Keeping track of CPU time

The adaptive partitioning thread scheduler throttles CPU usage by measuring the average CPU usage of each partition. The average is computed over an averaging window (typically 100 milliseconds), a value that is configurable.

However, the thread scheduler doesn't wait 100 milliseconds to compute the average. As soon as 1 millisecond passes, the usage for this 1 millisecond is added to the usage of the previous 99 milliseconds to compute the total CPU usage over the averaging window (i.e., 100 milliseconds).

Figure 1. The averaging window moves forward as time advances.

The window size defines the averaging time over which the thread scheduler attempts to balance the partitions to their guaranteed CPU limits. You can set the averaging window size to any value from 8 milliseconds to 400 milliseconds.

Different choices of the window size affect both the accuracy of load balancing and, in extreme cases, the maximum delays seen by ready-to-run threads. For more information, see the Considerations for the Thread Scheduler chapter.

The thread scheduler accounts for time spent to the actual fraction of clock ticks used. Time accounting is done not only on each timer tick, but also every time a thread starts or stops running. We refer to this as microbilling.