CUDA - operations on single elements of a matrix - getting ideas

I'm about writing a CUDA kernel to perform a single operation on every single element of a matrix (e.g. squarerooting every element, or exponentiation, or calculating the sine/cosine if all the numbers are between [-1;1], etc..)

I chose the blocks/threads grid dimensions and I think the code is pretty straightforward and simple, but I'm asking myself... what can I do to maximize coalescence/SM occupancy?

My first idea was: making all semiwarp (16 threads) load data ensemble from global memory and then putting them all to compute, but it finds out that there are no enough memory-transfer/calculations parallelization.. I mean all threads load data, then compute, then load again data, then calculate again.. this sounds really poor in terms of performance.

I thought using shared memory would be great, maybe using some sort of locality to make a thread load more data than it actually needs to facilitate other threads' work, but this sounds stupid too because the second woul开发者_运维技巧d wait for the former to finish loading data before starting its work.

I'm not really sure I gave the right idea regarding my problem, I'm just getting ideas before commencing to work on something concrete.

Every comment/suggestion/critic is well accepted, and thanks.

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If you have defined the grid so that threads read along the major dimension of the array containing your matrix, then you have already guaranteed coalesced memory access, and there is little else to be done to improve performance. These sort of O(N) complexity operations really do not contain sufficient arithmetic intensity to give good parallel speed up over an optimized CPU implementation. Often the best strategy is to fuse multiple O(N) operations together into a single kernel to improve the FLOP to memory transaction ratio.

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In my eyes your problem is this

load data ensemble from global memory

It seems that your algorithm idea is:

1. Do something on cpu - have some matrix
2. Transfer matrix from global to device memory
3. Perform your operation on every element
4. Transfer matrix back from device to global memory
5. Do something else on cpu - go sometimes back 1.

This kind of computations are almost everytime I/O-bandwidth limited (IO = memory IO), not computation power limited. GPGPU computations can sustain a very high memory bandwidth - but only from device memory to the gpu - transfer from global memory goes always over the very slow PCIe (slow compared to the device memory connection, that can deliver up to 160 GB/s + on fast cards). So one main thing to get good results is to keep the data (matrix) in device memory - preferable generate it even there if possible (depends on your problem). Never try to migrate data between cpu and gpu for and back as the transfer overhead eats all your speedup up. Also keep in mind that your matrix must have a certain size to amortize the transfer overhead, that you cant avoid (to compute a matrix with 10 x 10 elements would bring almost nothing, heck it would even cost more)

The interchanging transfer/compute/transfer is full ok, thats how such gpu algorithms work - but only if the the tranfer is from device memory.

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The GPU for something this trivial is overkill and will be slower than just keeping it on the CPU. Especially if you have a multicore CPU.

I have seen many projects showing the "great" advantages of the GPU over the CPU. They rarely stand up to scrutiny. Of course, goofy managers who want to impress their managers want to show how "leading edge" his group is.

Someone in the department toils months on getting silly GPU code optimized (which is generally 8x harder to read than equivalent CPU code), then have the "equivalent" CPU code written by some Indian sweat shop (the programmer whose last project was PGP), compile it with the slowest version of gcc they can find, with no optimization, then tout their 2x speed improvement. And BTW, many overlook I/O speed as somehow not important.