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Use linear scan for small bitmaps

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For small bitmaps, a linear scan of the bitmap is slightly faster than
a tree search - bitmap_t is more compact, and there are fewer writes
since we don't have to propogate state transitions up the tree.
On x86_64 with the current settings, I'm seeing ~.5%-1% CPU improvement
in production canaries with this change.

The old tree code is left since 32bit sizes are much larger (and ffsl
smaller), and maybe the run sizes will change in the future.

This resolves #339.
This commit is contained in:
Dave Watson
2016-02-24 08:04:43 -08:00
committed by Jason Evans
parent 01ecdf32d6
commit b8823ab026
2 changed files with 88 additions and 3 deletions
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41
src/bitmap.c
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@@ -3,6 +3,8 @@
/******************************************************************************/
#ifdef USE_TREE
void
bitmap_info_init(bitmap_info_t *binfo, size_t nbits)
{
@@ -32,6 +34,13 @@ bitmap_info_init(bitmap_info_t *binfo, size_t nbits)
binfo->nbits = nbits;
}
static size_t
bitmap_info_ngroups(const bitmap_info_t *binfo)
{
return (binfo->levels[binfo->nlevels].group_offset);
}
void
bitmap_init(bitmap_t *bitmap, const bitmap_info_t *binfo)
{
@@ -60,13 +69,43 @@ bitmap_init(bitmap_t *bitmap, const bitmap_info_t *binfo)
}
}
#else /* USE_TREE */
void
bitmap_info_init(bitmap_info_t *binfo, size_t nbits)
{
size_t i;
assert(nbits > 0);
assert(nbits <= (ZU(1) << LG_BITMAP_MAXBITS));
i = nbits >> LG_BITMAP_GROUP_NBITS;
if (nbits % BITMAP_GROUP_NBITS != 0)
i++;
binfo->ngroups = i;
binfo->nbits = nbits;
}
static size_t
bitmap_info_ngroups(const bitmap_info_t *binfo)
{
return (binfo->levels[binfo->nlevels].group_offset);
return (binfo->ngroups);
}
void
bitmap_init(bitmap_t *bitmap, const bitmap_info_t *binfo)
{
size_t extra;
memset(bitmap, 0xffU, bitmap_size(binfo));
extra = (binfo->nbits % (binfo->ngroups * BITMAP_GROUP_NBITS));
if (extra != 0)
bitmap[binfo->ngroups - 1] >>= (BITMAP_GROUP_NBITS - extra);
}
#endif /* USE_TREE */
size_t
bitmap_size(const bitmap_info_t *binfo)
{