ParallelExtend with map-reduce

[cuviper]

We were using a LinkedList<Vec<_>> intermediary for general ParallelExtend implementations, gathered using a fold to a vector and collect the list -- effectively a fold-fold-reduce. Now we explicitly fold-map-reduce it for better performance.

Thanks to @bluss for ridiculing the use of LinkedList, which got me to look at this again. At first I tried Vec<Vec<_>> instead, which did perform better, but I had to do this using fold-map-reduce to avoid infinite collect recursion. Then I tried that same tweak on the original code, finding it slightly better yet. So I stand by the use of LinkedList. 😄

test map_collect::i_mod_10_to_i::with_linked_list_collect_vec_sized     ... bench:   5,112,790 ns/iter (+/- 78,988)
test map_collect::i_mod_10_to_i::with_linked_list_map_reduce_vec_sized  ... bench:   4,954,336 ns/iter (+/- 103,285)
test map_collect::i_mod_10_to_i::with_vec_vec_sized                     ... bench:   4,965,241 ns/iter (+/- 80,797)
test map_collect::i_to_i::with_linked_list_collect_vec_sized            ... bench:   9,837,088 ns/iter (+/- 283,665)
test map_collect::i_to_i::with_linked_list_map_reduce_vec_sized         ... bench:   9,687,170 ns/iter (+/- 328,236)
test map_collect::i_to_i::with_vec_vec_sized                            ... bench:   9,690,253 ns/iter (+/- 242,379)
test vec_collect::vec_i::with_linked_list_collect_vec_sized             ... bench:  12,639,762 ns/iter (+/- 260,212)
test vec_collect::vec_i::with_linked_list_map_reduce_vec_sized          ... bench:   9,805,660 ns/iter (+/- 170,434)
test vec_collect::vec_i::with_vec_vec_sized                             ... bench:   9,904,740 ns/iter (+/- 226,382)
test vec_collect::vec_i_filtered::with_linked_list_collect_vec_sized    ... bench:  12,486,357 ns/iter (+/- 239,281)
test vec_collect::vec_i_filtered::with_linked_list_map_reduce_vec_sized ... bench:   9,794,605 ns/iter (+/- 306,361)
test vec_collect::vec_i_filtered::with_vec_vec_sized                    ... bench:   9,986,733 ns/iter (+/- 1,142,244)

[cuviper]

bors r+

[bors]

Build succeeded

• continuous-integration/appveyor/branch
• continuous-integration/travis-ci/push

[bluss]

That's awesome. Nice write-up.