How can I avoid a for-loop and speed up my code

RaviBee

I would consider using a 3rd party solution (eg: Extreme Optimization[^]) who offer a vector and matrix library[^] that provides a method[^] to do exactly this. (I'm not affiliated with ExtremeOptimization). Open source alternatives may also exist. /ravi

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Dan Neely

Behind the scenes, Matlab is creating a for loop for you. If you want a faster implementation that avoids the .net bounds checking, you could try writing the innermost layer of your code in c++ (with optional processor specific asm optimizations to use MMX/SSE instructions) and then using pinvoke to call it from the main c# portion of your app.

-- Rules of thumb should not be taken for the whole hand.

Luc Pattyn

There always will be a loop either in your code, or in code you call. 100K samples will not be processed by a linear sequence of instructions ! You can improve performance by using pointers; this is possible in C# too, it is considered unsafe, and requires the "unsafe" keyword (and a compiler switch to enable it). Look in Visual Studio's help for "unsafe code, sample [C#]" :)

Luc Pattyn

Judah Gabriel Himango

Unsafe C# can get around bounds checking as well, and would be a lot easier that having to code up a separate C++ lib and invoke that from C#.

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Judah Gabriel Himango

One way to speed up things like this on multi-processor or multi-core machines is to use multiple threads to do the processing. Say you have one of those new Intel Quad Core processors, you've got 4 hardware threads available. Split the array into 4 segments, then spawn 3 new threads, each the processes a segment of its own. Then have the current thread process a segment of its own (4 threads total). You've essentially increased performance 4x. Of course, this solution only works for processors that are hyperthreaded (i.e. 2 hardware threads per physical processor core) or for machines that have multiple processors or multiple cores. p.s. one should always question optimizing code like this unless you're absolutely, positively certain that there is a performance bottleneck here and the current performance is not acceptable.

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Luc Pattyn

For a job as simple as an inner product, I expect bus bandwidth limitations will be dominant over CPU limitations, so no much help from multi-threading... I do fully agree with the p.s. though :)

Luc Pattyn

Dan Neely

True, but IIRC native code is still faster than unsafe.

-- Rules of thumb should not be taken for the whole hand.

Judah Gabriel Himango

I'd be surprised at that. And given the P/Invoke overhead that would be required with a C++ lib on the side, unsafe C# may quite well out-perform.

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Judah Gabriel Himango

The job is simple, but the bottleneck is the linear time it takes to compute one operation, move onto the next, until finished. Meanwhile, potentially 1 or more cores are idle and could be doing these operations in the meantime. I'm quite certain you'd see a good speed up here. The MS Robotics team that built the CCR (concurrency and coordination runtime, a .NET library for threading and coordination among threads) found big speedups, often near a multiple of the number of cores in a machine, by utilizing multiple threads to do this kind of thing. Given, they are doing lots of IO, however. Joe Duffy, a CLR architect, is busy working on the PLinq (Parallel Language Integrated Query) project that will allow devs to easily parallelize queries and transformations on data. This is essentially the technique I described above: using 1 thread per hardware thread to parallelize queries and transformations on data. Eric Sink has an article[^] on his blog showing how C# can do Map, which utilizes this idea.

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Luc Pattyn

Sure, I believe multithreading can be great for any single job that is compute bound (including Map, the CCR stuff, and much more), as well as for most situations where a multitude of jobs come together. But my point is multiplying two arrays isnt much more than a data mover. And I expect the loop overhead will mostly be dealt with by the CPU's out-of-order capabilities. So lets wait and see. :)

Luc Pattyn

Dan Neely

I'm not sure if this counts or not, there'e //unsafe comments in the C# source, but I thought you needed an unsafe keyword as well which I didn't see. It does have c++ scoring better in most math related benchmarks. Bencharks[^]

-- Rules of thumb should not be taken for the whole hand.

Insincere Dave

There is a library at Microsoft Research called Accelerator that might be worth investigating. Accelerator provides a high-level data-parallel programming model as a library that is available for all .Net programming languages. The library translates the data-parallel operations on-the-fly to optimized GPU pixel shader code and API calls. Future versions will target multi-core cpus. Download, Channel9 Video

Judah Gabriel Himango

Yeah, it looks like he commented the unsafe portions, meaning he's back to normal array bounds checking and all that. I wonder why?

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lost in transition

Bet you didn't know you would get all this 'feed back' did you:) Just remember that no matter what you will still have the loop. So, think of it like this do I have a loop here with a little code or do I have a loop somewhere else with extra code to get there? Your call, but these guys are giving some real good knowledge that you should diffently try to learn. Good Luck, Jason

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Ennis Ray Lynch Jr

You can multithread the multiplication if the arrays are large enough. Another option is to drop into unsafe code and use pointer arithmetic. Antoher option is to use 64 bit multiplication but you might need a bitwise transform on the result. (Check out the the assembly for strlen for a non-application example) I don't know about intel chips but some processors provide op codes for array based operations. Check the MMX instruction set and you may be able to multiply the entire set in two or three op codes. http://web.cs.wpi.edu/~matt/courses/cs563/talks/powwie/p3/mmx.htm

On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question. - Charles Babbage

ComCoderCsharp

Hi coders Just wanted to say thanks for all your many inputs - As expected there seems to be no magic answer to this quistion, but I will take a closer look at using pointers - I did not know this was possible in C# so thanks alot... AL