Josef Machytka

Linux and PostgreSQL in the Multiverse of Connections

local CEST (Livestream 2)

PostgreSQL connections are expensive, their count is often the limiting factor for performance and stability. Connection pooling is a primary scaling tool for modern systems with microservices, short-lived clients, and bursty traffic. We all know it, but do we really understand why? This talk offers a deep dive into PostgreSQL and Linux architecture, explaining the concrete costs of “too many connections” in PostgreSQL’s process-per-connection model. We will discuss CPU overhead from context switching; kernel resources such as sockets and file descriptors; differences between the classic Completely Fair Scheduler (CFS) and its newer EEVDF-based scheduling approach; limitations of MVCC snapshot implementation across different PostgreSQL versions; accumulation of allocations in per-session memory contexts; and how much memory a query can allocate during execution. We will also look for guidance on “maximum active connections per CPU core” for OLTP vs. OLAP.

Key takeaways

• Pool when churn and burstiness dominate
• High connection counts amplify CPU context switching
• Idle sessions still consume memory
• Think in “active connections per core”
• Beware the thundering herd
• Version and platform choices influence limits

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