Re: [Qemu-devel] [PATCHv3 3/6] ui/vnc: optimize dirty bitmap tracking

[Wenchao Xia]

于 2014/1/6 2:02, Peter Lieven 写道:
> vnc_update_client currently scans the dirty bitmap of each client
> bitwise which is a very costly operation if only few bits are dirty.
> vnc_refresh_server_surface does almost the same.
> this patch optimizes both by utilizing the heavily optimized
> function find_next_bit to find the offset of the next dirty
> bit in the dirty bitmaps.
> 
> The following artifical test (just the bitmap operation part) running
> vnc_update_client 65536 times on a 2560x2048 surface illustrates the
> performance difference:
> 
> All bits clean - vnc_update_client_new: 0.07 secs
>                   vnc_update_client_old: 10.98 secs
> 
> All bits dirty - vnc_update_client_new: 11.26 secs
>                   vnc_update_client_old: 20.19 secs
> 
> Few bits dirty - vnc_update_client_new: 0.08 secs
>                   vnc_update_client_old: 10.98 secs
> 
> The case for all bits dirty is still rather slow, this
> is due to the implementation of find_and_clear_dirty_height.
> This will be addresses in a separate patch.
> 
> Signed-off-by: Peter Lieven <address@hidden>
> ---
>   ui/vnc.c |  154 
> +++++++++++++++++++++++++++++++++-----------------------------
>   ui/vnc.h |    4 ++
>   2 files changed, 87 insertions(+), 71 deletions(-)
> 
> diff --git a/ui/vnc.c b/ui/vnc.c
> index 1d2aa1a..6a0c03e 100644
> --- a/ui/vnc.c
> +++ b/ui/vnc.c
> @@ -572,6 +572,14 @@ void *vnc_server_fb_ptr(VncDisplay *vd, int x, int y)
>       ptr += x * VNC_SERVER_FB_BYTES;
>       return ptr;
>   }
> +/* this sets only the visible pixels of a dirty bitmap */
> +#define VNC_SET_VISIBLE_PIXELS_DIRTY(bitmap, w, h) {\
> +        int y;\
> +        memset(bitmap, 0x00, sizeof(bitmap));\
> +        for (y = 0; y < h; y++) {\
> +            bitmap_set(bitmap[y], 0, w / VNC_DIRTY_PIXELS_PER_BIT);\

  Will it be a problem when vnc's width % VNC_DIRTY_PIXELS_PER_BIT != 0?
Although it is a rare case, but I think it is better round it up, since
"v" and "VNC_DIRTY_PIXELS_PER_BIT" are variables. A macro computing it
would be nice:

#define VNC_DIRTY_BITS_FROM_WIDTH(w) (w + VNC_DIRTY_PIXELS_PER_BIT - 1/
VNC_DIRTY_PIXELS_PER_BIT)
#define VNC_DIRTY_BITS (VNC_DIRTY_BITS_FROM_WIDTH(VNC_MAX_WIDTH)

then here:
    bitmap_set(bitmap[y], 0, VNC_DIRTY_BITS_FROM_WIDTH(w));

Or simply warn or coredump when v % VNC_DIRTY_PIXELS_PER_BIT != 0.

Also, in vnc.h:
/* VNC_MAX_WIDTH must be a multiple of 16. */
#define VNC_MAX_WIDTH 2560
#define VNC_MAX_HEIGHT 2048

Maybe it should be updated as:
/* VNC_MAX_WIDTH must be a multiple of VNC_DIRTY_PIXELS_PER_BIT. */

