Which is faster - C# unsafe code or raw C++

Question

I'm writing an image processing program to perform real time processing of video frames. It's in C# using the Emgu.CV library (C#) that wraps the OpenCV library dll (unmanaged C++). Now I have to write my own special algorithm and it needs to be as fast as possible.

Which will be a faster implementation of the algorithm?

1. Writing an 'unsafe' function in C#

2. Adding the function to the OpenCV library and calling it through Emgu.CV

I'm guessing C# unsafe is slower because it goes throught the JIT compiler, but would the difference be significant?

Edit:

Compiled for .NET 3.5 under VS2008

Answer 1

It depends on the algorithm, the implementation, the C++ compiler and the JIT compiler. I guess in most cases the C++ implementation will be faster. But this may change.

The JIT compiler can optimize your code for the platform your code is running on instead of an average for all the platforms your code might run on as the C++ compiler does. This is something newer versions of the JIT compiler are increasingly good at and may in some cases give JITted code an advantage. So the answer is not as clear as you might expect. The new Java hotspot compiler does this very well for example.

Other situations where managed code may do better than C++ is where you need to allocate and deallocate lots of small objects. The .net runtime preallocates large chunks of memory that can be reused so it doesn't need to call into the os every time you need to allocate memory.

I'm not sure unsafe C# runs much faster than normal C#. You'll have to try this too.

If you want to know what's the best solution for your situation you'll have to try both and measure the difference. I dont think there will be more than

Answer 2

Languages don't have a "speed". It depends on the compiler and the code. It's possible to write inefficient code in any language, and a clever compiler will generate near-optimal code no matter the language of the source.

The only really unavoidable factor in performance between C# and C++ is that C# apps have to do more at startup (load the .NET framework and perhaps JIT some code), so all things being equal, they will launch a bit slower. After that, it depends, and there's no fundamental reason why one language must always be faster than another.

I'm also not aware of any reasons why unsafe C# should be faster than safe. In general, safe is good because it allows the compiler to make some much stronger assumptions, and so safe might be faster. But again, it depends on the code you're compiling, the compiler you're using and a dozen other factors.

In short, give up on the idea that you can measure the performance of a language. You can't. A language is never "fast" or slow". It doesn't have a speed.

Answer 3

it needs to be as fast as possible

Then you're asking the wrong question.

Code it in assembler, with different versions for each significant architecture variant you support.

Use as a guide the output from a good C++ compiler with optimisation, because it probably knows some tricks that you don't. But you'll probably be able to think of some improvements, because C++ doesn't necessarily convey to the compiler all information that might be useful for optimisation. For example, C++ doesn't have the C99 keyword restrict. Although in that particular case many C++ compilers (including MSVC) do now support it, so use it where possible.

Of course if you mean, "I want it to be fast, but not to the extent of going outside C# or C++", then the answer's different ;-)

I would expect C# to at least approach the performance of similar-looking C++ in a lot of cases. I assume of course that the program will be running long enough that the time the JIT itself takes is irrelevant, but if you're processing much video then that seems likely. But I'd also expect there to be certain things which if you do them in unsafe C#, will be far slower than the equivalent thing in C++. I don't know what they are, because all my experience of JITs is in Java rather than CLR. There might also be things which are slower in C++, for instance if your algorithm makes any calls back into C# code.

Unfortunately the only way to be sure how close it is is to write both and test them, which kind of misses the point that writing the C++ version is a bunch of extra effort. However, you might be able to get a rough idea by hacking some quick code which approximates the processing you want to do, without necessarily doing all of it or getting it right. If you algorithm is going to loop over all the pixels and do a few FP ops per pixel, then hacking together a rough benchmark should take all of half an hour.

Usually I would advise against starting out thinking "this needs to be as fast as possible". Requirements should be achievable, and by definition "as X as possible" is only borderline achievable. Requirements should also be testable, and "as X as possible" isn't testable unless you somehow know a theoretical maximum. A more friendly requirement is "this needs to process video frames of such-and-such resolution in real time on such-and-such a speed CPU", or "this needs to be faster than our main competitor's product". If the C# version does that, with a bit to spare to account for unexpected minor issues in the user's setup, then job done.

Answer 4

Running on the CPU is always going to be faster than running on a VM on the CPU. I can't believe people are trying to argue otherwise.

For example, we have some fairly heavy image processing work on our web server that's queued up. Initially to get it working, we used PHP's GD functions.

They were slow as hell. We rewrote the functionality we needed in C++.

Answer 5

C# is typically slower than C++. There are runtime checks in managed code. These are what make it managed, after all. C++ doesn't have to check whether the bounds of an array have been exceeded for example.

From my experience, using fixed memory helps a lot. There is a new System.IO.UnmanagedMemoryAccessor class in .NET 4.0 which may help in the future.

Answer 6

If you are going to implement your algorithm in a standard way I think it's irrelevant. But some languages have bindings to apis or libraries that can give you a non standart boost.

1. Consider if you can use GPU processing - nvidia and ati provide the CUDA and CTM frameworks and there is an ongoing standarization effort from the khronos group (openGL). A hunch tells me also that amd will add at least one streaming processor core in their future chips. So I think there is quite a promise in that area.

2. Try to see if you can exploit SSE instructions, there are libraries around -most in C++ or C- that provide handy apis, check Intel's site for handy optimized libraries I do recall "Intel Performance Primitives" and a "Math Kernel".

But on the politics side, do incorporate your algorithm in OpenCV so others may benefit too.

Answer 7

If you know your environment and you use a good compiler (for video processing on windows, Intel C++ Compiler is probably the best choice), C++ will beat C# hands-down for several reasons:

• The C++ runtime environment has no intrinsic runtime checks (the downside being that you have free reign to blow yourself up). The C# runtime environment is going to have some sanity checking going on, at least initially.
• C++ compilers are built for optimizing code. While it's theoretically possible to implement a C# JIT compiler using all of the optimizing voodo that ICC (or GCC) uses, it's doubtful that Microsoft's JIT will reliably do better. Even if the JIT compiler has runtime statistics, that's still not as good as profile-guided optimization in either ICC or GCC.
• A C++ environment allows you to control your memory model much better. If your application gets to the point of thrashing the data cache or fragmenting the heap, you'll really appreciate the extra control over allocation. Heck, if you can avoid dynamic allocations, you're already much better off (hint: the running time of malloc() or any other dynamic allocator is nondeterministic, and almost all non-native languages force heavier heap usage, and thus heavier allocation).

If you use a poor compiler, or if you can't target a good chipset, all bets are off.

Answer 8

I'm a little late in responding but I can give you some anecdotal experience. We had some matrix multiplication routines that were originally coded in C# using pointers and unsafe code. This proved to be a bottleneck in our application and we then used pinning+P/Invoke to call into a C++ version of the Matrix multiplication routine and got a factor of 2 improvement. This was a while ago with .NET 1.1, so things might be better now. As others point out, this proves nothing, but it was an interesting exercise.

I also agree with thAAAnos, if you algorithm really has to be "as fast as possible" leverage IPL or, if you must, consider a GPU implementation.

Answer 9

To be honest, what language you write it in is not nearly as important as what algorithms you use (IMO, anyway). Maybe by going to native code you might make your application faster, but it might also make it slower--it'd depend on the compiler, how the programs are written, what sort of interop costs you'd incur if you're using a mixed environment, etc. You can't really say without profiling it. (and, for that matter, have you profiled your application? Do you actually know where it's spending time?)

A better algorithm is completely independent of the language you choose.