improve worst-case performance of BTreeSet intersection v1

[ssomers]

Major performance boost when comparing tiny and huge sets. Probably with controversial changes and I sure have questions:

• Names and places of functions and types
• How many comments to write where
• Why does rustc tell me to ref mut and ref matches on the iterator, while the book says ref is old school: fixed in code, should take it up to compiler people.
• (Why) do I have to write out the clone like that (#[derive(Clone)] doesn't work): see improve worst-case performance of BTreeSet intersection v3 #59186 (comment)_
• Am I allowed to use #[derive(Debug)] there at all? Yes, but the actual Debug.fmt implementation should supersede it.
• I'd like to test function are_proportionate_for_intersection in test_intersection (or another test case next to it) itself, but I think the private function is inaccessible there. liballoc has other test cases not in the tests directory.

PS I don't list these questions to start a discussion here, just to inspire reviewers and to remember myself.

[rust-highfive]

r? @KodrAus

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[KodrAus]

Thanks @ssomers!

Would you be able to put together some benchmarks so we could get an idea of what the impact is for intersecting variously sized sets?

[ssomers]

put together some benchmarks so we could get an idea of what the impact is for intersecting variously sized sets?

I'm not sure how to contrast the various implementations in the same build. But I could first PR a benchmark in src\liballoc\benches\btree using the existing implementation, then merge this here and anyone looking closely enough would see times improve.

[ssomers]

Rough numbers on my system:

bset::tree ttest case	ns/iter before this PR	ns/iter after this PR
intersect_neg_vs_pos_100	500	=
intersect_neg_vs_pos_10_vs_10k	60	150
intersect_neg_vs_pos_10k	50,000	=
intersect_neg_vs_pos_10k_vs_10	50,000	400
intersect_pos_vs_neg_100	400	=
intersect_pos_vs_neg_10_vs_10k	40,000	150
intersect_pos_vs_neg_10k	40,000	=
intersect_pos_vs_neg_10k_vs_10	60	400
intersect_random_100	700	=
intersect_random_10_vs_10k	40,000	300
intersect_random_10k	200,000	=
intersect_random_10k_vs_10	50,000	300
intersect_staggered_100	700	=
intersect_staggered_10_vs_10k	80	150
intersect_staggered_10k	70,000	=
intersect_staggered_10k_vs_10	80	150

As you see, intersection of very asymmetric set (10 versus 10,000 elements) is mostly up, except if the set containing only numbers lower than any in the other set (neg = negative ints vs. pos=positive ints) is also a small set (i.e. the algorithm merely iterates over that small set and finishes).

The latter could be improved upon by limiting both sets to the range started with the smallest element of the other set. The whole algorithm could work like that: instead of iterating, find the next element >= the other set's lowest unseen value. But then sometimes/often the binary search towards the next element would be worse than iterating, in particular for the staggered test case.

[KodrAus]

Thanks for putting the numbers together @ssomers. That's much appreciated.

It looks like we aren't regressing in what I'd expect to be the common cases and smoothing out those worst cases is a really nice improvement.

@bors r+

[bors]

📌 Commit 4fd0cc1 has been approved by KodrAus

[bors]

🌲 The tree is currently closed for pull requests below priority 99, this pull request will be tested once the tree is reopened

[ssomers]

With the last commit added on top, rough numbers on my system are almost all down:

bset::tree ttest case	ns/iter before this PR	ns/iter after this PR
intersect_neg_vs_pos_100	500	60
intersect_neg_vs_pos_10_vs_10k	60	50
intersect_neg_vs_pos_10k	50,000	80
intersect_neg_vs_pos_10k_vs_10	50,000	400
intersect_pos_vs_neg_100	400	60
intersect_pos_vs_neg_10_vs_10k	40,000	50
intersect_pos_vs_neg_10k	40,000	90
intersect_pos_vs_neg_10k_vs_10	60	400
intersect_random_100	700	600
intersect_random_10_vs_10k	40,000	300
intersect_random_10k	200,000	170,000
intersect_random_10k_vs_10	50,000	300
intersect_staggered_100	700	=
intersect_staggered_10_vs_10k	80	170
intersect_staggered_10k	70,000	=
intersect_staggered_10k_vs_10	80	180

As to taking this further with "instead of iterating, find the next element >= the other set's lowest unseen value", no cigar: it slows down intersect_random_10k 2.5 times, and (absolute worst case) intersect_staggered_10k 14x.

[ssomers]

More tweaks:

• expose both intersection strategies as an unstable feature
• benchmark both at the same time, along with the actual strategy chosen
• seriously lower the threshold for the new strategy while still erring on the conservative side
• quantify are_proportionate_for_intersection with static assertions instead of unit test

No more ideas at the moment so that's going to be the last batch, unless there are remarks.

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travis_time:end:063bf03b:start=1551137061283394269,finish=1551137273491208861,duration=212207814592
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
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$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
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Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
---

[00:03:32] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:99: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:100: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:102: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:103: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:105: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:106: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:108: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:109: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:111: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:112: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:114: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/benches/btree/set.rs:115: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/collections/btree/set.rs:373: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/collections/btree/set.rs:374: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/collections/btree/set.rs:389: line longer than 100 chars
[00:03:33] tidy error: /checkout/src/liballoc/collections/btree/set.rs:390: line longer than 100 chars
[00:03:34] some tidy checks failed
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[00:03:34] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
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[00:03:34] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:03:34] Build completed unsuccessfully in 0:00:46
[00:03:34] Build completed unsuccessfully in 0:00:46
[00:03:34] make: *** [tidy] Error 1
[00:03:34] Makefile:68: recipe for target 'tidy' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:1fa8d203
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Mon Feb 25 23:31:37 UTC 2019
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$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
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cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
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$ dmesg | grep -i kill

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