Smarter HashMap/HashSet pre-allocation for extend/from_iter #38017
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r? @aturon (rust_highfive has picked a reviewer for you, use r? to override) |
| let iter = iter.into_iter(); | ||
| let hint = iter.size_hint().0; | ||
| self.reserve((hint / 2) + (hint & 1)); | ||
| for (k, v) in iter { |
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This will not compile :P
Might be simpler to just call down to self.map.extend(iter.into_iter().map(|k| (k, ())).
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I totally butchered the set part... will amend in a bit.
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I wonder if it should try to take into consideration the upper bound. |
| // will only resize twice in the worst case. | ||
| let iter = iter.into_iter(); | ||
| let hint = iter.size_hint().0; | ||
| self.reserve((hint / 2) + (hint & 1)); |
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Usually to round up you add (denom-1) before the division, as in (hint + 1) / 2.
(Note that the current code is correct, it is just a little unusual and it would require nontrivial changes if the divisor was changed)
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That makes much more sense, will fix.
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What I wanted to do here was to try this on our favourite hashmap workload (rustc) and see what the effect is. Haven't had time to do it yet. |
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That's a great idea, thanks! |
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It's noisy, results are a bit all over the place (So they are about the same). For example, we can retry piston-image and syntex: Why is syntex so quick, what happened with that? I also looked at memory usage in time-passes for piston-image and it is the same before and after. |
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Thanks @bluss! If we want to be extra sure not to regress we can revert the from_iter part, or, reserve the entire hint on extend if the hashmap is empty, since it's essentially the same runtime cost (except memory of course). |
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According to the result, something is regressing in the super small crates (hello world, issue-32062-equ, and syntex) and the rest are as good as unchanged. |
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I changed it to reserve everything if the map is empty, so if the regression was due to slower from_iter it should now be fixed. |
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@bors r+ Let's go with this; extend will reserve the size hint's lower bound by half. |
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📌 Commit 2c5d240 has been approved by |
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@bors r- Oops, I guess T-libs means that the libs team want to discuss it first? |
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@bors: r=bluss oh I actually just meant that as categorization, this is fine to go through without libs discussion :) |
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📌 Commit 2c5d240 has been approved by |
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great, thanks for the info |
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⌛ Testing commit 2c5d240 with merge 5f128ed... |
Smarter HashMap/HashSet pre-allocation for extend/from_iter HashMap/HashSet from_iter and extend are making totally different assumptions. A more balanced decision may allocate half the lower hint (rounding up). For "well defined" iterators this effectively limits the worst case to two resizes (the initial reserve + one resize). cc #36579 cc @bluss
HashMap/HashSet from_iter and extend are making totally different assumptions.
A more balanced decision may allocate half the lower hint (rounding up). For "well defined" iterators this effectively limits the worst case to two resizes (the initial reserve + one resize).
cc #36579
cc @bluss