[Qemu-devel] Re: [PATCH v2] qemu-kvm: Speed up of the dirty-bitmap-traveling

[Ulrich Drepper]

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On 02/10/2010 02:52 AM, OHMURA Kei wrote:

>      for (i = 0; i < len; i++) {
> -        c = bitmap[i];
> -        while (c > 0) {
> -            j = ffsl(c) - 1;
> -            c &= ~(1u << j);
> -            page_number = i * 8 + j;
> -            addr1 = page_number * TARGET_PAGE_SIZE;
> -            addr = offset + addr1;
> -            ram_addr = cpu_get_physical_page_desc(addr);
> -            cpu_physical_memory_set_dirty(ram_addr);
> -            n++;
> +        if (bitmap_ul[i] != 0) {
> +            c = le_bswap(bitmap_ul[i], HOST_LONG_BITS);
> +            while (c > 0) {
> +                j = ffsl(c) - 1;
> +                c &= ~(1ul << j);
> +                page_number = i * HOST_LONG_BITS + j;
> +                addr1 = page_number * TARGET_PAGE_SIZE;
> +                addr = offset + addr1;
> +                ram_addr = cpu_get_physical_page_desc(addr);
> +                cpu_physical_memory_set_dirty(ram_addr);
> +            }

If you're optimizing this code you might want to do it all.  The
compiler might not see through the bswap call and create unnecessary
data dependencies.  Especially problematic if the bitmap is really
sparse.  Also, the outer test is != while the inner test is >.  Be
consistent.  I suggest to replace the inner loop with

      do {
        ...
      } while (c != 0);

Depending on how sparse the bitmap is populated this might reduce the
number of data dependencies quite a bit.

- -- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖
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