Using libwsong time logger and fixing a performance issue for persistent stable get operations. #68
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+ prerequisite: libwsong
songweijia
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There are two major changes in this pull request.
Instead of using Cascade's own timestamp logger, this pull request shifts to wsong::perf timestamp logger. This facilitates performance tuning/debugging with the underlying library Derecho and external components.
When using persistent get with the stable flag, it will fall back to persistent.get(), which reconstructs the full state of the request version from the start of the log. That is way too expensive. In this pull request, this is optimized by leveraging the per-key backward chain and searching back starting from the current state. TODO: Although this is much faster than persistent::get(), an even faster approach is a per-key index with a little memory overhead.