swoole receive中 from_id 代表啥？

receive函数中有个 from_id参数， 这个是主进程中reactor线程id， 关于主进程中reactor线程说明参考swoole源码-主线程和reactor线程交互流程

$serv = new Swoole\Server("127.0.0.1", 9501);
$serv->on('connect', function ($serv, $fd){
    echo "Client:Connect.\n";
});
$serv->on('receive', function ($serv, $fd, $from_id, $data) {
    $serv->send($fd, 'Swoole: '.$data);
    $serv->close($fd);
});
$serv->on('close', function ($serv, $fd) {
    echo "Client: Close.\n";
});
$serv->start();