Built-in timers¶

Basic timers¶

Basic built-in timers keep track of the time NEST spent for network construction and actual simulation (propagation of the network state). These timers are active in all simulations with NEST, and the measured times can be checked by querying the corresponding kernel attributes. For example:

nest.time_simulate

The following basic time measurements are available:

Name	Explanation
`time_construction_create`	Cumulative time NEST spent creating neurons and devices
`time_construction_connect`	Cumulative time NEST spent creating connections
`time_simulate`	Time NEST spent in the last `Simulate()`

Note

While preparing the actual simulation after network construction, NEST needs to build the pre-synaptic part of the connection infrastructure, which requires MPI communication (Jordan et al. 2018). This happens only for the first call to Simulate() unless connectivity changed in the meantime, and it may cause significant overhead by adding to time_simulate. Therefore, the cumulative time NEST spent for building the pre-synaptic connection infrastructure is also tracked by a basic timer and available as kernel attribute time_communicate_prepare.

In the context of NEST performance monitoring, other useful kernel attributes are:

Name	Explanation
`biological_time`	Cumulative simulated time
`local_spike_counter`	Number of spikes emitted by the neurons represented on this MPI rank during the last `Simulate()`

Note

nest.ResetKernel() resets all time measurements as well as biological_time and local_spike_counter.

Detailed timers¶

Detailed built-in timers can be activated (and again deactivated) prior to compilation through the CMake flag -Dwith-detailed-timers=ON. They provide further insights into the time NEST spends in different phases of the simulation cycle, but they can impact the runtime. Therefore, detailed timers are by default inactive.

If detailed timers are active, the following time measurements are available as kernel attributes:

Name	Explanation	Part of
`time_gather_target_data`	Cumulative time for communicating connection information from postsynaptic to presynaptic side	`time_communicate_prepare`
`time_communicate_target_data`	Cumulative time for core MPI communication when gathering target data	`time_gather_target_data`
`time_update`	Time for neuron update	`time_simulate`
`time_gather_spike_data`	Time for complete spike exchange after update phase	`time_simulate`
`time_collocate_spike_data`	Time to collocate MPI send buffer from spike register	`time_gather_spike_data`
`time_communicate_spike_data`	Time for communicating spikes between compute nodes	`time_gather_spike_data`
`time_deliver_spike_data`	Time to deliver events from the MPI receive buffers to their local synaptic targets (including synaptic update, e.g. STDP synapses) and to the spike ring buffers of the corresponding postsynaptic neurons	`time_gather_spike_data`
`time_omp_synchronization_construction`	Synchronization time of threads during network construction.	`time_construction_create`, `time_construction_connect`, `time_communicate_prepare`
`time_omp_synchronization_simulation`	Synchronization time of threads during simulation.	`time_simulate`

MPI synchronization timer¶

In order to measure synchronization time between multiple MPI processes, an additional timer can be activated on demand via the -Dwith-mpi-sync-timer=ON CMake flag. This timer measures the time between the end of a process’ update phase (i.e., neuron state propagation) and start of collective communication of spikes between all MPI processes. This timer adds an additional MPI barrier right before the start of communication, which might affect performance.

Name	Explanation
`time_mpi_synchronization`	Time spent waiting for other processes.

Multi-threaded timers¶

In previous NEST versions, only the master thread measured timers. Since NEST 3.9, timers which measure time spent exclusively in multi-threaded environments are recorded by each thread individually.

The legacy timer behavior can be restored via the -Dwith-threaded-timers=OFF CMake flag.

Wall-time vs. CPU-time¶

All timers in NEST measure the actual wall-time spent between starting and stopping the timer. In order to only measure time spent on calculations, there is an additional variant for each of the timers above, suffixed with _cpu. They can be accessed in the exact same way. For example:

nest.time_simulate_cpu