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4.6.2. Sampling Techniques

The trace PMDA employs a rolling-window periodic sampling technique. The arrival time of the data at the trace PMDA in conjunction with the length of the sampling period being maintained by the PMDA determines the recency of the data exported by the PMDA. Through the use of rolling-window sampling, the trace PMDA is able to present a more accurate representation of the available trace data at any given time than it could through use of simple periodic sampling.
The rolling-window sampling technique affects the metrics in Example 4.6, “Rolling-Window Sampling Technique”:

Example 4.6. Rolling-Window Sampling Technique

trace.observe.rate
trace.counter.rate
trace.point.rate
trace.transact.ave_time
trace.transact.max_time
trace.transact.min_time
trace.transact.rate
The remaining metrics are either global counters, control metrics, or the last seen observation value. Section 4.7, “Trace API”, documents in more detail all metrics exported by the trace PMDA.

4.6.2.1. Simple Periodic Sampling

The simple periodic sampling technique uses a single historical buffer to store the history of events that have occurred over the sampling interval. As events occur, they are recorded in the working buffer. At the end of each sampling interval, the working buffer (which at that time holds the historical data for the sampling interval just finished) is copied into the historical buffer, and the working buffer is cleared. It is ready to hold new events from the sampling interval now starting.