are meaningless at best, and outright misleading at worst.

Bonus Topic: Benchmarks

Since quite a few blogs, web sites, magazine articles, etc., claim to provide "objective" evidence in one direction or another, I'll put in my two-cents worth on that subject as well.

Most of these benchmarks are a bit like teenagers deciding to race their cars, and whoever wins gets to keep both cars. The web sites differ in one crucial way though: they guy who's publishing the benchmark gets to drive both cars. By some strange chance, his car always wins, and everybody else has to settle for "trust me, I was really driving your car as fast as it would go."

It's easy to write a poor benchmark that produces results that mean next to nothing. Almost anybody with anywhere close to the skill necessary to design a benchmark that produces anything meaningful, also has the skill to produce one that will give the results he's decided he wants. In fact it's probably easier to write code to produce a specific result than code that will really produce meaningful results.

As my friend James Kanze put it, "never trust a benchmark you didn't falsify yourself."

Conclusion

There is no simple answer. I'm reasonably certain that I could flip a coin to choose the winner, then pick a number between (say) 1 and 20 for the percentage it would win by, and write some code that would look like a reasonable and fair benchmark, and produced that foregone conclusion (at least on some target processor--a different processor might change the percentage a bit).

As others have pointed out, for most code, speed is almost irrelevant. The corollary to that (which is much more often ignored) is that in the little code where speed does matter, it usually matters a lot. At least in my experience, for the code where it really does matter, C++ is almost always the winner. There are definitely factors that favor C#, but in practice they seem to be outweighed by factors that favor C++. You can certainly find benchmarks that will indicate the outcome of your choice, but when you write real code, you can almost always make it faster in C++ than in C#. It might (or might not) take more skill and/or effort to write, but it's virtually always possible.

Why would you want to use C# if its slower than C++?

Who exactly is this "bunch of people"? What are they comparing it against?

For the vast majority of things, C++ is not "much faster" than C#. It certainly has benefits in various situations, particularly where you want more deterministic memory handling, but in my experience the bottleneck in most applications isn't in places where C++ would help. As spoulson says, a lot of performance is in the design instead of the exact implementation - and there, it helps to be able to try different designs easily.

Why would we use C# when it's a bit slower than C++? Because it's generally reckoned (i.e. some disagree :) to be a lot easier to develop in without shooting yourself in the foot.

As for what C# can be used for... what do you want to use it for? Unless you want to develop drivers and kernels, it may well be fine for you. (Even OS development has some folks using C#...)

Job opportunities? Loads.

Downsides? Well, .NET itself is only available on Microsoft platforms. There's Mono, but it doesn't have quite the same degree of portability as Java (no doubt another "slow" language according to the same bunch of people).

How much faster is C++ than C#?

There is no strict reason why a bytecode based language like C# or Java that has a JIT cannot be as fast as C++ code. However C++ code used to be significantly faster for a long time, and also today still is in many cases. This is mainly due to the more advanced JIT optimizations being complicated to implement, and the really cool ones are only arriving just now.

So C++ is faster, in many cases. But this is only part of the answer. The cases where C++ is actually faster, are highly optimized programs, where expert programmers thoroughly optimized the hell out of the code. This is not only very time consuming (and thus expensive), but also commonly leads to errors due to over-optimizations.

On the other hand, code in interpreted languages gets faster in later versions of the runtime (.NET CLR or Java VM), without you doing anything. And there are a lot of useful optimizations JIT compilers can do that are simply impossible in languages with pointers. Also, some argue that garbage collection should generally be as fast or faster as manual memory management, and in many cases it is. You can generally implement and achieve all of this in C++ or C, but it's going to be much more complicated and error prone.

As Donald Knuth said, "premature optimization is the root of all evil". If you really know for sure that your application will mostly consist of very performance critical arithmetic, and that it will be the bottleneck, and it's certainly going to be faster in C++, and you're sure that C++ won't conflict with your other requirements, go for C++. In any other case, concentrate on first implementing your application correctly in whatever language suits you best, then find performance bottlenecks if it runs too slow, and then think about how to optimize the code. In the worst case, you might need to call out to C code through a foreign function interface, so you'll still have the ability to write critical parts in lower level language.

