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

In order of speed and power the asm > C > C++ >= C++/CLI > C# >= all others. But creating a web service in asm is a long pain. Then use the right langage for the right job and the right audience to do the best job, in the given time.

Answer 2

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 3

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.

Answer 4

Write fast code, is always a pain. The main issue that you can optimize just for one platform. This is really a case on Console, Embedded or other platform where Hardware is always the same. In real PC world this isnt the case. Different core, different istruction ecc ... make this a nightmare. This is the main issue, imho, that really make difference between man/unam code. Man. code can be optimistic optimizable for the new platform when its run. Unman code not, is written into the stone.

Answer 5

First, your statement "processes hundreds of events every millisecond." sounds quite unrealistic. Unless you have a special designed clock module in the computer, I don not think that you can achieve the goal with a generic PC (typical resolution is around 10 milliseconds). Secondly, Native C++ is vast better in terms of performance. There are a lot of optimization can be taken in term of C++ to speed up, while in managed code they are not possible. Also be aware that the garbage collection in managed code makes performance unpredictable -when GC fires up the whole process gets frozen. Once you run into the problem, the solution is more painful, now all the "nice style" offered by managed code will be gone.

As for the ability that manage code can optimize for CPU, it is true but you can take advantage of CPU features (SSE2, MMX etc.) in native C++ too. Based on my experience, the performance boost is negligible.

Answer 6

There are many good answers here, but one aspect of managed code that may give it an advantage in the long term is runtime analysis. Since the code generated by the managed compiler is an intermediate format, the machine code that actually executes can be optimized based on actual usage. If a particular subset of functionality is heavily used, the JIT'er can localize the machine code all on the same memory page, increasing locality. If a particular sub-call is made repeatedly from a particular method, a JIT'er can dynamically inline it.

This is an improvement over unmanaged code, where inlining must be "guessed" ahead of time, and excessive inlining is harmful because it bloats code size and causes locality issues that cause (very time-expensive) L2/L1 cache misses. That information is simply not available to static analysis, so it is only possible in a JIT'ing environment. There's a goody basket of possible wins from runtime analysis such as optimized loop unwinding, etc.

I'm not claiming the .NET JIT'er is as smart as it could be, but I know I've heard about global analysis features and I know a lot of research into runtime analysis has been done at Hewlett-Packard and other companies.

Answer 7

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 8

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 9

Isn't C++/CLI a half interpreted language like Java?

Also, didn't someone post a study just yesterday that showed that GC systems are always slower than non GC?