Performance of Managed C++ Vs UnManaged/native C++

Question

I am writing a very high performance application that handles and processes hundreds of events every millisecond.

Is Unmanaged C++ faster than managed c++? and why?

Managed C++ deals with CLR instead of OS and CLR takes care of memory management, which simplifies the code and is probably also more efficient than code written by "a programmer" in unmanaged C++? or there is some other reason? When using managed, how can one then avoid dynamic memory allocation, which causes a performance hit, if it is all transparent to the programmer and handled by CLR?

So coming back to my question, Is managed C++ more efficient in terms of speed than unmanaged C++ and why?

Answer 1

There is no one answer to this. As a really general rule, native code will usually be faster, but 1) that's not always the case, 2) sometimes the difference is too small to care about, and 3) how well the code is written will usually make more difference than managed vs. unmanaged.

Managed code runs in a virtual machine. Basically, you start with a compiler that produces byte codes as output, then feed that to the virtual machine. The virtual machine then re-compiles it to machine code and executes that. This can provide some real advantages under some circumstances. For one example, if you have a 64-bit processor running a 64-bit VM (pretty nearly a given any more) but and old program written before 64-bit processors were common, the VM will still compile that byte code to 64-bit machine code, which can give quite a substantial speed advantage for at least some code.

At the same time, it can also be a fairly noticeable disadvantage for some code. In particular, the compiler is running while the user waits. To accommodate that, the VM's compiler can't itself run very slowly. Although native code generators differ, there's a pretty fair chance that whatever native compiler you choose includes at least a few optimizations that were foregone in the VM's bytecode compiler to keep its resource usage reasonable.

The VM also uses a garbage collector. Garbage collectors have rather different characteristics from manually managing memory. With many manual managers, allocating memory is fairly expensive. Releasing memory is fairly cheap, but roughly linear on the number of items you release. Other manual managers roughly reverse that, doing extra work when freeing memory in order to make allocation faster. Either way, the cost structure is different from a typical collector.

With a garbage collector, allocating memory is typically very cheap. With a typical (copying) collector, the cost of releasing memory depends primarily upon the number of objects that have been allocated and are still (at least potentially) in use.

The allocations themselves also differ though. In native C++, you typically create most objects on the stack, where both allocating and releasing memory is extremely cheap. In managed code, you typically allocate a much larger percentage of memory dynamically, where it's garbage collected.

Answer 2

You can write slow code in any language; conversely, you can use decent algorithms that may well be fast is almost any language.

The common answer here would be to pick a language that you already know, use appropriate algorithms, then profile the heck out of it to determine the actual hot spots.

I am somewhat concerned about the hundreds of events every millisecond statement. That's an awful lot. Are you reasonably going to be able to do the processing you expect in any language?

As a C++ developer on high-performance systems, I tend to trust my ability to profile and optimize the emitted code. That said; there are very high performance .NET applications, where the writer has gone to great lengths to not do dynamic memory allocation inside the critical loops - mostly by using allocated pools of objects created beforehand.

So to repeat my previous comment: pick what you already know, then tune. Even if you hit a dead end; you will likely know much more about your problem space.

Answer 3

It all depends on the situation.

Things that make unmanaged code faster / managed code slower:

• the code needs to be converted to machine code before it can be executed
• garbage collection might cause an overhead
• calls from managed to unmanaged code have a serious overhead
• unmanaged compilers can optimize more since they directly generate machine code (seen myself)

Things that make managed code faster / unmanaged code slower:

• since the code is converted to machine code right before it's used, managed code can be optimized for the actual processor (with unmanaged code you have to target the 'minimum-supported' processor).

And probably there are many more reasons.

Answer 4

Managed code is in most cases slower than Unmanaged code, even though the .Net CLR always does a JIT-compilation before executing the code (it is not compiled multiple times while the program is running but it well never interpret the code).

The problem is rather with many checks the CLR does, e.g. to see if you run over the bounds of an array whenever you try to access it. This leads to fewer problems with buffer overflows, etc. but also means a performance hit due to the added overhead of those checks.

I've seen experiments where C# outperformed C++ but those were conducted with code taking heavily advantage of object hierarchies, etc. When it comes down to number crunching and you want to get the most out of your PC you will have to go with unmanaged code.

Another point was also already mentioned - the GC leads to somewhat unpredictable pauses in the programs execution when memory must be freed. You need this time as well when doing memory management in unmanaged code but it occurs more often and whenever you decide to destroy an object which means its not all done at once for the whole program, so you don't have a long pause.