[core] Read me on CO in the context of YCSB

core/COORDINATED-OMISSION.md

+YCSB in it's original form suffers from Coordinated Omission[1]:
+* Load is controlled by response time
+* Measurement does not account for missing time
+* Measurement starts at beginning of request rather than at intended beginning
+To provide a minimal correction patch I propose:
+1. Replace internal histogram implementation with HdrHistogram:
+HdrHistogram offers a dynamic range of measurement at a given precision and will
+improve the fidelity of reporting. It allows capturing a much wider range of latencies.
+We could add controls for corrected/uncorrected measurement. This is appropriate for a
+uniform load test, but not for other loads. The mixing of different Ops also makes
+this correction unreliable. This could work for a global histogram. 
+2. Track intended operation start and report latencies from that point in time:
+This will require the introduction of a new measurement point and will inevitably
+include measuring some of the internal preparation steps of the load generator.
+These overhead should be negligible in the context of a network hop, but could
+be corrected for by estimating the load-generator overheads (e.g. by measuring a
+no-op DB).
+
+Further correction suggestions:
+1. Correction load control: currently after a pause the load generator will do
+operations back to back to catchup, this leads to a flat out throughput mode
+of testing as opposed to controlled load.
+2. Move to async model: Scenarios where Ops have no dependency could delegate the
+Op execution to a threadpool and thus separate the request rate control from the
+synchronous execution of Ops. Measurement would start on queuing for execution.
+
+1. https://groups.google.com/forum/#!msg/mechanical-sympathy/icNZJejUHfE/BfDekfBEs_sJ
\ No newline at end of file