[Qemu-devel] Re: [PATCH] qemu-kvm: Speed up of the dirty-bitmap-traveling

[Jan Kiszka]

OHMURA Kei wrote:
> dirty-bitmap-traveling is carried out by byte size in qemu-kvm.c.
> But We think that dirty-bitmap-traveling by long size is faster than by byte
> size especially when most of memory is not dirty.

Sounds logical - do you have numbers on the improvement?

Would be great if you could provide a version for upstream as well
because it will likely replace this qemu-kvm code on day.

Jan

> 
> Signed-off-by: OHMURA Kei <address@hidden>
> ---
>  qemu-kvm.c |   49 ++++++++++++++++++++++++++++++++++++++++++-------
>  1 files changed, 42 insertions(+), 7 deletions(-)
> 
> diff --git a/qemu-kvm.c b/qemu-kvm.c
> index a305907..5459cdd 100644
> --- a/qemu-kvm.c
> +++ b/qemu-kvm.c
> @@ -2433,22 +2433,21 @@ int kvm_physical_memory_set_dirty_tracking(int enable)
>  }
>  
>  /* get kvm's dirty pages bitmap and update qemu's */
> -static int kvm_get_dirty_pages_log_range(unsigned long start_addr,
> -                                         unsigned char *bitmap,
> -                                         unsigned long offset,
> -                                         unsigned long mem_size)
> +static void kvm_get_dirty_pages_log_range_by_byte(unsigned int start,
> +                                                  unsigned int end,
> +                                                  unsigned char *bitmap,
> +                                                  unsigned long offset)
>  {
>      unsigned int i, j, n = 0;
>      unsigned char c;
>      unsigned long page_number, addr, addr1;
>      ram_addr_t ram_addr;
> -    unsigned int len = ((mem_size / TARGET_PAGE_SIZE) + 7) / 8;
>  
>      /* 
>       * bitmap-traveling is faster than memory-traveling (for addr...) 
>       * especially when most of the memory is not dirty.
>       */
> -    for (i = 0; i < len; i++) {
> +    for (i = start; i < end; i++) {
>          c = bitmap[i];
>          while (c > 0) {
>              j = ffsl(c) - 1;
> @@ -2461,13 +2460,49 @@ static int kvm_get_dirty_pages_log_range(unsigned 
> long start_addr,
>              n++;
>          }
>      }
> +}
> +
> +static int kvm_get_dirty_pages_log_range_by_long(unsigned long start_addr,
> +                                                 unsigned char *bitmap,
> +                                                 unsigned long offset,
> +                                                 unsigned long mem_size)
> +{
> +    unsigned int i;
> +    unsigned int len;
> +    unsigned long *bitmap_ul = (unsigned long *)bitmap;
> +
> +    /* bitmap-traveling by long size is faster than by byte size
> +     * especially when most of memory is not dirty.
> +     * bitmap should be long-size aligned for traveling by long.
> +     */
> +    if (((unsigned long)bitmap & (TARGET_LONG_SIZE - 1)) == 0) {
> +        len = ((mem_size / TARGET_PAGE_SIZE) + TARGET_LONG_BITS - 1) /
> +            TARGET_LONG_BITS;
> +        for (i = 0; i < len; i++)
> +            if (bitmap_ul[i] != 0)
> +                kvm_get_dirty_pages_log_range_by_byte(i * TARGET_LONG_SIZE, 
> +                    (i + 1) * TARGET_LONG_SIZE, bitmap, offset);
> +        /* 
> +         * We will check the remaining dirty-bitmap, 
> +         * when the mem_size is not a multiple of TARGET_LONG_SIZE. 
> +         */ 
> +        if ((mem_size & (TARGET_LONG_SIZE - 1)) != 0) {
> +            len = ((mem_size / TARGET_PAGE_SIZE) + 7) / 8;
> +            kvm_get_dirty_pages_log_range_by_byte(i * TARGET_LONG_SIZE, 
> +                len, bitmap, offset);
> +        }
> +    } else { /* slow path: traveling by byte. */
> +        len = ((mem_size / TARGET_PAGE_SIZE) + 7) / 8;
> +        kvm_get_dirty_pages_log_range_by_byte(0, len, bitmap, offset);
> +    }
> +
>      return 0;
>  }
>  
>  static int kvm_get_dirty_bitmap_cb(unsigned long start, unsigned long len,
>                                     void *bitmap, void *opaque)
>  {
> -    return kvm_get_dirty_pages_log_range(start, bitmap, start, len);
> +    return kvm_get_dirty_pages_log_range_by_long(start, bitmap, start, len);
>  }
>  
>  /* 
> -- 1.6.3.3 

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