The operating system dominates the CPU consumption when it's asked to work harder than it usually should. This means that too much CPU time is spent on resource and device management. This includes network and disk I/O, locks, memory management, or context switches.

If the CPU system time is more than a certain percentage value of the user time, the operating system is the dominating consumer. The jPDM identified 10% as a threshold value, based on the experience of analyzing innumerable production situations. That means if the CPU system time is more than 10% of the user time, the bottlenecks are contained in the OS subsystem.

In this case, we investigate the issue further using operating system tools, such as vmstat, perf, netstat, and others.

For example, an enterprise application that retrieves database entries with a huge number of individually executed queries puts lots of pressure on the operating system in managing all these database connections. The overhead spent on establishing each and every network connection will eventually dominate the overall system and represent the constraining resource in the system. Investigating this situation thus shows a big share of CPU time spent in the kernel where the network connections are established.