Enable processor-specific optimizations when available:
The SConstruct is modified to enable processor specific optimizations on clang and gcc toolchains. This improves the performance of RocksDB's CRC function. It might also enable other used libraries that are in the codebase now or in the future to apply cpu-specific optimisations. The mtune option ensures that a binary compiled on one machine will function on another,
Avoid zero initialization of Blob:
This seemed to improve the performance of the copy, although there did seem to be some byte by byte copying still present. Further investigation recommended.
Add repeatable NodeStore timing benchmark:
The timing test is changed to overcome possible file buffer cache effects by creating different read access patterns. The unittest-arg command line arguments allow running the benchmarks against any of the available backends and altering the parameters passed in the same format as rippled.cfg. The num_objects parameter permits variation of the number of key/values inserted. The data is random but matches reasonably well the values that rippled might generate.
Add new RocksDBQuickFactory for benchmarking:
This new factory is intended for benchmarking against the existing RocksDBFactory and has the following differences.
* Does not use BatchWriter
* Disables WAL for writes to memtable
* Uses a hash index in blocks
* Uses RocksDB OptimizeFor… functions
See Benchmarks.md for further discussion of some of the issues raised by investigation of RocksDB performance.