c - Is it possible to unpage all memory in Windows?

Question

I have plenty of RAM, however, after starting and finishing a large number of processes, it seems that most of the applications' virtual memory has been paged to disk, and switching to any of the older processes requires a very long time to load the memory back into RAM.

Is there a way, either via Windows API or via kernel call, to get Windows to unpage all (or as much as possible) memory? Maybe by stepping through the list of running processes and get the memory manager to unpage each process's memory?

Answer 1

Update 3: I've uploaded my complete program to github.

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OK, based on the replies so far, here's a naive suggestion for a tool that tries to get all applications back into physical memory:

1. Allocate a small chunk of memory X, maybe 4MB. (Should it be non-pageable?)
2. Iterate over all processes:
   • For each process, copy chunks of its memory to X. (Possibly suspending the process first?)

Suppose you have 2GB of RAM, and only 1GB is actually required by processes. If everything is in physical memory, you'd only copy 256 chunks, not the end of the world. At the end of the day, there's a good chance that all processes are now entirely in the physical memory.

Possible convenience and optimisation options:

• Check first that the total required space is no more than, say, 50% of the total physical space.
• Optionally only run on processes owned by the current user, or on a user-specified list.
• Check first whether each chunk of memory is actually paged to disk or not.

I can iterate over all processes using EnumProcesses(); I'd be grateful for any suggestions how to copy an entire process's memory chunk-wise.

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Update: Here is my sample function. It takes the process ID as its argument and copies one byte from each good page of the process. (The second argument is the maximal process memory size, obtainable via GetSystemInfo().)

void UnpageProcessByID(DWORD processID, LPVOID MaximumApplicationAddress, DWORD PageSize)
{
  MEMORY_BASIC_INFORMATION meminfo;
  LPVOID lpMem = NULL;

  // Get a handle to the process.
  HANDLE hProcess = OpenProcess(PROCESS_QUERY_INFORMATION | PROCESS_VM_READ, FALSE, processID);

  // Do the work
  if (NULL == hProcess )
  {
    fprintf(stderr, "Could not get process handle, skipping requested process ID %u.
", processID);
  }
  else
  {
    SIZE_T        nbytes;
    unsigned char buf;

    while (lpMem < MaximumApplicationAddress)
    {
      unsigned int stepsize = PageSize;

      if (!VirtualQueryEx(hProcess, lpMem, &meminfo, sizeof(meminfo)))
      {
        fprintf(stderr, "Error during VirtualQueryEx(), skipping process ID (error code %u, PID %u).
", GetLastError(), processID);
        break;
      }

      if (meminfo.RegionSize < stepsize) stepsize = meminfo.RegionSize;

      switch(meminfo.State)
      {
      case MEM_COMMIT:
        // This next line should be disabled in the final code
        fprintf(stderr, "Page at 0x%08X: Good, unpaging.
", lpMem);

        if (0 == ReadProcessMemory(hProcess, lpMem, (LPVOID)&buf, 1, &nbytes))
          fprintf(stderr, "Failed to read one byte from 0x%X, error %u (%u bytes read).
", lpMem, GetLastError(), nbytes);
        else
          // This next line should be disabled in the final code
          fprintf(stderr, "Read %u byte(s) successfully from 0x%X (byte was: 0x%X).
", nbytes, lpMem, buf);

        break;
      case MEM_FREE:
        fprintf(stderr, "Page at 0x%08X: Free (unused), skipping.
", lpMem);
        stepsize = meminfo.RegionSize;
        break;
      case MEM_RESERVE:
        fprintf(stderr, "Page at 0x%08X: Reserved, skipping.
", lpMem);
        stepsize = meminfo.RegionSize;
        break;
      default:
        fprintf(stderr, "Page at 0x%08X: Unknown state, panic!
", lpMem);
      }

      //lpMem = (LPVOID)((DWORD)meminfo.BaseAddress + (DWORD)meminfo.RegionSize);
      lpMem += stepsize;
    }
  }

  CloseHandle(hProcess);
}

Question: Does the region by whose size I increment consist of at most one page, or am I missing pages? Should I try to find out the page size as well and only increment by the minimum of region size and page size? Update 2: Page size is only 4kiB! I changed the above code to increment only in 4kiB steps. In the final code we'd get rid of the fprintf's inside the loop.