Solon Server Threads: Zero-Config Auto-Tuning by CPU Cores — ioBound, coreThreads, maxThreads

It was 2 AM, and the on-call chat was on fire again: the order service was healthy on every dashboard, but throughput had flatlined at ~800 req/s while P99 climbed past 4 seconds. The usual suspect? A thread pool sized by guesswork during a late-night deploy, six months earlier. We'd hand-tuned maxThreads to "something that felt right," and it wasn't right anymore. That's the moment I started appreciating a different default: in Solon, all of those knobs ship as 0 — meaning auto , derived from your machine's actual CPU cores at runtime. You can go months without thinking about a single thread-pool property. This post walks through the five knobs that exist, how the auto-tuning math works, and the three failure modes that tell you it's time to touch them. The five knobs under the hood Solon exposes these on app.yml (all values are the documented defaults): # Minimum threads for the http server (0 = auto; also accepts fixed values like 2, or core multiples like x2) server.http.coreThreads : 0 # Maximum threads for the http server (0 = auto; also accepts fixed values like 32, or core multiples like x32) server.http.maxThreads : 0 # Idle thread timeout in ms (0 = auto) # supported since v1.10.13 server.http.idleTimeout : 0 # Is this an IO-bound service? (default true) # supported since v1.12.2 server.http.ioBound : true # Enable the virtual thread pool (default false) # supported since v2.7.3 solon.threads.virtual.enabled : false Notice what's missing: no hard-coded defaults for coreThreads or maxThreads . 0 means "figure it out from the hardware." That single decision removes a whole class of "copy-pasted tuning values" problems — the ones that were right for someone else's 32-core box and wrong for your 2-core container. CPU-bound or IO-bound: the one question that matters The auto-tuner only needs you to answer one question: is your workload CPU-bound or IO-bound? CPU-bound : the work happens entirely in CPU and memory — think a "hello world" handler that returns a s