# GVT-based Sampling

This collects data immediately after each GVT. By default, the data is collected on a PE basis, but some metrics can be changed to tracking on a KP or LP basis (depending on the metric).

You can also choose to sample less often at GVT using the `--num-gvt=n`

command, where n is the number of GVT computations to complete between each sampling point.

### Simulation engine data sampling

To turn on simulation engine sampling, use the option `--engine-stats=n`

, where n = 1 for GVT-based sampling, 2 for real time sampling, 3 for virtual time sampling, or 4 to turn on all modes.
No model code modifications are needed to collect simulation engine data for any instrumentation mode.

### Model-level data sampling

To turn on the model-level data sampling, you need to use `--model-stats=n`

.
You can turn it on for the GVT-based sampling (`--model-stats=1`

), real time sampling (`--model-stats=2`

), virtual time sampling (`--model-stats=3`

), or all modes (`--model-stats=4`

).
It will sample the model data at the same time as the simulation engine data, depending on the modes you have turned on.
You do not actually need to turn on the simulation engine instrumentation though.
For instance, setting `--model-stats=1`

with no other ROSS instrumentation options specified, will sample model data at each GVT, but will not sample the simulation engine data.