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Port the single byte COW heuristic from the 1.8 code path to 1.9. This fixes jruby#168.
ghost
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Great find, btw! Thank you! |
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Had a peek at #168 and determined the problem is indeed a pathological copy-on-write case: the 1.9 code path was missing a heuristic that the 1.8 code uses around not sharing for single character substrings. For 1.9, each read of a character in bench_threaded_reverse.rb was marking the (very long) source string shared, causing an expensive copy of it for each character written in the bench. Lots of extra work.
This pull ports that heuristic to the 1.9 case, taking into account that the string is probably non-ascii encoded.
My before numbers looks very similar to the ones @headius posted in #168. Here are the after numbers on my machine:
Natural questions would be "Is this optimization too localized to the benchmark?" and "How do we know this doesn't degrade performance elsewhere". Here are my thoughts:
Hopefully I haven't missed anything here... let me know if there's any questions!