[Qemu-devel] [PATCHv4 3/6] ui/vnc: optimize dirty bitmap tracking

[Peter Lieven <pl@kamp.de>]

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 |  155 ++++++++++++++++++++++++++++++++++----------------------------
 ui/vnc.h |    4 ++
 2 files changed, 88 insertions(+), 71 deletions(-)

diff --git a/ui/vnc.c b/ui/vnc.c
index f42398d..3412cdf 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -572,6 +572,15 @@ 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,\
+                       DIV_ROUND_UP(w, VNC_DIRTY_PIXELS_PER_BIT));\
+        } \
+    }
 
 static void vnc_dpy_switch(DisplayChangeListener *dcl,
                            DisplaySurface *surface)
@@ -597,7 +606,9 @@ 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 +616,9 @@ 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));
     }
 }
 
@@ -891,10 +904,9 @@ 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;
@@ -910,39 +922,27 @@ 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);
         }
 
         vnc_job_push(job);
@@ -2680,8 +2680,8 @@ 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;
+    int guest_stride = 0, server_stride;
     int cmp_bytes;
     VncState *vs;
     int has_dirty = 0;
@@ -2706,44 +2706,57 @@ 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);
+        x = 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 + x * cmp_bytes;
+
+        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;
+        }
+        guest_ptr += x * cmp_bytes;
+
+        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) {
+                continue;
+            }
+            memcpy(server_ptr, guest_ptr, cmp_bytes);
+            if (!vd->non_adaptive) {
+                vnc_rect_updated(vd, x * VNC_DIRTY_PIXELS_PER_BIT,
+                                 y, &tv);
             }
+            QTAILQ_FOREACH(vs, &vd->clients, next) {
+                set_bit(x, 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)
-- 
1.7.9.5