Stopping long-running simulations

Sometimes you have a simulation that is running longer than it needs to run or longer than you want it to run. Some examples include

• you create a large input data set and send it off for simulation, but you discovered an error in the code while the simulation was running
• maybe the simulation is running longer than you thought it would and you want to kill it to take a closer look
• you have run the system until some critical event happened (the patient was cured!) and you really don’t need to simulate that last 5 year of treatment
• you want to let a simulation run for 3 minutes maximum; if it is not done by then, just give up and move on

This blog post is about stopping those runs and most of the time, this requires writing a bit of code into the model.

1 User interrupt

When mrgsolve is simulating a data set, it stops every so often to check for the user interrupt signal (Control-C or Esc). So if you have some regrets after starting a simulation, hit Control-C or Esc a bunch of times and that will eventually stop the simulation.

mrgsolve has to stop to check for user interrupt; we don’t want to stop to frequently so as to hurt performance, but we don’t want to stop too infrequently either - that would make you have to wait too long to stop the simulation. By default, mrgsolve counts the records that is processes and stops every256 records to look for the interrupt. This default value can be checked or modified as the interrupt argument to mrgsim() and friends. If you have both observations and records in your data set, this check interval will roughly correspond to every 256 output records. I say roughly because mrgsolve also counts some records that aren’t in your data set but still get implemented (like turning off an infusion). You might want it to look more frequently if you have a model that is very difficult to solve or if you have long spans of time between records. You might want to check less often if the model is very easy to solve. You can set interrupt to a negative number to have is avoid checking at all. Most of the time, it is not necessary to change the check interval.

2 Stopping a model early

mrgsolve has some C++ functions that you can write into your model that will stop the simulation once some condition is met. The functions are

• self.stop()
• self.stop_id()
• self.stop_id_cf()

We’ll walk through each of them.

To stop the simulation and throw an error, use

self.stop();

This can be called in $MAIN or $TABLE and it will just stop the simulation with an error message. You can catch this condition by wrapping the simulation call in try()

output <- try(mrsim(model, data))

and then check to see if the simulation failed

if(inherits(output, "try-error")) {
  # bail out  
} else {
  # keep going  
}

If you want to just stop simulating the current individual, you can use

self.stop_id();

or

self.stop_id_cf();

Both of these will stop simulating the current subject and then move on to the next subject. The difference in behavior comes from what gets filled in for the remaining records for that subject.

• self.stop_id() fills in the remaining rows with NA_real_ so that an easy way to find the stopping point is to select records where ID is not NA
• self.stop_id_cf() fills in the remaining rows with the last simulated value; this approach might be useful in more limited circumstances, but available if needed

3 Setting a maximum run time

It is possible to limit the simulation time for an individual or for the entire problem. This isn’t handled automatically by mrgsolve, but we’ll show you how to write a small bit of code to do this in a very flexible way.

Recall that the $PREAMBLE block gets called once at the start of the problem.

$GLOBAL
#include <time.h>

$PREAMBLE
time_t tstart = time(0);

In $PREAMBLE, I got the current time with time(0) and saved it to tstart The units for start are seconds. Also note that we had to #include the header file (<time.h>) to get that function.

Now, we’ll write some code to check the elapsed time in $TABLE and set a maximum run duration in $PARAM

$PARAM MAXTIME = 180

$TABLE
if( (tstart - time(0) ) > MAXTIME) {
  self.stop_id();
}

So once the run time exceeds MAXTIME (180 seconds), we will stop the run graciously using self.stop_id(). You could make a case for using self.stop() here too, but you get the picture: you can determine (1) when to check the duration (2) what is the maximum duration (3) what to do when the maximum duration is exceeded, etc. The behavior is really up to you with a little bit of coding.

I like this pattern and have plans to write a [ plugin ] that will make coding this a little easier. Stay tuned.

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mrgsolve: mrgsolve.github.io | metrum research group: metrumrg.com