* Add Power-of-Two-Choices Peak-EWMA load balancer (p2c)
Implements the p2c load balancing policy proposed in #3340: each
selection samples two random servers (configurable via choices=N) and
routes to the lower peak-EWMA latency * (inflight+1) / weight score.
Upward latency spikes take effect immediately while recovery decays
over tau_ms (default 10s), so a degraded server is shed within one
observation at O(1) selection cost.
Uses only existing LoadBalancer hooks (SelectServer/Feedback,
need_feedback) with DoublyBufferedData membership like rr/la; per-node
stats are shared_ptr-owned by both buffers. Registered as "p2c" in
global.cpp. Includes unit tests (functional, weighted, exclusion,
error punishment, concurrency churn) and docs in cn/en client.md.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
* Reuse SelectIn.begin_time_us to avoid extra clock read per selection
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
* Address review: gflags for tunables, cap error punishment, drop FlatMap init
- p2c_default_choices/p2c_default_tau_ms gflags as channel-overridable defaults
- p2c_max_punish_ms(default 30s) caps the doubled-EWMA error punishment so a
persistently failing server recovers in bounded time after turning healthy
- Servers::server_map relies on FlatMap small-map auto-init
- tests: error_punish_is_capped + feedback_lock_overhead benchmark
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---------
Co-authored-by: rajvarun77 <287367605+rajvarun77@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Bright Chen