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강성진 : What is Memory Coalescing

Memory coalescing is a technique used in computer memory management to improve memory bandwidth and reduce memory access latency.

Memory coalescing occurs when multiple memory access requests are combined into a single, larger request, so that the memory can be accessed more efficiently. This can reduce the number of memory transactions, which can help to reduce memory access latency and increase memory bandwidth.

Memory coalescing is often used in high-performance computing and graphics applications, where the processing of large amounts of data requires efficient memory access. By combining multiple memory requests into a single, larger request, memory coalescing can help to reduce the number of memory transactions and improve memory performance.

Memory coalescing is typically performed by the memory controller, which is the component in the computer system that manages access to memory. The memory controller can track memory access requests and combine them into larger requests to optimize memory performance. The technique can also be used in parallel processing systems, where multiple processors access shared memory simultaneously, to reduce contention and improve performance.

강성진 : Show me the example that utilize memory coalescing in c code

Here is an example in C that demonstrates the use of memory coalescing to improve memory bandwidth:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define ARRAY_SIZE 1000000

void sum(float *a, float *b, float *c) {
  int i;
  for (i = 0; i < ARRAY_SIZE; i++) {
    c[i] = a[i] + b[i];
  }
}

int main() {
  float *a, *b, *c;
  int i;

  a = (float*)malloc(ARRAY_SIZE * sizeof(float));
  b = (float*)malloc(ARRAY_SIZE * sizeof(float));
  c = (float*)malloc(ARRAY_SIZE * sizeof(float));

  for (i = 0; i < ARRAY_SIZE; i++) {
    a[i] = sinf(i);
    b[i] = cosf(i);
  }

  sum(a, b, c);

  free(a);
  free(b);
  free(c);

  return 0;
}

In this example, the function sum calculates the sum of two arrays a and b, and stores the result in the array c. The arrays are allocated using the malloc function, and the values in the arrays are initialized in the main function.

By accessing the arrays a, b, and c in a contiguous manner, this code can benefit from memory coalescing. The memory controller can combine multiple memory access requests into a single, larger request, which can reduce the number of memory transactions and improve memory bandwidth.

Note that the exact behavior of memory coalescing can vary depending on the specific memory architecture and memory controller implementation. However, accessing arrays in a contiguous manner is generally a good practice for optimizing memory performance.