As part of our API tour of C++ AMP, we looked recently at parallel_for_each. I ended that post by saying we would revisit parallel_for_each after introducing array and array_view. Now is
the time, so this is part 2 of parallel_for_each, and also a post that brings together everything we've seen until now.
The code for serial and accelerated Consider a naïve (or brute force) serial implementation of matrix multiplication 0: void MatrixMultiplySerial(std::vector<float>& vC,
const std::vector<float>& vA,
const std::vector<float>& vB, int M, int N, int W)
1: {
2: for (int row = 0; row < M; row++)
3: {
4: for (int col = 0; col < N; col++)
5: {
6: float sum = 0.0f;
7: for(int i = 0; i < W; i++)
8: sum += vA[row * W + i] * vB[i * N + col];
9: vC[row * N + col] = sum;
10: }
11: }
12: }
We notice that each loop iteration is independent from each other and so can be parallelized. If in addition we have really large amounts of data, then this is a good candidate to offload to an accelerator. First, I'll just show you an example of what that code may look like with C++ AMP, and then we'll analyze it. It is assumed that you included at
the top of your file #include <amp.h>
13: void MatrixMultiplySimple(std::vector<float>& vC,
const std::vector<float>& vA,
const std::vector<float>& vB, int M, int N, int W)
14: {
15: concurrency::array_view<const float,2> a(M, W, vA);
16: concurrency::array_view<const float,2> b(W, N, vB);
17: concurrency::array_view<concurrency::writeonly<float>,2> c(M, N, vC);
18: concurrency::parallel_for_each(c.grid,
19: [=](concurrency::index<2> idx) restrict(direct3d) {
20: int row = idx[0]; int col = idx[1];
21: float sum = 0.0f;
22: for(int i = 0; i < W; i++)
23: sum += a(row, i) * b(i, col);
24: c[idx] = sum;
25: });
26: }
First a visual comparison, just for fun:
The beginning and end is
the same, i.e. lines 0,1,12 are identical to lines 13,14,26.
The double nested loop (lines 2,3,4,5 and 10,11) has been transformed into a parallel_for_each call (18,19,20 and 25).
The core algorithm (lines 6,7,8,9) is essentially
the same (lines 21,22,23,24). We have extra lines in
the C++ AMP version (15,16,17). Now let's dig in deeper.
Using array_view and extent
When we decided to convert this function to run on an accelerator, we knew we couldn't use
the std::vector objects in
the restrict(direct3d) function. So we had a choice of copying
the data to
the the concurrency::array<T,N> object, or wrapping
the vector container (and hence its data) with a concurrency::array_view<T,N> object from amp.h – here we used
the latter (lines 15,16,17). Now we can access
the same data through
the array_view objects (a and b) instead of
the vector objects (vA and vB), and
the added benefit is that we can capture
the array_view objects in
the lambda (lines 19-25) that we pass to
the parallel_for_each call (line 18) and
the data will get copied on demand for us to
the accelerator.
Note that line 15 (and ditto for 16 and 17) could have been written as two lines instead of one:
extent<2> e(M, W);
array_view<const float, 2> a(e, vA);
In other words, we could have explicitly created
the extent object instead of letting
the array_view create it for us under
the covers through
the constructor overload we chose.
The benefit of
the extent object in this instance is that we can express that
the data is indeed two dimensional, i.e a matrix. When we were using a vector object we could not do that, and instead we had to track via additional unrelated variables
the dimensions of
the matrix (i.e. with
the integers M and W) – aren't you loving C++ AMP already?
Note that
the const before
the float when creating a and b, will result in
the underling data only being copied to
the accelerator and not be copied back – a nice optimization. A similar thing is happening on line 17 when creating array_view c, where we have indicated that we do not need to copy
the data to
the accelerator, only copy it back.
The kernel dispatch
On line 18 we make
the call to
the C++ AMP entry point (parallel_for_each) to invoke our parallel loop or, as some may say, dispatch our kernel.
The first argument we need to pass describes how many threads we want for this computation. For this algorithm we decided that we want exactly
the same number of threads as
the number of elements in
the output matrix, i.e. in array_view c which will eventually update
the vector vC. So each thread will compute exactly one result. Since
the elements in c are organized in a 2-dimensional manner we can organize our threads in a two-dimensional manner too. We don't have to think too much about how to create
the first argument (a grid) since
the array_view object helpfully exposes that as a property. Note that instead of c.grid we could have written grid<2>(c.extent) or grid<2>(extent<2>(M, N)) –
the result is
the same in that we have specified M*N threads to execute our lambda.
The second argument is a restrict(direct3d) lambda that accepts an index object. Since we elected to use a two-dimensional extent as
the first argument of parallel_for_each,
the index will also be two-dimensional and as covered in
the previous posts it represents
the thread ID, which in our case maps perfectly to
the index of each element in
the resulting array_view.
The kernel itself
The lambda body (lines 20-24), or as some may say,
the kernel, is
the code that will actually execute on
the accelerator. It will be called by M*N threads and we can use those threads to index into
the two input array_views (a,b) and write results into
the output array_view ( c ).
The four lines (21-24) are essentially identical to
the four lines of
the serial algorithm (6-9).
The only difference is how we index into a,b,c versus how we index into vA,vB,vC.
The code we wrote with C++ AMP is much nicer in its indexing, because
the dimensionality is a first class concept, so you don't have to do funny arithmetic calculating
the index of where
the next row starts, which you have to do when working with vectors directly (since they store all
the data in a flat manner).
I skipped over describing line 20. Note that we didn't really need to read
the two components of
the index into temporary local variables. This mostly reflects my personal choice, in some algorithms to break down
the index into local variables with names that make sense for
the algorithm, i.e. in this case row and col. In other cases it may i,j,k or x,y,z, or M,N or whatever. Also note that we could have written line 24 as: c(idx[0], idx[1])=sum or c(row, col)=sum instead of
the simpler c[idx]=sum
Targeting a specific accelerator
Imagine that we had more than one hardware accelerator on a system and we wanted to pick a specific one to execute this parallel loop on. So there would be some code like this anywhere before line 18:
vector<accelerator> accs = MyFunctionThatChoosesSuitableAccelerators();
accelerator acc = accs[0];
…and then we would modify line 18 so we would be calling another overload of parallel_for_each that accepts an accelerator_view as
the first argument, so it would become:
concurrency::parallel_for_each(acc.default_view, c.grid,
...and
the rest of your code remains
the same… how simple is that?
Comments about this post by Daniel Moth welcome at
the original blog.
