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    SubjectRe: [PATCH] Optimize bitmap_weight
    From
    2012/5/12 Andrew Morton <akpm@linux-foundation.org>:
    > On Fri, 11 May 2012 23:10:14 +0900
    > Akinobu Mita <akinobu.mita@gmail.com> wrote:
    >
    >> The current implementation of bitmap_weight simply evaluates the
    >> population count for each long word of the array, and adds.
    >>
    >> The subsection "Counting 1-bits in an Array" in the revisions of
    >> the book 'Hacker's Delight' explains more superior methods than
    >> the naive method.
    >>
    >> http://www.hackersdelight.org/revisions.pdf
    >> http://www.hackersdelight.org/HDcode/newCode/pop_arrayHS.c.txt
    >>
    >> My benchmark results on Intel Core i3 CPU with 32-bit kernel
    >> showed 50% faster for 8192 bits bitmap.  However, it is not faster
    >> for small bitmap (< BITS_PER_LONG * 8) than the naive method.
    >> So if the bitmap size is known to be small at compile time,
    >> use the naive method.
    >>
    >> ...
    >>
    >>  extern void bitmap_clear(unsigned long *map, int start, int nr);
    >> @@ -277,7 +278,9 @@ static inline int bitmap_weight(const unsigned long *src, int nbits)
    >>  {
    >>       if (small_const_nbits(nbits))
    >>               return hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
    >
    > Why do we require a constant_p `nbits' for this case?
    >
    >> -     return __bitmap_weight(src, nbits);
    >> +     else if (__builtin_constant_p(nbits) && (nbits) < BITS_PER_LONG * 8)
    >> +             return __bitmap_weight(src, nbits);
    >> +     return __bitmap_weight_fast(src, nbits);
    >>  }
    >
    > BITS_PER_LONG*8 sounds like a large bitmap: 256 or 512 entries.  Will
    > the kernel call __bitmap_weight_fast() sufficiently often to make this
    > extra code worth merging?
    >

    I roughly checked the call sites of bitmap_weight() and picked up some
    outstanding usages below.

    Some filesystems (udf, omfs, ntfs, and hpfs) use bitmap_weight() to
    the block size bytes region in statfs() path.

    num_online_cpus() and the variants are bitmap_weight() to the NR_CPUS
    bitmap and num_online_nodes() and the variants are to the MAX_NUMNODES
    bitmap. So these bitmaps could be large on extremely large system.

    bm_count_bits() in drivers/block/drbd/drbd_bitmap.c computes the
    population count for multiple pages. But it is currently open-coded
    loops with hweight_long() which can be converted to bitmap_weight().

    I consider introducing bitmap_weight_large() which is specialized for
    the large bitmap instead of optimizing bitmap_weight() and replace the
    call sites like above.
    --
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