Re: [Qemu-devel] [PATCH] Reducing X communication bandwidth, take 2

[Anthony Liguori <anthony@codemonkey.ws>]

Hi Julian,

Julian Seward wrote:
> Here is a somewhat revised version of a patch I first made nearly
> three years ago.  The original thread is 
>
> http://lists.gnu.org/archive/html/qemu-devel/2004-07/msg00263.html
>
> It still uses a shadow frame buffer.  Fabrice mentioned this is not
> necessary
>   

I thought about this a little and here's what I came up with.

I think we could change vga_draw_line* so that as part of the drawing 
process, it compared the newly generated pixel with the previous pixel 
value and returned back the min, max x-coordinate that changed.

Since we tend to only extend the vertical dirty range by a couple 
pixels, this should be a relatively cheap way of reducing the size of 
the update region.

If there still is concern about performance, then we could introduce a 
flag that make this behavior conditional.  This would be nice since 
there are already certain scenarios where I want to disable this for VNC 
(using a shared memory transport for instance).

It also helps for cases where this behavior is totally unneeded such as 
emulating the VMware VGA device.

The more I think about it, the more convinced I am that this 
minimization has to occur in the VGA devices themselves.

What do you think?

Regards,

Anthony Liguori

>  http://lists.gnu.org/archive/html/qemu-devel/2004-07/msg00279.html
>
> but I can't see how to get rid of it and still check for redundant
> updates in NxN pixel blocks (N=32 by default).  The point of checking
> NxN squares is that mouse pointer pixmaps are square and so the most
> common display updates - mouse pointer movements - are often reduced
> to transmission of a single 32x32 block using this strategy.
>
> The shadow framebuffer is only allocated when -remote-x11 is present,
> so the patch has no effect on default memory use.
>
> I measured the bandwidth saving roughly by resuming a vm snapshot
> containing a web browser showing a page with a lot of links.  I moved
> the pointer slowly over the links (so they change colour) and scrolled
> up and down a bit; about 1/2 minute of activity in total.  I tried to do
> the same with and without -remote-x11.  Without -remote-x11, 163Mbyte
> was transmitted to the X server; with it, 20.6Mbyte was, about an 8:1
> reduction.
>
> J
>
>
> diff -u -r Orig/qemu-0.9.0/sdl.c qemu-0.9.0/sdl.c
> --- Orig/qemu-0.9.0/sdl.c	2007-02-05 23:01:54.000000000 +0000
> +++ qemu-0.9.0/sdl.c	2007-03-13 22:16:40.000000000 +0000
> @@ -29,6 +29,8 @@
>  #include <signal.h>
>  #endif
>  
> +#include <assert.h>
> +
>  static SDL_Surface *screen;
>  static int gui_grab; /* if true, all keyboard/mouse events are grabbed */
>  static int last_vm_running;
> @@ -44,17 +46,232 @@
>  static SDL_Cursor *sdl_cursor_hidden;
>  static int absolute_enabled = 0;
>  
> +/* Mechanism to reduce the total amount of data transmitted to the X
> +   server, often quite dramatically.  Keep a shadow copy of video
> +   memory in alt_pixels, and when asked to update a rectangle, use
> +   the shadow copy to establish areas which are the same, and so do
