Ngspice started with batch option (-b) and rawfile output (-r rawfile) will store all simulation data immediately into the rawfile without keeping them in memory. Thus very large circuits may be simulated, the memory requested upon ngspice start up will depend on the circuit size, but will not increase during simulation.

If you start ngspice in interactive mode or interactively with control section, all data will be kept in memory, to be available for later evaluation. A large circuit may outgrow even Gigabytes of memory. The same may happen after a very long simulation run with many vectors and many time steps to be stored. Issuing the save <nodes> command will help to reduce memory requirements by saving only the data defined by the command. You may alos choose option INTERP (15.1.4) to reduce memory usage.