Dovecot and Linux TCP Stack: Connection Latency under High Concurrency

Concurrency Management and OS Interoperability

Dovecot employs a multi-process architecture where a master process manages worker processes to handle IMAP connections. In environments with high connection bursts, latency may emerge at the boundary between the Dovecot listener and the operating system's TCP stack.

When the volume of concurrent requests exceeds the configured process limits or the OS listen backlog, connections may be queued or delayed before reaching a worker process. This creates a bottleneck that is distinct from standard I/O wait or authentication delays.

Given the interaction between default_process_limit and the system-level TCP backlog, how does Dovecot prioritize the hand-off of queued connections to available worker processes during a burst? Does the listener mechanism introduce additional latency when the process limit is reached but the TCP backlog is not yet full?