Copyright by William Lloyd Bircher 2010 - The Laboratory for ...

2.3 Performance Counter Sampling
To sample performance monitoring counters a small kernel that provides periodic
sampling of processor performance counters is developed. This kernel uses a device
driver to provide ring-0 access to user-mode applications. This approach is preferred
over existing user-mode performance counter libraries as it affords more precise control
of sampling and lower overhead. In all experiments, the worst-case sampling overhead
(% CPU time sampling) for performance counter access averages less than 1% for
sampling intervals as low as 16ms. In addition to the performance impact of counter
sampling, there is a power impact which must be minimized. A common problem with
periodically scheduled code, such as performance counter sampling, is excessive
scheduler activity. This activity causes CPUs to frequently exit the idle state to service
interrupts, thus increasing power consumption. The sampling kernel avoids this issue by
explicitly requesting a scheduling interval that exactly matches the required sampling
interval. As a result the scheduler only runs enough to schedule the performance counter
sampling events and background operating system activity.
2.4 Workloads
Workload selection is a critical part of dynamic power management analysis. The focus
on power accounting and prediction requires workloads with widely varying power and
performance levels. Unlike microarchitectural analysis that considers phases within an
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