lxdm-greeter-gtk excessive CPU usage

I'm sorry, forgot to mention. I've found reported issue in Maverick package version lxdm-0.2.0-0ubuntu3.

Thank you for your bug report. Could you try with Ubuntu or Lubuntu Natty (11.04). There was some fixes regarding lxdm and the background rendering in lxdm 0.3.0.
You can also test lxdm 0.3.0 in LXDE PPA : https://launchpad.net/~lxde/+archive/ppa

High cpu usage still occurring in Natty with lxdm 0.3.0. 60% cpu with a 2 GHz P4.

Both versions are affected because on_expose function was not changed. If you check serc/greeter.c::on_expose, you will see that this function redraws entire screen on each call:

    if( bg_img )
    {
        cairo_matrix_t matrix;
        double x = 0, y = 0, sx, sy;
        cairo_get_matrix(cr, &matrix);
        sx = (double)gdk_screen_width() / (double)gdk_pixbuf_get_width(bg_img);
        sy = (double)gdk_screen_height() / (double)gdk_pixbuf_get_height(bg_img);
        cairo_scale(cr, sx, sy);
        gdk_cairo_set_source_pixbuf(cr, bg_img, x, y);
        cairo_paint(cr);
        cairo_set_matrix(cr, &matrix);
    }

Witn src/greeter.c::on_timeout function which changes system time every second, it leads to full screen redraw every second. In my case it means to scale 800x800 pixels wallpaper image and put it on 1600x900 pixel screen. This algorithm just cannot work fast or consume ow CPU.

It need to use information from evt and redraw only affected area. Also, I think it should cache pre-scaled bitmap.

Sorry, I have no experience in GTK/Cairo programming and cannot provide a patch right now.

Thank your for the analysis. Deconnecting the on_expose function to expose-event fix the flickering and I suppose the CPU usage. IMO, background need only to be draw on creation of the window, it should not change during teh "life" of the window. I'll try to make a proper patch.

Could you test with the lxdm package in my PPA : https://launchpad.net/~gilir/+archive/lubuntu ?
There are still flickering in natty, but it seems to be another problem.

PPA version lxdm_0.3.0-0ubuntu5~ppa3 still uses about 20-30% of CPU on my box.

Thanks for the testing. Unfortunatly, I was not enable to do a better solution to improve the drawing. I think the better solution is just to remove this on_expose function. I'm not even sure it's really necessary because the background should be managed by the theme.
I'll publish another package in the PPA with this fix (version ~ppa5).

Version lxdm_0.3.0-0ubuntu5~ppa5 does not use CPU, but now it also ignores any background image settings, as if bg= was not set at all. Probably on_expose should be called at least once.

Also, there is on_screen_size_changed, function in greeter.c. I don't know what it is, but we probably should redraw on screen size changes as well. Can't imagine when screen size can change, may be on mobile devices because of rotation?

Upstream proposed another fix for this problem. Could you test version lxdm - 0.3.0-0ubuntu5~ppa8 in my PPA ?

I'll test ppa8 version a bit later and get back with results, but I'm afraid it won't help.

I've tried to implement fix proposed by upstream myself based on ppa3 version (using gdk_cairo_rectangle + cairo_fill). Looks like it is cairo performance problem. The following implementation of on_expose function produces 20-30%CPU:

static gboolean on_expose(GtkWidget* widget, GdkEventExpose* evt, gpointer user_data)
{
    cairo_t *cr = gdk_cairo_create(evt->window);
    gdk_cairo_set_source_pixbuf (cr, bg_img, 0, 0);
    gdk_cairo_rectangle(cr, &evt->area);
    cairo_fill(cr);
    cairo_destroy(cr);
    return FALSE;
}

Falling back to deprecated api uses only 1-2% CPU (need -DGDK_ENABLE_DEPRECATED and -DGDK_PIXBUF_ENABLE_DEPRECATED compile flags):

static gboolean on_expose(GtkWidget* widget, GdkEventExpose* evt, gpointer user_data)
{
    GdkGC *gc = gdk_gc_new(evt->window);
    gdk_draw_pixbuf(evt->window,
                    gc,
                    bg_img,
                    evt->area.x, evt->area.y,
                    evt->area.x, evt->area.y,
                    evt->area.width, evt->area.height,
                    GDK_RGB_DITHER_NONE, 0, 0);
    g_object_unref(gc);
    return FALSE;
}

Looks like a new API 20-30 time slower than an old one. May be this was fixed in never versions of gdk. Here in maverick I have:

libgdk2.0-0 version 2.22.0-0ubuntu1
libgdk-pixbuf2.0-0 version 2.22.0-0ubuntu1
libcairo2 version 1.10.0-1ubuntu3

Yes, difficult to say where the bug is in the stack.

Steve, could you test the last version in my PPA and see if the CPU usage is still high on Natty ?

3% CU usage with ppa10 version down from 27% with ppa8 version.
1 GHz Duron and ATI Rage 128.

Thanks Steve.
I'll upload upstream fix, and assume the other part of the performance problem is fixed in newer versions of gtk + cairo, available in Natty.

This bug was fixed in the package lxdm - 0.3.0-0ubuntu5

---------------
lxdm (0.3.0-0ubuntu5) natty; urgency=low

  * debian/patches/90_simple_expose.patch
   - From upstream, rework the expose-event callback which have a high CPU
     usage (LP: #733912)
  * debian/patches/20_disable_resize_grip.patch
   - Disable resize grip which break the background drawing (LP: #703658)
 -- Julien Lavergne <email address hidden> Sun, 20 Mar 2011 00:31:24 +0100