Re: [Qemu-devel] [PATCH 2/3] ui/vnc: optimize dirty bitmap tracking

[Peter Lieven <pl@kamp.de>]

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On 18.11.2013 17:27, Anthony Liguori wrote:
>
>
> On Nov 18, 2013 12:20 AM, "Peter Lieven" <pl@kamp.de <mailto:pl@kamp.de>> wrote:
> >
> > 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.
> >
> > Signed-off-by: Peter Lieven <pl@kamp.de <mailto:pl@kamp.de>>
>
> Can you include performance data?
>

I made some aritificial analysis of vnc_update_client with the attached test code.

$ gcc -O2  -o vnc_perf vnc_perf.c
$ ./vnc_perf
All bits clean - vnc_update_client_new: 0.07 secs
                  vnc_update_client_old: 10.82 secs

All bits dirty - vnc_update_client_new: 9.81 secs
                  vnc_update_client_old: 20.00 secs

Few bits dirty - vnc_update_client_new: 0.08 secs
                  vnc_update_client_old: 10.62 secs

find_and_clear_dirty_height() is still very slow, but I will look at this separately.

Peter

---

#define ITERATIONS 16*1024

#define VNC_MAX_WIDTH 2560
#define VNC_MAX_HEIGHT 2048

#define VNC_DIRTY_PIXELS_PER_BIT 16

/* 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_BITS_PER_LINE might be greater than VNC_DIRTY_BITS due to alignment */
#define VNC_DIRTY_BITS_PER_LINE(x) (sizeof(x) / VNC_MAX_HEIGHT * BITS_PER_BYTE)

#define BITS_PER_BYTE           8
#define BITS_PER_LONG           (sizeof (unsigned long) * BITS_PER_BYTE)

#define BITOP_WORD(nr)        ((nr) / BITS_PER_LONG)

#define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))

#define BIT(nr)            (1UL << (nr))
#define BIT_MASK(nr)        (1UL << ((nr) % BITS_PER_LONG))
#define BIT_WORD(nr)        ((nr) / BITS_PER_LONG)
#define BITS_TO_LONGS(nr)    DIV_ROUND_UP(nr, BITS_PER_BYTE * sizeof(long))

#define DECLARE_BITMAP(name,bits)                  \
     unsigned long name[BITS_TO_LONGS(bits)]

#define ctzl ctz64

#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include "time.h"

static inline int ctz64(uint64_t val)
{
     return val ? __builtin_ctzll(val) : 64;
}

DECLARE_BITMAP(dirty[VNC_MAX_HEIGHT], VNC_DIRTY_BITS);

unsigned long find_next_bit(const unsigned long *addr, unsigned long size,
                 unsigned long offset)
{
     const unsigned long *p = addr + BITOP_WORD(offset);
     unsigned long result = offset & ~(BITS_PER_LONG-1);
     unsigned long tmp;

     if (offset >= size) {
         return size;
     }
     size -= result;
     offset %= BITS_PER_LONG;
     if (offset) {
         tmp = *(p++);
         tmp &= (~0UL << offset);
         if (size < BITS_PER_LONG) {
             goto found_first;
         }
         if (tmp) {
             goto found_middle;
         }
         size -= BITS_PER_LONG;
         result += BITS_PER_LONG;
     }
     while (size >= 4*BITS_PER_LONG) {
         unsigned long d1, d2, d3;
         tmp = *p;
         d1 = *(p+1);
         d2 = *(p+2);
         d3 = *(p+3);
         if (tmp) {
             goto found_middle;
         }
         if (d1 | d2 | d3) {
             break;
         }
         p += 4;
         result += 4*BITS_PER_LONG;
         size -= 4*BITS_PER_LONG;
     }
     while (size >= BITS_PER_LONG) {
         if ((tmp = *(p++))) {
             goto found_middle;
         }
         result += BITS_PER_LONG;
         size -= BITS_PER_LONG;
     }
     if (!size) {
         return result;
     }
     tmp = *p;

found_first:
     tmp &= (~0UL >> (BITS_PER_LONG - size));
     if (tmp == 0UL) {        /* Are any bits set? */
         return result + size;    /* Nope. */
     }
found_middle:
     return result + ctzl(tmp);
}

static inline int test_bit(int nr, const unsigned long *addr)
{
     return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1)));
}

static inline void clear_bit(int nr, unsigned long *addr)
{
     unsigned long mask = BIT_MASK(nr);
         unsigned long *p = addr + BIT_WORD(nr);

     *p &= ~mask;
}

static inline void set_bit(int nr, unsigned long *addr)
{
     unsigned long mask = BIT_MASK(nr);
         unsigned long *p = addr + BIT_WORD(nr);

     *p  |= mask;
}

static inline int test_and_clear_bit(int nr, unsigned long *addr)
{
     unsigned long mask = BIT_MASK(nr);
         unsigned long *p = addr + BIT_WORD(nr);
     unsigned long old = *p;

