On Tue, Oct 13, 2020 at 12:40:44AM +0000, Elijah Newren via GitGitGadget wrote:
> merge-ort is a heavy user of strmaps, which are built on hashmap.[ch].
> reset_maps() in merge-ort was taking about 12% of overall runtime in my
> testcase involving rebasing 35 patches of linux.git across a big rename.
> reset_maps() was calling hashmap_free() followed by hashmap_init(),
> meaning that not only was it freeing all the memory associated with each
> of the strmaps just to immediately allocate a new array again, it was
> allocating a new array that wasy likely smaller than needed (thus
s/wasy/was/
> resulting in later need to rehash things). The ending size of the map
> table on the previous commit was likely almost perfectly sized for the
> next commit we wanted to pick, and not dropping and reallocating the
> table immediately is a win.
>
> Add some new API to hashmap to clear a hashmap of entries without
> freeing map->table (and instead only zeroing it out like alloc_table()
> would do, along with zeroing the count of items in the table and the
> shrink_at field).
This seems like a reasonable optimization to make, and doesn't make the
API significantly more complicated. I'd expect the allocation of actual
entry objects to dwarf the table allocation, but I guess:
- you'll deal with the individual entries later using a mempool
- it's not just the allocation, but the re-insertion of the entries as
we grow
It would be nice if we had some actual perf numbers to report here, so
we could know exactly how much it was buying us. But I guess things are
a bit out-of-order there. You want to do this series first and then
build merge-ort on top as a user. We could introduce the basic data
structure first, then merge-ort, and then start applying optimizations
with real-world measurements. But I'm not sure it's worth the amount of
time you'd have to spend to reorganize in that way.
> hashmap.c | 39 +++++++++++++++++++++++++++------------
> hashmap.h | 13 ++++++++++++-
The implementation itself looks correct to me. I already mentioned my
thoughts on naming in patch 1.
-Peff
