rajvarun77 opened a new pull request, #3526:
URL: https://github.com/apache/brpc/pull/3526
# What & Why
A server that just joined the cluster or restarted is "cold" (empty caches,
unwarmed JIT, unestablished connection pools). Every load balancer hands it a
full traffic share immediately, causing cold-start tail-latency spikes — and
latency-feedback schemes (la, p2c_ewma from #3367) can even oscillate: the cold
server scores badly, gets starved, its stats decay, it gets slammed again. This
PR adds an opt-in cross-scheme slow-start ramp, the same remedy Envoy ships as
`slow_start`.
# Usage
```
-lb_warmup_ms=30000 # ramp window; 0 (default) disables warm-up
entirely
-lb_warmup_curve=1.0 # shape: share = max(min_weight, progress^curve)
-lb_warmup_min_weight=0.1 # initial share, validated (0, 1]
```
# Design
- Join timestamp is stamped per server at `AddServer` (deliberately not on
`Socket` — sockets are shared across channels, so a socket-level stamp would
leak one channel's membership change into another's ramp).
- Ramp math centralized in `load_balancer.{h,cpp}` (`WarmupMultiplier` /
`WarmupAccept`), one implementation for all policies.
- `la` and `p2c_ewma` multiply the ramp into their weight so it composes
with latency scoring instead of fighting it; `rr`, `wrr`, `random`, and
consistent hashing divert probabilistically (chash moves to the next ring node,
temporarily diverting part of the hash affinity).
- The ramp is never applied on the last-chance path: a warming server that
is the only choice still gets picked, so warm-up cannot manufacture EHOSTDOWN.
- Re-adding a server after membership removal restarts the ramp; transient
disconnects do not; all servers ramping together at channel init is a no-op
(shares stay equal).
# Tests & Docs
8 cases in `test/brpc_lb_warmup_unittest.cpp` (disabled-by-default, ramp
math incl. curve shaping and configurable floor, per-policy share convergence
for rr/wrr/random/la/p2c/chash, last-chance exemption, re-add restart); full
`brpc_load_balancer_unittest` (17/17) passing. Documented in
`docs/cn/client.md` and `docs/en/client.md`.
cc @chenBright @wwbmmm
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