> +        } \
> +    }
> 
>   static void vnc_dpy_switch(DisplayChangeListener *dcl,
>                              DisplaySurface *surface)
> @@ -597,7 +605,9 @@ static void vnc_dpy_switch(DisplayChangeListener *dcl,
>       qemu_pixman_image_unref(vd->guest.fb);
>       vd->guest.fb = pixman_image_ref(surface->image);
>       vd->guest.format = surface->format;
> -    memset(vd->guest.dirty, 0xFF, sizeof(vd->guest.dirty));
> +    VNC_SET_VISIBLE_PIXELS_DIRTY(vd->guest.dirty,
> +                                 surface_width(vd->ds),
> +                                 surface_height(vd->ds));
> 
>       QTAILQ_FOREACH(vs, &vd->clients, next) {
>           vnc_colordepth(vs);
> @@ -605,7 +615,9 @@ static void vnc_dpy_switch(DisplayChangeListener *dcl,
>           if (vs->vd->cursor) {
>               vnc_cursor_define(vs);
>           }
> -        memset(vs->dirty, 0xFF, sizeof(vs->dirty));
> +        VNC_SET_VISIBLE_PIXELS_DIRTY(vs->dirty,
> +                                     surface_width(vd->ds),
> +                                     surface_height(vd->ds));
>       }
>   }
> 
> @@ -889,10 +901,9 @@ static int vnc_update_client(VncState *vs, int has_dirty)
>           VncDisplay *vd = vs->vd;
>           VncJob *job;
>           int y;
> -        int width, height;
> +        int height;
>           int n = 0;
> 
> -
>           if (vs->output.offset && !vs->audio_cap && !vs->force_update)
>               /* kernel send buffers are full -> drop frames to throttle */
>               return 0;
> @@ -908,39 +919,27 @@ static int vnc_update_client(VncState *vs, int 
> has_dirty)
>            */
>           job = vnc_job_new(vs);
> 
> -        width = MIN(pixman_image_get_width(vd->server), vs->client_width);
>           height = MIN(pixman_image_get_height(vd->server), 
> vs->client_height);
> 
> -        for (y = 0; y < height; y++) {
> -            int x;
> -            int last_x = -1;
> -            for (x = 0; x < width / VNC_DIRTY_PIXELS_PER_BIT; x++) {
> -                if (test_and_clear_bit(x, vs->dirty[y])) {
> -                    if (last_x == -1) {
> -                        last_x = x;
> -                    }
> -                } else {
> -                    if (last_x != -1) {
> -                        int h = find_and_clear_dirty_height(vs, y, last_x, x,
> -                                                            height);
> -
> -                        n += vnc_job_add_rect(job,
> -                                              last_x * 
> VNC_DIRTY_PIXELS_PER_BIT,
> -                                              y,
> -                                              (x - last_x) *
> -                                              VNC_DIRTY_PIXELS_PER_BIT,
> -                                              h);
> -                    }
> -                    last_x = -1;
> -                }
> -            }
> -            if (last_x != -1) {
> -                int h = find_and_clear_dirty_height(vs, y, last_x, x, 
> height);
> -                n += vnc_job_add_rect(job, last_x * VNC_DIRTY_PIXELS_PER_BIT,
> -                                      y,
> -                                      (x - last_x) * 
> VNC_DIRTY_PIXELS_PER_BIT,
> -                                      h);
> +        y = 0;
> +        for (;;) {
> +            int x, h;
> +            unsigned long x2;
> +            unsigned long offset = find_next_bit((unsigned long *) 
> &vs->dirty,
> +                                                 height * VNC_DIRTY_BPL(vs),
> +                                                 y * VNC_DIRTY_BPL(vs));
> +            if (offset == height * VNC_DIRTY_BPL(vs)) {
> +                /* no more dirty bits */
> +                break;
>               }
> +            y = offset / VNC_DIRTY_BPL(vs);
> +            x = offset % VNC_DIRTY_BPL(vs);
> +            x2 = find_next_zero_bit((unsigned long *) &vs->dirty[y],
> +                                    VNC_DIRTY_BPL(vs), x);
> +            bitmap_clear(vs->dirty[y], x, x2 - x);
> +            h = find_and_clear_dirty_height(vs, y, x, x2, height);
> +            n += vnc_job_add_rect(job, x * VNC_DIRTY_PIXELS_PER_BIT, y,
> +                                  (x2 - x) * VNC_DIRTY_PIXELS_PER_BIT, h);
>           }

Minor comments:
  VNC_DIRTY_BPL(vs) is accessing memory by pointer, should we use a
variable instead of VNC_DIRTY_BPL(vs) in every place, in case of
compiler didn't optimize it for us?

> 
>           vnc_job_push(job);
> @@ -2678,8 +2677,8 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
>       int width = pixman_image_get_width(vd->guest.fb);
>       int height = pixman_image_get_height(vd->guest.fb);
>       int y;
> -    uint8_t *guest_row;
> -    uint8_t *server_row;
> +    uint8_t *guest_row0 = NULL, *server_row0;
  Any reason that rename those variable?