> +   not need updating.
> +*/
> +
> +static void* alt_pixels = NULL;
> +
> +#define THRESH 32
> +
> +/* Return 1 if the area [x .. x+w-1, y .. y+w-1] is different from
> +   the old version and so needs updating. */
> +static int cmpArea_16bit ( int x, int y, int w, int h )
> +{
> +           int    i, j;
> +  unsigned int    sll;
> +  unsigned short* p1base = (unsigned short*)screen->pixels;
> +  unsigned short* p2base = (unsigned short*)alt_pixels;
> +  assert(screen->format->BytesPerPixel == 2);
> +  if (w == 0 || h == 0)
> +     return 0;
> +  assert(w > 0 && h > 0);
> +  sll = ((unsigned int)screen->pitch) >> 1;
> +  for (j = y; j < y+h; j++) {
> +    unsigned short* p1p = & p1base[j * sll + x];
> +    unsigned short* p2p = & p2base[j * sll + x];
> +    for (i = 0; i < w-5; i += 5) {
> +      if (p1p[i+0] != p2p[i+0]) return 1;
> +      if (p1p[i+1] != p2p[i+1]) return 1;
> +      if (p1p[i+2] != p2p[i+2]) return 1;
> +      if (p1p[i+3] != p2p[i+3]) return 1;
> +      if (p1p[i+4] != p2p[i+4]) return 1;
> +    }
> +    for (/*fixup*/; i < w; i++) {
> +      if (p1p[i+0] != p2p[i+0]) return 1;
> +    }
> +  }
> +  return 0;
> +}
> +static void copyArea_16bit ( int x, int y, int w, int h )
> +{
> +           int    i, j;
> +  unsigned int    sll;
> +  unsigned short* p1base = (unsigned short*)screen->pixels;
> +  unsigned short* p2base = (unsigned short*)alt_pixels;
> +  assert(screen->format->BytesPerPixel == 2);
> +  sll = ((unsigned int)screen->pitch) >> 1;
> +  if (w == 0 || h == 0)
> +     return;
> +  assert(w > 0 && h > 0);
> +  for (j = y; j < y+h; j++) {
> +    unsigned short* p1p = & p1base[j * sll + x];
> +    unsigned short* p2p = & p2base[j * sll + x];
> +    for (i = 0; i < w-5; i += 5) {
> +      p2p[i+0] = p1p[i+0];
> +      p2p[i+1] = p1p[i+1];
> +      p2p[i+2] = p1p[i+2];
> +      p2p[i+3] = p1p[i+3];
> +      p2p[i+4] = p1p[i+4];
> +    }
> +    for (/*fixup*/; i < w; i++) {
> +      p2p[i+0] = p1p[i+0];
> +    }
> +  }
> +}
> +
> +static int cmpArea_32bit ( int x, int y, int w, int h )
> +{
> +  int           i, j;
> +  unsigned int  sll;
> +  unsigned int* p1base = (unsigned int*)screen->pixels;
> +  unsigned int* p2base = (unsigned int*)alt_pixels;
> +  assert(screen->format->BytesPerPixel == 4);
> +  sll = ((unsigned int)screen->pitch) >> 2;
> +  for (j = y; j < y+h; j++) {
> +    for (i = x; i < x+w; i++) {
> +      if (p1base [j * sll +i] != p2base [j * sll +i])
> +	return 1;
> +    }
> +  }
> +  return 0;
> +}
> +static void copyArea_32bit ( int x, int y, int w, int h )
> +{
> +  int           i, j;
> +  unsigned int  sll;
> +  unsigned int* p1base = (unsigned int*)screen->pixels;
> +  unsigned int* p2base = (unsigned int*)alt_pixels;
> +  assert(screen->format->BytesPerPixel == 4);
> +  sll = ((unsigned int)screen->pitch) >> 2;
> +  for (j = y; j < y+h; j++) {
> +    for (i = x; i < x+w; i++) {
> +      p2base [j * sll +i] = p1base [j * sll +i];
> +    }
> +  }
> +}
> +
> +
> +static void econoUpdate (DisplayState *ds, int x, int y, 
> +                         unsigned int w, unsigned int h)
> +{
> +   static int tested_total = 0;
> +   static int update_total = 0;
> +
