Re: XP increased processor utilization

"Doug Harrison [MVP]" <>
Tue, 15 Apr 2008 17:44:06 -0500
On Mon, 14 Apr 2008 08:45:01 -0700, FLevel8 <FLevel8@nospam.nospam> wrote:

I stumbled across the following this weekend:

This guy sounds like he has a "very" similar problem and code environment to
what I'm seeing. This led me down the path of checking the OnIdle processing
of the CWinThread class.

I tried overridding the IsIdleMessage in my main app as follows:
 BOOL DDG_STUI::IsIdleMessage(MSG* pMsg)
    if (!CWinApp::IsIdleMessage(pMsg) || pMsg->message == WM_TIMER ||
        pMsg->message == WM_MOUSEMOVE)
       return FALSE;
       return TRUE;

I lifted the bulk of this code from the MSDN CWinThread::IsIdleMessage page
but with a little trial and error (and Spy++) added the WM_MOUSEMOVE to the
condition also. This seems to fix the process utilization issue when the
mouse is moving. It does not resolve the general across the board increase
in processor utilization but I'm guessing the OnIdle processing is still the

I couldn't find a response to the question from the link above though it
appears his analysis is fairly accurate. Anyone here have any thoughts on
why XP would handle the SendMessageToDescendants of the WM_IDLEUPDATECMDUI
message so poorly compared to 2K?

I don't know how many windows you have, but the message you linked to
talked about several hundred. The "excessive-mouse" guy is implicitly doing
a lot of SendMessage in the idle time handler, and it appears to me that
::SendMessage is a lot less efficient in XP SP2 than in Vista (and
presumably 2K as well). After a lot of experimenting to rule out slowness
in things like the SendMessageToDescendents looping, I finally ended up

BOOL CMdi_btnApp::OnIdle(LONG lCount)
#if 1
    if (CWnd* pMainWnd = AfxGetMainWnd())
        static HWND hWnd;
        if (hWnd == 0)
            static CButton btn;
            btn.Create(_T("1"), BS_PUSHBUTTON | WS_VISIBLE,
                    CRect(0, 0, 100, 100), pMainWnd, 2000);
#if 1 // A
            hWnd = btn.UnsubclassWindow();
            hWnd = btn.m_hWnd;
        if (hWnd)
#if 1 // B
            static int n;
            for (int i = 0; i < 1000; ++i)
                SendMessage(hWnd, WM_NULL, 0, 0);
            // TRACE(_T("Count = %d\n"), ++n);
    return false;
    return CWinApp::OnIdle(lCount);


This does not call CWinApp::OnIdle, but it does call SendMessage 1000 times
on a single window, which is comparable to what the "excessive-mouse" guy
is doing implicitly with his hundreds of windows. Here are my results,
which I obtained by rapidly moving the mouse over the MDI child window for
approximately 15 sec and viewing the CPU utilization in the Process
Explorer Performance Graph tab, which belongs to the individual process
properties dialog box (the TRACE ensured I was comparing a like number of
OnIdle calls and was disabled thereafter):

1. (A) and (B) active:
XP SP2: 3.5%
XP SP2 + Themes: 8%
Vista SP1: 1%

2. (A) inactive, (B) active:
XP SP2: 9%
XP SP2 + Themes: 12%
Vista SP1: 4%

3. (A) inactive, (B) inactive:
XP SP2: 0%
XP SP2 + Themes: 0.5%
Vista SP1: 0%

Case (1) times SendMessage to a window not mapped to an MFC window, and it
is clear that this uses significantly more CPU in XP. Turning themes on
makes it even worse.

Case (2) times SendMessage to a window that is mapped to an MFC window, and
while MFC's message map processing imposes a substantial increase in CPU
utilization, Vista is still beating XP by a large margin.

Case (3) simply disables everything.

Your program is most like Case (2).

To try to quantify the difference in SendMessage efficiency, I created a
program to time a tight SendMessage loop. My results were:

XP SP2: 9.73e+005 SendMessage/sec
XP SP2 + Themes: 9.64e+005 SendMessage/sec
Vista SP1: 4.21e+006 SendMessage/sec

So, it would seem Vista is about 4x faster at SendMessage than XP. Here's
the program (derived from Raymond Chen's "scratch" program):

#define STRICT
#include <windows.h>
#include <tchar.h>

#include <stdio.h>
#include <time.h>

HINSTANCE g_hinst; /* This application's HINSTANCE
HWND g_hwndChild; /* Optional child window */

 * Window procedure
WndProc(HWND hwnd, UINT uiMsg, WPARAM wParam, LPARAM lParam)
    switch (uiMsg) {

    return DefWindowProc(hwnd, uiMsg, wParam, lParam);

    WNDCLASS wc; = 0;
    wc.lpfnWndProc = WndProc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = g_hinst;
    wc.hIcon = NULL;
    wc.hCursor = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wc.lpszMenuName = NULL;
    wc.lpszClassName = _T("Scratch");

    if (!RegisterClass(&wc)) return FALSE;

    return TRUE;

int WINAPI WinMain(HINSTANCE hinst, HINSTANCE hinstPrev,
                   LPSTR lpCmdLine, int nShowCmd)
    MSG msg;
    HWND hwnd;

    g_hinst = hinst;

    if (!InitApp()) return 0;

        hwnd = CreateWindow(
            _T("Scratch"), /* Class Name */
            _T("Scratch"), /* Title */
            WS_OVERLAPPEDWINDOW, /* Style */
            CW_USEDEFAULT, CW_USEDEFAULT, /* Position */
            CW_USEDEFAULT, CW_USEDEFAULT, /* Size */
            NULL, /* Parent */
            NULL, /* No menu */
            hinst, /* Instance */
            0); /* No special parameters */

        ShowWindow(hwnd, nShowCmd);

const int N = 50*1000*1000;
clock_t t0 = clock();
for (int i = 0; i < N; ++i)
   SendMessage(hwnd, WM_NULL, 0, 0);
clock_t t1 = clock();
double sec = double(t1-t0)/CLOCKS_PER_SEC;
TCHAR buf[100];
_stprintf(buf, _T("%.2e SendMessage/sec (%.2f sec)"),
   N/sec, sec);
MessageBox(0, buf, 0, 0);

    return 0;

Also, any thoughts on the implications of the quick fix I implemented above?
The documentation I have says the OnIdle updates toolbars (and I don't have
any) and deletes temporary objects. I would think that the triggering of
OnIdle by messages other than MOUSEMOVE should be sufficient to clean up
these temporary objects, but I'm not sure.

Certainly, it can be postponed, but as you realize, parts of the UI that
depend on idle time processing due to WM_MOUSEMOVE might not update
properly. To be safer, and if your UI isn't suffering any ill effects, you
might allow (say) every tenth WM_MOUSEMOVE to invoke idle processing.

Finally, any thoughts on just
overridding OnIdle and not calling the base class version? Would those
temporary objects eventually bring my app to a crashing halt if left
undeleted? I'm not sure which temporary objects it is deleting or the
frequency of those objects well enough to make an informed decision.

Leaking temporary objects is not a good plan. Definitely don't do that.

Doug Harrison
Visual C++ MVP

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