Lab 5: Data Locality and Cache Blocking

Updated September 27th, 9:23 AM


Run cp -R ~cs61c/labs/05 . to copy this lab to your current directory.

Background Information

Matrix Multiplication

If you recall, matrices are 2-dimensional data structures wherein each data element is accessed via two indices. To multiply two matrices, we can simply use 3 nested loops, assuming that matrices A, B, and C are all n-by-n and stored in one-dimensional column-major arrays:

    for (int i = 0; i < n; i++)
        for (int j = 0; j < n; j++)
            for (int k = 0; k < n; k++)
                C[i+j*n] += A[i+k*n] * B[k+j*n];

Matric multiplication operations are at the heart of many linear algebra algorithms, and efficient matrix multiplication is critical for many applications within the applied sciences.

In the above code, note that the loops are ordered i, j, k. Thus, considering the innermost "k" loop, we move through B with stride 1, A with stride n and C with stride 0. To compute the matrix multiplication correctly, the loop order doesn't matter. However, how we choose to stride though the matrices can have a large impact on performance as it can break the assumption of spatial locality that is critical for good cache performance.

Matrix Transposition

Sometimes, we wish to swap the rows and columns of a matrix. This operation is called a "transposition" and an efficient implementation can be quite helpful while performing more-complicated linear algebra operations. The transpose of matrix A is often denoted as AT.

Cache Blocking

In the above code for matrix multiplication, note that we are striding across the entire matrices to compute a single value of C. As such, we are constantly accessing new values from memory and obtain very little reuse of cached data! We can improve the amount of data reuse in cache by implementing a technique called cache blocking. More formally, cache blocking is a technique that attempts to reduce the cache miss rate by improving the temporal and/or spatial locality of memory accesses. In the case of matrix transposition we consider completing the transposition one block at a time.

In the above image, each block Aij of matrix A is transposed into its final location in the output matrix. With this scheme, we significantly reduce the magnitude of the working set in cache at any one point in time. This (if implemented correctly) will result in a substantial improvement in performance. For this lab, you will implement a cache blocking scheme for matrix transposition and analyze its performance. As a side note, you will be required to implement several levels of cache blocking for matrix multiplication for Project 3.

Exercise 1: Matrix multiply

Take a glance at matrixMultiply.c. You'll notice that the file contains multiple implementations of matrix multiply with 3 nested loops. Compile and run this code with the following command:     make ex1

Note that it is important here that we use the '-O3' flag to turn on compiler optimizations. The makefile will run matrixMultiply twice. Copy the results somewhere so that you do not have to run this program again and use them to help you answer the following questions:

  1. Why does performance drop for large values of n?
  2. The second run of matrixMultiply runs all 6 different loop orderings. Which ordering(s) perform best for 1000-by-1000 matrices? Which ordering(s) perform the worst? How does the way we stride through the matrices with respect to the innermost loop affect performance?

Checkoff: Be prepared to explain your responses to the above questions to your TA.

Exercise 2: Matrix transposition

Compile and run the naive matrix transposition implemented in transpose.c (make sure to use the '-O3' flag: gcc -o transpose -O3 transpose.c).

  1. Note the time required to perform the naive transposition for a matrix of size 2000-by-2000.
  2. Modify the function called transpose in transpose.c to implement a single level of cache blocking. That is, loop over all matrix blocks and transpose each into the destination matrix. Make sure to handle special cases of the transposition: What if we tried to transpose the 5-by-5 matrix above with a blocksize of 2?
  3. Try block sizes of 2-by-2, 100-by-100, 200-by-200, 400-by-400 and 1000-by-1000. Which performs best on the 2000-by-2000 matrix? Which performs worst?

Checkoff: Show and explain your code to your TA. Answer the questions in part c above and then run your code with a block size of 30-by-30 to verify your algorithm works.