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

change. Two major issues were found: minimum OS scheduling quanta and
increase/decrease filter.
Performance
100%
80%
60%
40%
20%
0%
Cannot change
p-state faster
than scheduling
quanta
1 10
TimeCheck (ms)
100 1000
Figure 6.4 Varying OS P-state Transition Rates
First, performance remains constant when scaling from 1 us to 10 ms (< 1 ms not
depicted). This is attributable to the OS implementation of scheduling. For Microsoft
Windows Vista, all processes are scheduled on the 10 ms timer interrupt. Setting
TimeCheck to values less than 10 ms will have no impact since p-state changes, like all
process scheduling, occur only on 10-ms boundaries. Second, even at the minimum
TimeCheck value, performance loss is at 80 percent. The reason is that other settings
become dominant below 10 ms. In order for a p-state transition to occur the workload
must overcome the in-built low-pass filter. This filter is implemented as a combination
of two thresholds: increase/decrease percent and increase/decrease time. The percent
threshold represents the utilization level that must be crossed in order to consider a p-
state change. The threshold must be exceeded for a fixed amount of time specified by
114
Performance
loss increases
as p-state lag
increases