> +   int  (*cmpArea) (int, int, int, int) = NULL;
> +   void (*copyArea)(int, int, int, int) = NULL;
> +
> +   int xi, xj, xlim, yi, yj, ylim, ntest, nupd;
> +   if (w == 0 || h == 0) 
> +      return;
> +
> +   if (screen->format->BytesPerPixel == 4) {
> +      cmpArea = cmpArea_32bit;
> +      copyArea = copyArea_32bit;
> +   }
> +   else
> +   if (screen->format->BytesPerPixel == 2) {
> +      cmpArea = cmpArea_16bit;
> +      copyArea = copyArea_16bit;
> +   }
> +   else
> +      assert(0); /* should not be reached - sdl_update guards against this */
> +
> +   xlim = x + w;
> +   ylim = y + h;
> +
> +   ntest = nupd = 0;
> +   for (xi = x; xi < xlim; xi += THRESH) {
> +      xj = xi + THRESH;
> +      if (xj > xlim) xj = xlim;
> +      for (yi = y; yi < ylim; yi += THRESH) {
> +         yj = yi + THRESH;
> +         if (yj > ylim) yj = ylim;
> +         if (xj-xi == 0 || yj-yi == 0)
> +            continue;
> +	 ntest++;
> +	 if (cmpArea(xi, yi, xj-xi, yj-yi)) {
> + 	    nupd++;
> +            copyArea(xi, yi, xj-xi, yj-yi);
> +            SDL_UpdateRect(screen, xi, yi, xj-xi, yj-yi);
> +	 }
> +      }
> +   }
> +   tested_total += ntest;
> +   update_total += nupd;
> +   if (0)
> +   printf("(tested, updated):   total (%d, %d),   this time (%d, %d)\n", 
> +          tested_total, update_total, ntest, nupd);
> +}
> +
> +
>  static void sdl_update(DisplayState *ds, int x, int y, int w, int h)
>  {
> -    //    printf("updating x=%d y=%d w=%d h=%d\n", x, y, w, h);
> -    SDL_UpdateRect(screen, x, y, w, h);
> +   int warned = 0;
> +
> +   //printf("updating x=%d y=%d w=%d h=%d\n", x, y, w, h);
> +   //   printf("Total Size    %d %d\n", screen->w, screen->h);
> +   //printf("BytesPerPixel %d\n", screen->format->BytesPerPixel);
> +   //printf("pitch         %d (%d)\n", screen->pitch, 
> +   //                       screen->w * screen->format->BytesPerPixel);
> +
> +   if (!remote_x11) {
> +      SDL_UpdateRect(screen, x, y, w, h);
> +      return;
> +   }
> +
> +   if ( (screen->format->BytesPerPixel != 4 
> +         && screen->format->BytesPerPixel != 2)
> +       || screen->pitch != screen->w * screen->format->BytesPerPixel) {
> +       if (!warned) {
> +          warned = 1;
> +          fprintf(stderr, "qemu: SDL update optimisation disabled\n"
> +                          "      (wrong bits-per-pixel, or wrong pitch)\n");
> +       }
> +       SDL_UpdateRect(screen, x, y, w, h);
> +       return;
> +    }
> +
> +    assert(screen->pitch == screen->w * screen->format->BytesPerPixel);
> +    assert(sizeof(int) == 4);
> +    assert(sizeof(short) == 2);
> +    if (alt_pixels == NULL) {
> +       /* First time through (at this resolution).  
> +          Copy entire screen. */
> +       if (screen->format->BytesPerPixel == 4) {
> +          int i, word32s = screen->w * screen->h;
> +          if (0) printf("copying init screen - 32 bit\n");
> +          alt_pixels = malloc(word32s * sizeof(int));
> +          assert(alt_pixels);
> +          for (i = 0; i < word32s; i++) 
> +             ((unsigned int*)alt_pixels)[i] 
> +                = ((unsigned int*)(screen->pixels))[i];
> +          SDL_UpdateRect(screen, x, y, w, h);
