Add Power-of-Two-Choices Peak-EWMA load balancer (p2c)#3367
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Implements the p2c load balancing policy proposed in apache#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>
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What problem does this PR solve?
Issue Number: resolve #3340
Problem Summary: brpc has no Power-of-Two-Choices load balancer — the most widely deployed tail-latency-aware policy (Envoy LEAST_REQUEST, Finagle/linkerd Peak-EWMA). A single degraded backend keeps receiving 25% of
rrtraffic until humans intervene;la's averaging window reacts slower than one observation.What is changed and the side effects?
Changed: New
p2cpolicy (src/brpc/policy/p2c_ewma_load_balancer.{h,cpp}): each selection samples two random servers (p2c:choices=Nwidens; evaluates all when N ≥ cluster size, with random tie-breaking to avoid herding) and routes to the lowerpeak_ewma_latency_us * (inflight + 1) / weight. Latency spikes replace the average immediately; recovery decays overtau_ms(default 10s); failures are punished with at least the RPC timeout. Uses only existing hooks (SelectServer/Feedback,DoublyBufferedDatamembership likerr); per-node stats are stable pointers shared by both buffers as inla. Registered inglobal.cpp; 10 unit tests;p2cdocs indocs/{cn,en}/client.md.Benchmark (rpc_press, 4 echo backends, slow = +5ms
bthread_usleep, qps=4000, 15s, 50 threads, 3 reps averaged, zero errors; p99/p999 µs):lap2c(best-of-2)p2c:choices=4rrSlow-node traffic share (1 slow of 4):
rr25%,p2c1.3%,p2c:choices=40.6%,la0.0% — consistent with the numbers on #3340.Side effects:
p2cselection is O(1) (two score evaluations).Check List:
brpc_p2c_ewma_load_balancer_unittestpasses 10/10 andbrpc_load_balancer_unittestpasses 16/16.cc @chenBright (thanks for the green light on #3340!) @zyearn
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