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Improve memory footprint of JobOptionsSvc

Marco Clemencicrequested to merge
194-improve-jos-mem-footprint into master

Reduce the memory footprint of JobOptionsSvc by replacing the internal unordered_map<string,PropertyRef> to unordered_map<PropertyId,PropertyRef> where PropertyId uses a vector of pointers to global string instances to save space.

The implementation consists two classes: SharedString and PropertyId.

SharedString translates string to const string* where the actual string is stored in an unordered_set<string> so that we use always the same chunk of memory for duplicated strings. In this way the memory used by a string is one pointer per copy plus the unordered_map metadata plus the actual size of the string.

PropertyId translates a string in the format "chunk.chunk.chunk" in a vector of SharedString instances, one per chunk, plus the std::hash of the original string.

I did not manage yet to measure (or calculate) how much memory we save in this way.

Closes #194 (closed)

Edited by Marco Clemencic

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