Why doesn't RR load balancing appear to be even?

Envoy utilizes a siloed :ref:threading model <arch_overview_threading>. This means that worker threads and the load balancers that run on them do not coordinate with each other. When utilizing load balancing policies such as :ref:round robin <arch_overview_load_balancing_types_round_robin>, it may thus appear that load balancing is not working properly when using multiple workers. The :option:--concurrency option can be used to adjust the number of workers if desired.

The siloed execution model is also the reason why multiple HTTP/2 or HTTP/3 connections may be established to each upstream; :ref:connection pools <arch_overview_conn_pool> are not shared between workers.