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hmap_random_node: Improve distribution

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Improve random distribution for an hmap with a small number of nodes
with the expense of the increased cpu cost.
It would be a fair trade-off because the situation is rather common
for bond, which is currently the only consumer of this API in tree.

Consider 2 items, 4 buckets, no collision.

    bucket 0   item 0
    bucket 1
    bucket 2
    bucket 3   item 1

The old algorithm picks item 0 if rand % 4 == 0.  (25%)
Otherwise it picks item 1.  (75%)

This change makes them 50%.

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
This commit is contained in:
YAMAMOTO Takashi
2014-04-22 13:34:36 +09:00
parent 7d1700980b
commit e58f91a15a
1 changed files with 2 additions and 2 deletions
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4
lib/hmap.c
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@@ -214,8 +214,8 @@ hmap_random_node(const struct hmap *hmap)
size_t n, i;
/* Choose a random non-empty bucket. */
for (i = random_uint32(); ; i++) {
bucket = hmap->buckets[i & hmap->mask];
for (;;) {
bucket = hmap->buckets[random_uint32() & hmap->mask];
if (bucket) {
break;
}