/*
 * You can explore how reserved and committed memory appear in TaskManager, PerfMon and so on 
 * with the following small program. This uses the Windows VirtualAlloc call to first reserve, 
 * and then commit memory. Finally it starts to touch the committed memory, writing a value 
 * into it every 4096 bytes, that is, into every memory page. This brings the pages into the 
 * working set. If you run this program and watch it with Task Manager or PerfMon, 
 * you will see how the committed and reserved memory belonging to the process changes.
 * For example when this program commits memory you will see the Task Manager VMSize value go up; 
 * as it touches memory you will see the Task Manager MemUsage value go up. 
 *
 * This program has been compiled with Microsoft Visual Studio's C compiler as follows:
 * cl experiment.c
 * This generates experiment.exe.
 */


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

 
#define MB_TO_RESERVE 200   /* reserving  200 MB */
#define MB_TO_COMMIT  100   /* committing 100 MB */
#define MB_TO_TOUCH    30   /* touching    30 MB */
#define PAGE_SIZE    4096   /* page size is almost always 4K    */
                            /* could use GetSystemInfo to check */
void main () {

  void * reserved_p;    /* the starting address of the block we shall reserve */
  void * committed_p;   /* the starting address of the block we shall commit  */ 
  int mb;               /* will be used to iterate through memory             */
  
  /****************************************************************************
   * reserve a stretch of the address space
   ****************************************************************************/
  reserved_p = VirtualAlloc(NULL,
                            MB_TO_RESERVE*1024*1024, 
                            MEM_RESERVE, 
                            PAGE_READWRITE);
  printf("reserved a stretch of the address space...press enter to commit...\n");
  getchar();

  /****************************************************************************
   * commit part of the reserved address space
   ****************************************************************************/
  committed_p = VirtualAlloc(reserved_p,
                             MB_TO_COMMIT*1024*1024,
                             MEM_COMMIT,
                             PAGE_READWRITE);
  printf("committed part of the reserved address space...press enter to touch...\n");
  getchar();

  /*****************************************************************************
   * gradually touch more and more of the committed memory
   *****************************************************************************/
  printf("gradually touching more and more of the committed memory...\n");
  for (mb = 1; mb < MB_TO_TOUCH ; mb++) {
    char * p;	// used to iterate down committed pages, touching them all
    printf("...touching %d MB...\n",mb);

    /***************************************************************************
     * march down memory touching every page
     ***************************************************************************/
    for (p = committed_p; p < (char*)committed_p + mb * 1024 * 1024; p += PAGE_SIZE) {
      *p = 'x';
    }

    Sleep(1000);
  }
  printf("...finished\n");

}