Allocate and acccess 512MB of memory using 512 1MB VirtualAllocs

test.cpp

#include <stdio.h>
#include <stdint.h>
#include <windows.h>

static int64_t timer()
{
   LARGE_INTEGER li;
   QueryPerformanceCounter(&li);
   return li.QuadPart;
}

static int64_t timer_freq()
{
   LARGE_INTEGER li;
   QueryPerformanceFrequency(&li);
   return li.QuadPart;
}

volatile uint32_t g_sink;

static const int size = 512 << 20;
#if 1
static const int num_allocs = 512;
static unsigned char *allocs[num_allocs];
#else
static const int num_allocs = 1;
static unsigned char global_buf[size];
static unsigned char *allocs[1] = { global_buf };
#endif

static void time_it()
{
   int64_t start = timer();
   int size_per_alloc = size / num_allocs;

   for (int i = 0; i < num_allocs; i++)
   {
      if (!allocs[i])
         allocs[i] = (unsigned char *)VirtualAlloc(NULL, size_per_alloc, MEM_COMMIT, PAGE_READWRITE);

      unsigned char *p = allocs[i];

      // access every cache line once
      for (int i = 0; i < size_per_alloc; i += 64)
         g_sink += p[i];
   }
   int64_t end = timer();

   int64_t freq = timer_freq();
   double duration = 1.0 * (end - start) / freq;

   printf("%.1fms (%.2f GB/s)\n", 1000.0 * duration, size / (1048576.0 * 1024.0 * duration));
}

int main()
{
   for (int run = 0; run < 5; run++)
   {
      printf("run=%d ", run);
      time_it();
   }

   return 0;
}