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

[Peter Lieven]

On 18.11.2013 17:27, Anthony Liguori wrote:

On Nov 18, 2013 12:20 AM, "Peter Lieven" <address@hidden> 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 <address@hidden>

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;
}