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3.3.1.2
Window Sampling
Window sampling is used to circumvent sampling overhead cost when small sampling windows
are required when high-resolution data is desired, as is normally the case for instantaneous
bandwidth measurements. The sampling window programmed small (<100µs), and then a period
rate is set in order to space the samples out over the workload. At the beginning of the programmed
window, the performance counters are started. At the end of the window, the performance counters
are sampled, and then reset. Some fixed periodic time later, the cycle repeats. This methodology is
generally used for homogeneous workloads.
Figure 56. TBS Window Sampling
3.3.2
Random Based, or Statistical Sampling
Random based sampling (RBS) is generally more desirable for analysis of non-homogeneous
workloads, where the periodic nature of TBS may miss transactions. Rather than programming a
period, a random range is programmed, which bounds the average amount of time between
windows. As with windowed TBS, the performance counters are started at the beginning of a
window, and then sampled and reset at the end of the window.
Figure 57. Random Based Sampling
Development Tools User's Guide
Sample, Reset
Counters
Start
Counters
T
F
Time
Sample, Reset
Counters
Start
Counters
T
R
T
Time
®
Intel
IXP2400/IXP2800 Network Processors
Performance Monitoring Unit
T
F
is Sample Window
is Fixed Delay
T
F
T
F
T
T
R
is Sample Window
is Random Delay
R
T
R
T
F
T
F
B2740-01
R
T
B2741-01
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