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

[Peter Lieven <pl@kamp.de>]

On 06.01.2014 11:08, Wenchao Xia wrote:
> 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 <pl@kamp.de>
>> ---
>>   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)
>>

If you or anyone else further want to test. The x offset calculation in vnc_refresh_server_surface was wrong.
I will send an updated series addresseing Wenchaos comments later this week.

diff --git a/ui/vnc.c b/ui/vnc.c
index cf9aa4a..3b076ee 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -2717,8 +2717,9 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
break;
}
y = offset / VNC_DIRTY_BPL(&vd->guest);
-
- server_ptr = server_row0 + y * server_stride;
+ x = offset % VNC_DIRTY_BPL(&vd->guest);
+
+ server_ptr = server_row0 + y * server_stride + x * cmp_bytes;

if (vd->guest.format != VNC_SERVER_FB_FORMAT) {
qemu_pixman_linebuf_fill(tmpbuf, vd->guest.fb, width, 0, y);
@@ -2726,13 +2727,11 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
} else {
guest_ptr = guest_row0 + y * guest_stride;
}
+ guest_ptr += x * cmp_bytes;

- 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])) {
+ for (;x < DIV_ROUND_UP(width, VNC_DIRTY_PIXELS_PER_BIT);
+ x++, guest_ptr += cmp_bytes, server_ptr += cmp_bytes) {
+ if (!test_and_clear_bit(x, vd->guest.dirty[y])) {
continue;
}
if (memcmp(server_ptr, guest_ptr, cmp_bytes) == 0) {
@@ -2740,10 +2739,11 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
}
memcpy(server_ptr, guest_ptr, cmp_bytes);
if (!vd->non_adaptive) {
- vnc_rect_updated(vd, x, y, &tv);
+ vnc_rect_updated(vd, x * VNC_DIRTY_PIXELS_PER_BIT,
+ y, &tv);
}
QTAILQ_FOREACH(vs, &vd->clients, next) {
- set_bit((x / VNC_DIRTY_PIXELS_PER_BIT), vs->dirty[y]);
+ set_bit(x, vs->dirty[y]);
}
has_dirty++;
}