function close( error ) {
		var pids;
		var pid;
		var i;
		if ( error && !err ) {
			err = error;
		}
		debug( 'Instructing child processes to close...' );
		pids = objectKeys( workers );
		for ( i = 0; i < pids.length; i++ ) {
			pid = pids[ i ];
			debug( 'Instructing child process (pid: %d) to close...', pid );
			workers[ pid ].send( 'close' );
		}
	}
Closes all workers.

@private
@param {Error} [error] - error object