> +    int guest_stride = 0, server_stride;
>       int cmp_bytes;
>       VncState *vs;
>       int has_dirty = 0;
> @@ -2704,44 +2703,57 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
>       if (vd->guest.format != VNC_SERVER_FB_FORMAT) {
>           int width = pixman_image_get_width(vd->server);
>           tmpbuf = qemu_pixman_linebuf_create(VNC_SERVER_FB_FORMAT, width);
> -    }
> -    guest_row = (uint8_t *)pixman_image_get_data(vd->guest.fb);
> -    server_row = (uint8_t *)pixman_image_get_data(vd->server);
> -    for (y = 0; y < height; y++) {
> -        if (!bitmap_empty(vd->guest.dirty[y], VNC_DIRTY_BITS)) {
> -            int x;
> -            uint8_t *guest_ptr;
> -            uint8_t *server_ptr;
> -
> -            if (vd->guest.format != VNC_SERVER_FB_FORMAT) {
> -                qemu_pixman_linebuf_fill(tmpbuf, vd->guest.fb, width, 0, y);
> -                guest_ptr = (uint8_t *)pixman_image_get_data(tmpbuf);
> -            } else {
> -                guest_ptr = guest_row;
> -            }
> -            server_ptr = server_row;
> +    } else {
> +        guest_row0 = (uint8_t *)pixman_image_get_data(vd->guest.fb);
> +        guest_stride = pixman_image_get_stride(vd->guest.fb);
> +    }
> +    server_row0 = (uint8_t *)pixman_image_get_data(vd->server);
> +    server_stride = pixman_image_get_stride(vd->server);
> +
> +    y = 0;
> +    for (;;) {
> +        int x;
> +        uint8_t *guest_ptr, *server_ptr;
> +        unsigned long offset = find_next_bit((unsigned long *) 
> &vd->guest.dirty,
> +                                             height * 
> VNC_DIRTY_BPL(&vd->guest),
> +                                             y * VNC_DIRTY_BPL(&vd->guest));
> +        if (offset == height * VNC_DIRTY_BPL(&vd->guest)) {
> +            /* no more dirty bits */
> +            break;
> +        }
> +        y = offset / VNC_DIRTY_BPL(&vd->guest);
> 
> -            for (x = 0; x + VNC_DIRTY_PIXELS_PER_BIT - 1 < width;
> -                 x += VNC_DIRTY_PIXELS_PER_BIT, guest_ptr += cmp_bytes,
> -                 server_ptr += cmp_bytes) {
> -                if (!test_and_clear_bit((x / VNC_DIRTY_PIXELS_PER_BIT),
> -                    vd->guest.dirty[y])) {
> -                    continue;
> -                }
> -                if (memcmp(server_ptr, guest_ptr, cmp_bytes) == 0) {
> -                    continue;
> -                }
> -                memcpy(server_ptr, guest_ptr, cmp_bytes);
> -                if (!vd->non_adaptive)
> -                    vnc_rect_updated(vd, x, y, &tv);
> -                QTAILQ_FOREACH(vs, &vd->clients, next) {
> -                    set_bit((x / VNC_DIRTY_PIXELS_PER_BIT), vs->dirty[y]);
> -                }
> -                has_dirty++;
> +        server_ptr = server_row0 + y * server_stride;
> +
> +        if (vd->guest.format != VNC_SERVER_FB_FORMAT) {
> +            qemu_pixman_linebuf_fill(tmpbuf, vd->guest.fb, width, 0, y);
> +            guest_ptr = (uint8_t *)pixman_image_get_data(tmpbuf);
> +        } else {
> +            guest_ptr = guest_row0 + y * guest_stride;
> +        }
> +
> +        for (x = offset % VNC_DIRTY_BPL(&vd->guest);
> +             x + VNC_DIRTY_PIXELS_PER_BIT - 1 < width;
> +             x += VNC_DIRTY_PIXELS_PER_BIT, guest_ptr += cmp_bytes,
> +             server_ptr += cmp_bytes) {
> +            if (!test_and_clear_bit((x / VNC_DIRTY_PIXELS_PER_BIT),
> +                vd->guest.dirty[y])) {
> +                continue;
> +            }
> +            if (memcmp(server_ptr, guest_ptr, cmp_bytes) == 0) {
> +                continue;
> +            }
> +            memcpy(server_ptr, guest_ptr, cmp_bytes);
> +            if (!vd->non_adaptive) {
> +                vnc_rect_updated(vd, x, y, &tv);
>               }
> +            QTAILQ_FOREACH(vs, &vd->clients, next) {
> +                set_bit((x / VNC_DIRTY_PIXELS_PER_BIT), vs->dirty[y]);
> +            }
> +            has_dirty++;
>           }
> -        guest_row  += pixman_image_get_stride(vd->guest.fb);
> -        server_row += pixman_image_get_stride(vd->server);
> +
> +        y++;
>       }
>       qemu_pixman_image_unref(tmpbuf);
>       return has_dirty;
> diff --git a/ui/vnc.h b/ui/vnc.h
> index 4a8f33c..07e1f59 100644
> --- a/ui/vnc.h
> +++ b/ui/vnc.h
> @@ -88,6 +88,10 @@ typedef void VncSendHextileTile(VncState *vs,
>   /* VNC_DIRTY_BITS is the number of bits in the dirty bitmap. */
>   #define VNC_DIRTY_BITS (VNC_MAX_WIDTH / VNC_DIRTY_PIXELS_PER_BIT)
> 
> +/* VNC_DIRTY_BPL (BPL = bits per line) might be greater than
> + * VNC_DIRTY_BITS due to alignment */
> +#define VNC_DIRTY_BPL(x) (sizeof((x)->dirty) / VNC_MAX_HEIGHT * 
> BITS_PER_BYTE)
> +
>   #define VNC_STAT_RECT  64
>   #define VNC_STAT_COLS (VNC_MAX_WIDTH / VNC_STAT_RECT)
>   #define VNC_STAT_ROWS (VNC_MAX_HEIGHT / VNC_STAT_RECT)
>