     *p = old & ~mask;
     return (old & mask) != 0;
}

static int find_and_clear_dirty_height(int y, int last_x, int x, int height)
{
     int h;

     for (h = 1; h < (height - y); h++) {
         int tmp_x;
         if (!test_bit(last_x, dirty[y + h])) {
             break;
         }
         for (tmp_x = last_x; tmp_x < x; tmp_x++) {
             clear_bit(tmp_x, dirty[y + h]);
         }
     }

     return h;
}

#define VNC_CLIENT_UPDATE_RECT() \
     if (last_x != -1) {\
         int h = find_and_clear_dirty_height(y, last_x, x, height);\
     }

void vnc_update_client_new() {
         int width = VNC_MAX_WIDTH;
         int height = VNC_MAX_HEIGHT;
         int y = 0;
         for (;;) {
             int x;
             int last_x = -1;
             unsigned long offset = find_next_bit((unsigned long *) &dirty,
                                                  height * VNC_DIRTY_BITS_PER_LINE(dirty),
                                                  y * VNC_DIRTY_BITS_PER_LINE(dirty));
             if (offset == height * VNC_DIRTY_BITS_PER_LINE(dirty)) {
                 /* no more dirty bits */
                 break;
             }
             y = offset / VNC_DIRTY_BITS_PER_LINE(dirty);

             for (x = offset % VNC_DIRTY_BITS_PER_LINE(dirty);
                  x < width / VNC_DIRTY_PIXELS_PER_BIT; x++) {
                 if (test_and_clear_bit(x, dirty[y])) {
                     if (last_x == -1) {
                         last_x = x;
                     }
                 } else {
                     VNC_CLIENT_UPDATE_RECT();
                     last_x = -1;
                 }
             }
             VNC_CLIENT_UPDATE_RECT();
             y++;
         }}

void vnc_update_client_old() {
         int width = VNC_MAX_WIDTH;
         int height = VNC_MAX_HEIGHT;
         int y;
         for (y = 0; y < height; y++) {
             int x;
             int last_x = -1;
             for (x = 0; x < width / 16; x++) {
                 if (test_and_clear_bit(x, dirty[y])) {
                     if (last_x == -1) {
                         last_x = x;
                     }
                 } else {
                     VNC_CLIENT_UPDATE_RECT();
                     last_x = -1;
                 }
             }
             VNC_CLIENT_UPDATE_RECT();
         }
}

void main() {
     int i;
     clock_t start, end;
     start = clock();
     for (i = 0; i < ITERATIONS; i++) {
          memset(dirty, 0x00, sizeof(dirty));
          vnc_update_client_new();
     }
     end = clock();
     printf("All bits clean - vnc_update_client_new: %.2f secs\n", (double) (end-start)/CLOCKS_PER_SEC);
     start = clock();
     for (i = 0; i < ITERATIONS; i++) {
          memset(dirty, 0x00, sizeof(dirty));
          vnc_update_client_old();
     }
     end = clock();
     printf("                 vnc_update_client_old: %.2f secs\n\n", (double) (end-start)/CLOCKS_PER_SEC);
     start = clock();
     for (i = 0; i < ITERATIONS; i++) {
          memset(dirty, 0xff, sizeof(dirty));
          vnc_update_client_new();
     }
     end = clock();
     printf("All bits dirty - vnc_update_client_new: %.2f secs\n", (double) (end-start)/CLOCKS_PER_SEC);
     start = clock();
     for (i = 0; i < ITERATIONS; i++) {
          memset(dirty, 0xff, sizeof(dirty));
          vnc_update_client_old();
     }
     end = clock();
     printf("                 vnc_update_client_old: %.2f secs\n\n", (double) (end-start)/CLOCKS_PER_SEC);
     start = clock();
     for (i = 0; i < ITERATIONS; i++) {
          int y;
          memset(dirty, 0x00, sizeof(dirty));
          for (y = VNC_MAX_HEIGHT/2-8; y < VNC_MAX_HEIGHT/2+8; y++) {
              set_bit(VNC_DIRTY_BITS/2,dirty[y]);
          }
          vnc_update_client_new();
     }
     end = clock();
     printf("Few bits dirty - vnc_update_client_new: %.2f secs\n", (double) (end-start)/CLOCKS_PER_SEC);
     start = clock();
     for (i = 0; i < ITERATIONS; i++) {
          int y;
          memset(dirty, 0x00, sizeof(dirty));
          for (y = VNC_MAX_HEIGHT/2-8; y < VNC_MAX_HEIGHT/2+8; y++) {
              set_bit(VNC_DIRTY_BITS/2,dirty[y]);
          }
          vnc_update_client_old();
     }
     end = clock();
     printf("                 vnc_update_client_old: %.2f secs\n\n", (double) (end-start)/CLOCKS_PER_SEC);
     return;
}


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