If the CPU time didn't identify the OS to be the dominating consumer, we follow the decision tree further and analyze whether the CPU is idling. If that is the case, it means there is still CPU time available that cannot be consumed.

Since we are analyzing a situation where the SLA is not met, that is, the overall system is in a situation where it doesn't perform well enough for the given load, a well-saturated situation would fully utilize the CPU. Idling CPU times thus indicates a liveliness issue.

What needs to be investigated is why the threads are not scheduled by the operating system. This can have multiple causes, such as empty connection or thread pools, deadlock situations, or slow responding external systems. The state of the threads will indicate the cause of the constraint. We again use operating system tooling to investigate the situation.

An example for this category of issues is when an external system that responds slowly is accessed synchronously. It will lead to threads that are waiting for network I/O and can't run. This is the difference to dominating OS consumption, that the thread is not actively executing work but waiting to get scheduled.