const workerFarm = require('worker-farm')
...

// Number of virtual clients/errors already launched/gathered
let n = 0
let nbErrors = 0
const cpus = require('os').cpus().length
// Perform some maths so that we get the closest possible of the concurrency with the same number of clients per CPU
const options = {
  maxConcurrentWorkers: (concurrency < cpus ? concurrency : cpus),
  maxCallsPerWorker: Infinity,
  maxConcurrentCallsPerWorker: (concurrency < cpus ? 1 : Math.floor(concurrency / cpus)),
  maxConcurrentCalls: Infinity
}
// Total durations of client operations
let durations = {}
// Our farm of virtual clients
const workers = workerFarm(options, require.resolve('./client'))

for (var i = 0; i < total; i++) {
  // Default options include URL, client ID and the # of scenarios it has to execute
  let workerOptions = { url: program.url, index: i + 1, nbScenarios }
  // For the first/last clients include the ramp option as well
  if (i < concurrency) workerOptions.rampUp = ramp
  if (i > total - concurrency) workerOptions.rampDown = ramp
  workers(workerOptions, (err, client) => {
    n++
    if (!err) {
      Object.entries(client.durations).forEach(([key, value]) => {
        // Add client durations to total durations
        if (!durations[key]) durations[key] = 0
        durations[key] += client.durations[key]
      })
    } else {
      nbErrors++
      logger.error(err)
    }
    if (n === total) {
      workerFarm.end(workers)
      // Display total/averaged durations and error ratio
      Object.entries(durations).forEach(([key, value]) => {
        logger.info('Total ' + key + ' time = ' + durations[key].toFixed(2) + ' (s)')
        logger.info('Average ' + key + ' time = ' + (durations[key] / total).toFixed(2) + ' (s)')
      })
      logger.info('Error ratio = ' + (100 * nbErrors / total).toFixed(2) + ' %')
    }
  })
}