Keep in mind that it's relatively easy to optimize a correct program, but much harder to correct an optimized program.

Giving actual percentages of speed advantages is impossible, it largely depends on your code. In many cases, the programming language implementation isn't even the bottleneck. Take the benchmarks at http://benchmarksgame.alioth.debian.org/ with a great deal of scepticism, as these largely test arithmetic code, which is most likely not similar to your code at all.

Why is C# running faster than C++?

I think you've not been careful enough in your evaluation. I recreated your test with C++ proving massively faster as detailed below:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CSScratch
{
    class Program
    {
        static void Main(string[] args)
        {
            ulong i = 0;
            while (i < 1000000)
            {
                Console.WriteLine(i);
                i++;
            }
        }
    }
}

I built the above in VS2013 Release mode to CSScratch.exe, which I then timed (under cygwin bash) with output redirected so file-system writing time wasn't counted. Results were fairly consistent, the fastest of five runs being:

time ./CSScratch.exe > NUL

real    0m17.175s
user    0m0.031s
sys     0m0.124s

The C++ equivalent:

#include <iostream>

int main()
{
    std::ios_base::sync_with_stdio(false);

    unsigned long i = 0;
    while (i < 1000000)
    {
        std::cout << i << '\n';
        i++;
    }
}

Also compiled with VS2013:

cl /EHsc /O2 output.cc
time ./output > NUL

The slowest of five runs:

real    0m1.116s
user    0m0.000s
sys     0m0.109s

which is still faster (1.116 seconds) than the fastest of C# runs (17.175 seconds).

Some of the time for both versions is taken by loading / dynamic linking, initialisation etc.. I modified the C++ version to loop 10x more, and it still only took 9.327 seconds - about half the time C# needed for a tenth of the workload.

(You could further tune the C++ version by setting a larger output buffer, but that's not normally needed).

C# vs C - Big performance difference

Since you never use 'root', the compiler may have been removing the call to optimize your method.

You could try to accumulate the square root values into an accumulator, print it out at the end of the method, and see what's going on.

Edit : see Jalf's answer below

Why is C# much slower than Java and C++ in my prime number testing

About C#:

First of all, instead of datetime you should use stopwatch.
Datetime is not reliable for code timing.

Second, are you sure you are executing it in release mode with visual studio closed?
If visual studio is open or you are launching with F5 the JIT will not optimize the code!

So... use stopwatch and close all instances of visual studio.
You have to change project options, you should have a combobox, in the top toolbar, where you can read "Debug", just click on it and select "Release" or go to right click on your project, properties, and change it to release mode. Then to avoid every kind of problems, close all instances of visual studio and launch with double click on the executable.

See http://msdn.microsoft.com/en-us/library/wx0123s5.aspx

CTRL+F5 don't compile in release mode, it just launch the executable in selected compilation mode without attacching the process for debugging, so if it was compiled in debug, it will launch the executable compiled in debug mode without debugging it.

Then i would suggest to you to avoid the use of the boolean variable, each branch condition can slow down the CPU, you can do it using an integer. This is valid for all languages, not only C#.

static void Main()
{
    const int NumberOfPrimesToFind = 100000;
    const int NumberOfRuns = 1;

    System.Diagnostic.Stopwatch sw = new System.Diagnostic.Stopwatch();

    sw.Start();
    for (int k = 0; k < NumberOfRuns; k++)
    {
        FindPrimes(NumberOfPrimesToFind);
    }
    sw.Stop();

