presto-main/src/main/java/io/prestosql/sql/planner/LocalExecutionPlanner.java/LocalExecutionPlanner/setDriverInstanceCount
class LocalExecutionPlanner:        public void setDriverInstanceCount(int driverInstanceCount)
        {
            checkArgument(driverInstanceCount > 0, "driverInstanceCount must be > 0");
            if (this.driverInstanceCount.isPresent()) {
                checkState(this.driverInstanceCount.getAsInt() == driverInstanceCount, "driverInstance count already set to " + this.driverInstanceCount.getAsInt());
            }
            this.driverInstanceCount = OptionalInt.of(driverInstanceCount);
        }