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SubjectRe: [PATCH] Optimize bitmap_weight
On Mon, 14 May 2012 06:50:15 +0900
Akinobu Mita <akinobu.mita@gmail.com> wrote:

> 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?

^^ this?

> >> - 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.

I don't see much advantage to that - it would be better if
bitmap_weight() Just Works.


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