Understanding Processor: Ready Queue

The Processor Queue Length counter in the System object is an extremely important indicator of processor performance. It is an instantaneous peek at the number of Ready threads that are currently waiting to run. Even though reporting processor utilization is much more popular, the Processor Queue Length is actually a more important indicator of a processor bottleneck. It shows that work is being delayed, and the delay is directly proportional to the length of the queue.

Since there is one Scheduler Dispatch Queue that services all processors, the Queue Length counter is only measured at System level. The Thread State counter in the Thread object indicates precisely which threads are waiting for service at the processor(s). In other words, the Processor Queue Length counter indicates how many threads are waiting in the Scheduler dispatch Ready Queue, while the Thread State counter tells which thread are in the queue. A good working assumption is that when the processor is very busy queuing delays impact the performance of executing threads. The longer the queue, the longer the delays that threads encounter.

By monitoring the Thread State counter for all instances of the Thread object at the same time, it is possible to determine which threads are waiting in the Processor Ready Queue at the time measurements are taken. Note that the instantaneous measure of the size of the Ready Queue is influenced by the dispatchability of the performance monitor application itself. In other words, the Ready Queue at the time the Scheduler was finally able to dispatch the System Monitor Measurement thread. The System Monitor runs at high priority, but by no means at the highest priority in the system.

Priority Scheduling is the general solution designed to cope with situations where processor is very busy. Priority scheduling orders the Ready Queue, ensuring that under conditions of scarcity, the highest-priority work gains favored access to the resource.

(Source: Windows 2000 Performance Guide by Mark Friedman & Odysseas Pentakalos)

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