Reuse scratch space for Sabre best-swap choice #10783
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This re-uses the same growable scratch space for storing the temporary swaps for each choice. This particular space typically needed to grow several times for each swap choice, which was taking up a non-negligible amount of the time we spent in Rust space, especially for large circuits on big coupling maps.
jakelishman
added
performance
Changelog: None
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mtreinish
reviewed
Sep 6, 2023
mtreinish
approved these changes
Sep 6, 2023
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Summary
This re-uses the same growable scratch space for storing the temporary swaps for each choice. This particular space typically needed to grow several times for each swap choice, which was taking up a non-negligible amount of the time we spent in Rust space, especially for large circuits on big coupling maps.
Details and comments
This is a super simple change, but drastically reduces the number of allocations / re-allocations we have to make during the inner Rust-space loops. Using the same test example as #10782, I had the timings go from 2.01(3)s on
mainto 1.88(2)s s with this PR (the numbers are larger because I've got webex open at the moment lol), and the improvements will stack almost perfectly with the improvements of #10782.This PR will conflict slightly with #10782, and I'll update whichever merges second.