# Deciding Optimal Session Length¶

On a high level, a benchmark session comprises many rounds. We calculate the CI after collecting new data from each round. The session ends when the CIs of all PIs reach the target. Because each round can have non-stable phases that are not contributing samples, we should maximize the length of each round and minimize the number of rounds. This also has the extra benefit of including those work units that are far from the beginning of the initial work amount.

But we cannot begin the first round using the maximum work amount, because in many cases the maximum work amount can be very large. This is typical in storage benchmarks where the limit can be the total size of a storage device, and it would take several dozens of hours to finish one round that fully writes a device. In network benchmarks, we can even set the maximum work amount to unlimited because these workloads can keep running forever. If we start the first round of workload with the full work amount, we risk letting the user wait too long a time before showing the first result. Therefore, we begin the benchmark session with a few short trial rounds to learn the duration to work unit ratio.

## Workloads that provide unit readings¶

We treat each unit reading as one measurement. One workload round can usually provide hundreds or thousands of measurements, making it faster to reach the required sample size. For instance, if a sequential write workload writes 500 MB data using 1 MB I/O, we can get 500 throughput measurements if the workload saves the duration of each write. Pilot sends the unit reading results through the non-stable phases removal, performs auto-correlation reduction, and uses the rest of the unit readings to calculate the CI. Pilot keeps running new rounds of the workload until the desired width of the CI is reached.

## Workloads that cannot provide unit reading¶

These workloads are handled using the WPS method (see Workloads that cannot provide unit reading), which also performs non-stable phases detection and removal, subsession analysis, and decides the optimal number of rounds to achieve the desired width of CI.