An event-driven approach to measuring processor execution state.
The limitations of the legacy approach to measuring CPU busy in Windows and the need for more precise measurements of CPU utilization are recognized in many quarters across the Windows development organization at Microsoft. The legacy sampling approach is doubtless very efficient, and this measurement facility was deeply embedded in the OS kernel’s Scheduler facility, a chunk of code that is very risky to tamper with. But, for example, more efficient power management, something that is crucial for battery-powered Windows devices, strongly argues for an event-driven alternative. You do not want the OS to wake up from a low power state regularly on an idle machine just to perform its CPU usage accounting duties, for example.
A straightforward alternative to periodically sampling the processor execution state is to measure the time spent in each processor state directly. This is accomplished by instrumenting the phase state transitions themselves. Processor state transitions in Windows are known as context switches. A context switch occurs in Windows whenever the processor switches the processor execution context to run a different thread. Processor state transitions also occur as a result of high priority Interrupt Service Routines (ISRs) gaining control following a device interrupt, as well as the Deferred Procedure Calls (DPCs) that ISRs schedule to complete the interrupt processing. By recording the time that each context switch occurs, it is possible to construct a complete and an accurate picture of CPU consumption.
(See a two-part article in MSDN Magazine, entitled “Core OS Events in Windows 7,” written by Insung Park and Alex Bendetovers and published beginning in September 2009. The authors are, respectively, the architect and lead developer of the ETW infrastructure. The article provides a conceptual overview describing how to use the various OS kernel events to reconstruct a state machine for processor execution, along with other diagnostic scenarios. Park and Bendetovers report, “In state machine construction, combining Context Switch, DPC and ISR events enables a very accurate accounting of CPU utilization.”)
It helps to have a good, general understanding of thread scheduling in the OS in order to interpret this stream of events. Figure 3 is a diagram depicting the state machine associated with thread execution. At any point in time, a thread can be in only one of the three states indicated: Waiting, Ready, or Running. The state transition diagram shows the changes in execution state that can occur. A Waiting thread is usually waiting for some event to occur, perhaps a Wait timer to expire, an IO operation to complete, a mouse or keyboard click that signals user interaction with the application, or a synchronization event from another thread that indicates it is OK to continue processing.
A thread that is Ready to run is placed in the Dispatcher’s Ready Queue, which is ordered by priority. When a processor becomes available, the OS Scheduler selects the highest priority thread on the Ready Queue and schedules it for execution on that processor. Once it is running, a thread remains in the Running state until it completes its execution cycle and transitions back to the Wait state. An executing thread can also be interrupted because a higher priority execution unit needs to run (this is known as preemptive scheduling) or it is interrupted by the OS Scheduler because its time-slice has expired. A Running thread can also be delayed because of a page fault, accessing data or an instruction in virtual memory that is not currently resident in physical memory. These thread execution time delays are often referred to as involuntary waits.
Figure 4 associates these thread execution state transitions with the ETW events that record when these transitions occur. The most important of these is the CSwitch event record that is written on every processor context switch. The CSwitch event record indicates the thread ID of the thread that is entering the Running state (the new thread id), the thread ID that was displaced (the old thread ID) and provides the Wait Reason code associated with an old thread ID that is transitioning from Running back to the Wait state. The processor number indicating which logical CPU has undergone this state change is provided in an ETW_Buffer_Context structure associated with the ETW standard record header. In addition, it is necessary to know that Thread 0 from Process 0 indicates the Idle thread, which is dispatched on a processor whenever there are no Ready threads waiting for execution. While a thread other than the Idle thread is “active,” the CPU is considered busy.
Conceptually, a context switch event is something like a processor state switch(oldThreadId, newThreadId), with a time stamp identifying when the context switch occurred. The CPU time of a thread is precisely the amount of time it spends in the Running state. It can be measured using the CSwitch events that show the thread transitioning from Ready to the Running state and the CSwitch events that show that thread transitioning back from the Running state to Waiting. To calculate processor busy, you summarize the amount of time each processor spends when the Idle thread is active and subtract from 100% over the measurement interval.
One complication in this approach is that the ETW infrastructure does not guarantee delivery of every event to a Listener application. If the Listener application cannot keep up with the stream of events, then ETW will drop memory-resident buffers filled with events rather than queue them for delivery later. CSwitch events can occur at very high rates, 20,000-40,000 times per second per CPU are not unusual on busy machines, so there is definitely potential to miss enough of the context switch events to bias the calculations that result. In practice, handling the events efficiently in the Listener application and making appropriate adjustments to the ETW record buffering options can be used to minimize the potential for missing events.
To see this event-driven processor execution state measurement facility at work, access the Resource Monitor application (resmon.exe) that is available beginning in Vista and Windows Server 2008. Resource Monitor can be launched directly from the command line, or from either Performance Monitor plug-in or Task Manager Performance tab. Figure 5 displays a screen shot that shows Resource Monitor in action on a Windows 7 machine, calculating CPU utilization over the last 60 seconds of operation, breaking out that utilization by process. The CPU utilization measurements that ResMon calculates are based on the context switch events. These measurements are very accurate, about as good as it gets from a vantage point inside the OS.
The Resource Monitor measures CPU busy in real time by listening to the ETW event stream that generates an event every time a context switch occurs. It also produces similar reports from memory, disk, and network events.
To summarize these developments, this trace-driven measurement source positions the Windows OS so it could replace its legacy CPU measurement facility with something more reliable and accurate sometime in the near future. Unfortunately, converting all existing features in Windows, including Perfmon and Task Manager, to support the new measurements is a big job, not without its complications and not always as straightforward as one would hope. But we can look forward to future versions of the Windows OS where an accurate, event-driven approach to measuring processor utilization supplants the legacy sampling approach that Task Manager and Perfmon rely on today.
In the next blog entry in this series, I will show a quick example using xperf to calculate the same CPU utilization metrics from the ETW event stream. I will point xperfview at an .etl file gathered during the same measurement interval as the one illustrated in Figure 5 using ResMon..