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

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

于 2014/1/6 21:31, Peter Lieven 写道:
> 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:
> Good point, I will use DIV_ROUND_UP here.
>>
>> #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. */
> correct. will fix as well.
>>
>>> +        } \
>>> +    }
>>>
>>>    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?
> I am pretty sure that sizeof is evaluated at compile time or do you have other
> evidence?

  You are right, it is sizeof((x)->dirty), I missed the bracket before.

>>
>>>            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?
> Its actually a pointer to row0 and not to any specific row. This is why
> I renamed it.

  I see, thanks for the explanation.

>>
>>> +    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)
>>>
> 
> 
>