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Placeholder for unrolled small maps enhancement (companion for vectors at CLJ-1517).
Comment made by: jafingerhut
Is there an expectation that these would perform better that PersistentArrayMap?
Comment made by: ztellman
Yes, in some cases significantly so, for three reasons (in rough order of importance):
There are a series of benchmarks at https://github.com/ztellman/cambrian-collections/blob/master/test/cambrian_collections/map_test.clj#L64-L148, which compare operations against maps with both keywords (which don't benefit from the hash comparisons) and symbols (which do). The 7-entry map cases cause the unrolled maps to overflow, so they only exist to test the overflow mechanism.
I've run the benchmark suite on my laptop, and the results are at https://gist.github.com/ztellman/961001e1a77e4f76ee1d. Some notable results:
The rest of the benchmarks are marginally faster due to unrolling, but most of the performance benefits are from the above behaviors. In a less synthetic benchmark, I found that Cheshire JSON decoding (which is 33% JSON lexing and 66% data structure building) was sped up roughly 30-40%.