> +          if (0) printf("done- 32 bit\n");
> +       }
> +       else
> +       if (screen->format->BytesPerPixel == 2) {
> +          int i, word16s = screen->w * screen->h;
> +          if (0) printf("copying init screen - 16 bit\n");
> +          alt_pixels = malloc(word16s * sizeof(int));
> +          assert(alt_pixels);
> +          for (i = 0; i < word16s; i++) 
> +             ((unsigned short*)alt_pixels)[i] 
> +                = ((unsigned short*)(screen->pixels))[i];
> +          SDL_UpdateRect(screen, x, y, w, h);
> +          if (0) printf("done - 16 bit\n");
> +       }
> +       else 
> +          assert(0); /* guarded above */
> +    } else {
> +       assert(w >= 0);
> +       assert(h >= 0);
> +       econoUpdate(ds, x, y, w, h);
> +    }
>  }
>  
>  static void sdl_resize(DisplayState *ds, int w, int h)
>  {
>      int flags;
>  
> -    //    printf("resizing to %d %d\n", w, h);
> +    if (alt_pixels) {
> +        assert(remote_x11);
> +        free(alt_pixels);
> +	alt_pixels = NULL;
> +    }
>  
>      flags = SDL_HWSURFACE|SDL_ASYNCBLIT|SDL_HWACCEL;
>      if (gui_fullscreen)
> diff -u -r Orig/qemu-0.9.0/vl.c qemu-0.9.0/vl.c
> --- Orig/qemu-0.9.0/vl.c	2007-02-05 23:01:54.000000000 +0000
> +++ qemu-0.9.0/vl.c	2007-03-13 22:12:39.000000000 +0000
> @@ -173,6 +173,7 @@
>  int nb_option_roms;
>  int semihosting_enabled = 0;
>  int autostart = 1;
> +int remote_x11 = 0;
>  
>  /***********************************************************/
>  /* x86 ISA bus support */
> @@ -6121,6 +6122,7 @@
>  	   "-vnc display    start a VNC server on display\n"
>  #ifndef _WIN32
>  	   "-daemonize      daemonize QEMU after initializing\n"
> +           "-remote-x11     improve graphics responsiveness when X11 
> client != server\n"
>  #endif
>  	   "-option-rom rom load a file, rom, into the option ROM space\n"
>             "\n"
> @@ -6204,6 +6206,7 @@
>      QEMU_OPTION_no_acpi,
>      QEMU_OPTION_no_reboot,
>      QEMU_OPTION_daemonize,
> +    QEMU_OPTION_remote_x11,
>      QEMU_OPTION_option_rom,
>      QEMU_OPTION_semihosting
>  };
> @@ -6288,6 +6291,7 @@
>      { "no-acpi", 0, QEMU_OPTION_no_acpi },
>      { "no-reboot", 0, QEMU_OPTION_no_reboot },
>      { "daemonize", 0, QEMU_OPTION_daemonize },
> +    { "remote-x11", 0, QEMU_OPTION_remote_x11 },
>      { "option-rom", HAS_ARG, QEMU_OPTION_option_rom },
>  #if defined(TARGET_ARM)
>      { "semihosting", 0, QEMU_OPTION_semihosting },
> @@ -6947,6 +6951,9 @@
>  	    case QEMU_OPTION_daemonize:
>  		daemonize = 1;
>  		break;
> +	    case QEMU_OPTION_remote_x11:
> +		remote_x11 = 1;
> +		break;
>  	    case QEMU_OPTION_option_rom:
>  		if (nb_option_roms >= MAX_OPTION_ROMS) {
>  		    fprintf(stderr, "Too many option ROMs\n");
> diff -u -r Orig/qemu-0.9.0/vl.h qemu-0.9.0/vl.h
> --- Orig/qemu-0.9.0/vl.h	2007-02-05 23:01:54.000000000 +0000
> +++ qemu-0.9.0/vl.h	2007-03-13 22:11:24.000000000 +0000
> @@ -159,6 +159,7 @@
>  extern int no_quit;
>  extern int semihosting_enabled;
>  extern int autostart;
> +extern int remote_x11;
>  
>  #define MAX_OPTION_ROMS 16
>  extern const char *option_rom[MAX_OPTION_ROMS];
>
>
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