[issue33205] GROWTH_RATE prevents dict shrinking
Inada Naoki
report at bugs.python.org
Wed Jan 26 03:54:49 EST 2022
Inada Naoki <songofacandy at gmail.com> added the comment:
We do not have *fill* since Python 3.6.
There is a `dk_nentries` instead. But when `insertion_resize()` is called, `dk_nentries` is equal to `USABLE_FRACTION(dk_size)` (dk_size is `1 << dk_log2_size` for now). So it is different from *fill* in the old dict.
I chose `dk_used*3` as GROWTH_RATE because it reserves more spaces when there are dummies than when there is no dummy, as I described in the same comment:
> In case of dict growing without deletion, dk_size is doubled for each resize as current behavior.
> When there are deletion, dk_size is growing aggressively than Python 3.3 (used*2 -> used*3). And it allows dict shrinking after massive deletions.
For example, when current dk_size == 16 and USABLE_FRACTION(dk_size) == 10, new dk_size is:
* used = 10 (dummy=0) -> 32 (31.25%)
* used = 9 (dummy=1) -> 32 (28.125%)
(snip)
* used = 6 (dummy=4) -> 32 (18.75%)
* used = 5 (dummy=5) -> 16 (31.25%)
* used = 4 (dummy=6) -> 16 (25%)
(snip)
* used = 2 (dummy=8) -> 8 (25%)
As you can see, dict is more sparse when there is dummy than when there is no dummy, except used=5/dummy=5 case.
There may be a small room for improvement, especially for `used=5/dummy=5` case. But I am not sure it is worth enough to use more complex GROWTH_RATE than used*3.
Any good idea?
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<https://bugs.python.org/issue33205>
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