Processes are allotted a maximum number of threads by the operating system, and this limit is typically difficult or impossible to change. Therefore, this section is more informational than advisory.

If a Kudu tablet server’s thread count exceeds the OS limit, it will crash, usually with a message in the logs like "pthread_create failed: Resource temporarily unavailable". If the system thread count limit is exceeded, other processes on the same node may also crash.

Threads and thread pools are used all over Kudu for various purposes, but the number of threads found in nearly all of these does not scale with load or data/tablet size; instead, the number of threads is either a hard coded constant, a constant defined by a configuration parameter, or based on a static dimension (such as the number of CPU cores).

The only exception to this is the WAL append thread, one of which exists for every "hot" replica.

Note that all replicas may be considered hot at startup, so tablet servers' thread usage will generally peak when started and settle down thereafter.