Optimize string memory #48
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…d TBB race condition
…into lthomaso/opt-string-mem
fosterbrereton
approved these changes
Aug 31, 2022
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Description
This PR moves the string pools from being per-thread (thread_local) to static & global.
With @fosterbrereton FAH PR, ORC is now using much less memory, and the strings are again big contributors to the memory allocation. It was worth re-visiting. Careful use of a tbb::unordered_map and a mutex array (a trick we use elsewhere in the code) allows us to store strings uniquely.
Having unique strings 1) saves memory and 2) allows equality to be determined by simple pointer comparison. (Which means you don't even have to load the string itself into cache.)
Comparing main to this branch:
memory for 20.3 GB to 12.6 GB. (About 8 GB savings)
runtime from 62s to 53s. And that's on a 128GB machine where it doesn't thrash - if the change gets a machine below the threshold for thrashing, it helps even more.
How Has This Been Tested?
Tested the output to make sure it is stable. No issues found.