    Console.WriteLine(sw.Elapsed.TotalMilliseconds);
    Console.ReadLine();
}

static void FindPrimes(int NumberOfPrimesToFind)
{
    int NumberOfPrimes = 0;
    int CurrentPossible = 2;

    while (NumberOfPrimes < NumberOfPrimesToFind)
    {
        int IsPrime = 1;

        for (int j = 2; j < CurrentPossible; j++)
        {
            if (CurrentPossible % j == 0)
            {
                IsPrime = 0;
                break;
            }
        }

        NumberOfPrimes += IsPrime;
        CurrentPossible++;
    }
}

When you compile it with C++ in release mode however since input parameters are constants C++ compiler is smart enough to perform some of the computation at compile time (the power of modern C++ compilers!). This magic is usually used with templates too, STL (standard template library) for example is very, very slow in debug mode but very fast in release mode.

In this case the compiler is totally ruling out your function because the output of your function is not used. Try to make it return an integer, the number of found primes, and print it.

int FindPrimes(int NumberOfPrimesToFind)
{
    int NumberOfPrimes = 0;
    int CurrentPossible = 2;
    while (NumberOfPrimes < NumberOfPrimesToFind)
    {
        int IsPrime = 1;

        for (int j = 2; j < CurrentPossible; j++)
        {
            if (CurrentPossible % j == 0)
            {
                IsPrime = 0;
                break;
            }
        }

        NumberOfPrimes += IsPrime;
        CurrentPossible++;
    }
    return NumberOfPrimes ;
}

If you are curious about this aspect of the C++ compiler, take a look at the template metaprogramming for example, there exists a formal proof that C++ compiler is turing complete. As wikipedia quotes "In addition, templates are a compile time mechanism in C++ that is Turing-complete, meaning that any computation expressible by a computer program can be computed, in some form, by a template metaprogram prior to runtime." http://en.wikipedia.org/wiki/C%2B%2B

However I really hope you are using this algorithm only to try to understand how the three different compilers\systems behave, because, of course, this is the worst algorithm you can use to find prime numbers, as pointed out in other answers :)

Why C# is twice as slow as C++ even though the generated machine code is nearly identical?

The reason is JIT overhead. When benchmarking .NET code, you should always discard the first measure, because it includes time the runtime spent to produce x86 code out of the IL.

Here’s what the test app prints after I’ve measured 3 times instead of just 1 (for 511M pixels):

#1 391.1885 ms, #2 216.985 ms, #3 235.5549 ms

Source code: https://gist.github.com/Const-me/0f0c283a0b998aa9977550d85fa33958

These ~220 ms is pretty close to the performance of the equivalent C++ code. So the C# SIMD is not that bad after all.

Compile C#, so that it runs with the speed of C++

What makes you think that translating your C# code to C++ would magically make it faster?

Languages don't have a speed. Assuming that C# code is slower (I'll get back to that), it is because of what that code does (including the implicit requirements placed by C#, such as bounds checking on arrays), and not because of the language it is written in.

If you converted your C# code to C++, it would still need to do bounds checking on arrays, because the original source code expected this to happen, so it would have to do just as much work.

Moreover, C# often isn't slower than C++. There are plenty of benchmarks floating around on the internet, generally showing that for the most part, C# is as fast as (or faster than) C++. Only when you spend a lot of time optimizing your code, does C++ become faster.

If you want faster code, you need to write code that requires less work to execute, not try to change the source language. That's just cargo-cult programming at its worst. You once saw some efficient code, and that was written in C++, so now you try to make things C++, in the hope of attracting any efficiency that might be passing by.

It just doesn't work that way.

C# running faster than C++?

Without source code it's difficult to say anything about the performance of your encryption algorithm/program.
I reckon though that you made a "mistake" while porting it to C++, meaning that you used it in a inefficient way (e.g. lots of copying of objects happens). Maybe you also used VC 6, whereas VC 9 would/could produce much better code.

As for the "x >> 3" optimization... modern compilers do convert integer division to bitshifts by themselves. Needless to say that this optimization may not be the bottleneck of your program at all. You should profile it first to find out where you're spending most of your time :)

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