Re: [PATCH RESEND v3] bitmap: speed up bitmap_find_free_region

[anish singh]

On Tue, May 7, 2013 at 1:45 PM, Chanho Min <chanho.min@xxxxxxx> wrote:
> In bitmap_find_free_region, If we skip the all-ones words and find bits
> in a not-all-ones word, we can improve performance of it.
>
> For example, If bitmap_find_free_region() is called with order=0, First,
> It scans bitmap array by the increment of long type, then find 1 free bit
> within 1 long type value. In 32 bits system and 1024 bits size, in the worst
> case, We need 1024 for-loops to find 1 free bit. But, If this is applied,
> it takes 64 for-loops. Instead, It will be needed additional if-comparison
> of every word and It can take time slightly as 'Test case 3'. But, In many
> cases, It will speed up significantly.
>
> Test cases bellows show the time taken to execute bitmap_find_free_region()
> before and after patch.
>
> Test condition : ARMv7 1GHZ 32 bits system, 1024 bits size, gcc 4.6.2
>
> Test case 1: order is 0. all bits are one except that last one bit is zero.
>  before patch: 29727 ns
>  after patch: 2349 ns
How did you find out this time?Can  you please post your code
for that?
>
> Test case 2: order is 1. all bits are one except that last 2 contiguous bits
> are zero.
>  before patch: 15475 ns
>  after patch: 2225 ns
>
> Test case 3: order is 1. all words are not-all-ones and don't have 2 contiguous
> bits except that last 2 contiguous are zero.
>  before patch: 15475 ns
>  after patch: 16131 ns
>
> Changes compared to v1:
>  - Modified unnecessarily complicated code.
>  - Fixed the buggy code if `bits' is not an multiple of BITS_PER_LONG.
>
> Changes compared to v2:
>  - Removed bitmap_find_free_one and merged it to bitmap_find_free_region.
>
> Signed-off-by: Chanho Min <chanho.min@xxxxxxx>
> ---
>  lib/bitmap.c |   23 +++++++++++++++--------
>  1 file changed, 15 insertions(+), 8 deletions(-)
>
> diff --git a/lib/bitmap.c b/lib/bitmap.c
> index 06f7e4f..95e8efc 100644
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -1114,14 +1114,21 @@ done:
>   */
>  int bitmap_find_free_region(unsigned long *bitmap, int bits, int order)
>  {
> -       int pos, end;           /* scans bitmap by regions of size order */
> -
> -       for (pos = 0 ; (end = pos + (1 << order)) <= bits; pos = end) {
> -               if (!__reg_op(bitmap, pos, order, REG_OP_ISFREE))
> -                       continue;
> -               __reg_op(bitmap, pos, order, REG_OP_ALLOC);
> -               return pos;
> -       }
> +       int pos, end, nbit, i;  /* scans bitmap by regions of size order */
> +       int nlongs = BITS_TO_LONGS(bits);
> +
> +       for (i = 0; i < nlongs; i++)
> +               if (bitmap[i] != ~0UL) {
> +                       pos = i * BITS_PER_LONG;
> +                       nbit = min(bits, pos + BITS_PER_LONG);
> +                       for (; (end = pos + (1 << order)) <= nbit; pos = end) {
> +                               if (!__reg_op(bitmap, pos, order,
> +                                       REG_OP_ISFREE))
> +                                       continue;
> +                               __reg_op(bitmap, pos, order, REG_OP_ALLOC);
> +                               return pos;
> +                       }
> +               }
>         return -ENOMEM;
>  }
>  EXPORT_SYMBOL(bitmap_find_free_region);
> --
> 1.7.9.5
>
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