Concurrency & Asynchronous Performance Questions

How concurrency choices change throughput, latency and resource use in server-side software. Covers sizing and choosing thread pools and executors for CPU-bound versus I/O-bound work, including bounded queues, rejection policies and work-stealing pools, event-loop behavior and what blocks it, offloading blocking calls, the GIL and process-versus-thread trade-offs, connection and worker pool limits and exhaustion under a slow dependency, lock contention and how to find it (perf, eBPF, flamegraphs), lock sharding and striping, lock convoys, spinning versus blocking and backoff, head-of-line blocking, request coalescing so concurrent callers share one fetch, scheduling pipeline stages across pools, and the coordination costs that cap parallel speedup. Focuses on the performance consequences of concurrency design rather than the correctness theory of synchronization or any one language's API. Primitives and deadlock theory, per-language concurrency APIs, async syntax and promise semantics, message-driven system design, and database isolation are covered elsewhere.