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bpo-33205: dict: Change GROWTH_RATE to `used*3` (GH-6350) 

(cherry picked from commit 5fbc511f56)

Co-authored-by: INADA Naoki <methane@users.noreply.github.com>
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Miss Islington (bot) 2018-04-17 10:17:19 -07:00 committed by GitHub
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Misc/NEWS.d/next/Core and Builtins/2018-04-03-00-58-41.bpo-33205.lk2F3r.rst Normal file
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@ -0,0 +1,3 @@
Change dict growth function from ``round_up_to_power_2(used*2+hashtable_size/2)`` to
``round_up_to_power_2(used*3)``. Previously, dict is shrinked only when ``used == 0``.
Now dict has more chance to be shrinked.

11
Objects/dictobject.c
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@ -396,17 +396,16 @@ dk_set_index(PyDictKeysObject *keys, Py_ssize_t i, Py_ssize_t ix)
*/
/* GROWTH_RATE. Growth rate upon hitting maximum load.
* Currently set to used*2 + capacity/2.
* Currently set to used*3.
* This means that dicts double in size when growing without deletions,
* but have more head room when the number of deletions is on a par with the
* number of insertions.
* Raising this to used*4 doubles memory consumption depending on the size of
* the dictionary, but results in half the number of resizes, less effort to
* resize.
* number of insertions. See also bpo-17563 and bpo-33205.
*
* GROWTH_RATE was set to used*4 up to version 3.2.
* GROWTH_RATE was set to used*2 in version 3.3.0
* GROWTH_RATE was set to used*2 + capacity/2 in 3.4.0-3.6.0.
*/
#define GROWTH_RATE(d) (((d)->ma_used*2)+((d)->ma_keys->dk_size>>1))
#define GROWTH_RATE(d) ((d)->ma_used*3)
#define ENSURE_ALLOWS_DELETIONS(d) \
if ((d)->ma_keys->dk_lookup == lookdict_unicode_nodummy) { \