Multithreading with libssh

[Simon Moselewski <s.moselewski@xxxxxxxx>]

Hi everyone,

I am using libssh to connect to a remote host, run commands on it and read the results. Since I am doing this for 1500 commands I want to use threads to parallelize that.

I am using gcc compiler with flags -libssh -pthread and -lssh_threads.

My threadless approach was:
open session
repeat 1500 times {
    open channel
    run remote command
    close channel
}
close session

I read in the docu, that sessions cannot be shared over multiple threads, so with the threadful approach I do it differently:
repeat 1500 times {
    open session
    open channel
    run remote command
    close channel
    close session
}

The threadful programm takes longer than my threadless approach so I guess something with the threading is not working correctly.

I am very inexperienced with threads and would need some help here.

Thanks in advance for any hints!!!
Regards
Simon



My threadful code (shortened):

ssh_threads_set_callbacks(ssh_threads_get_pthread());
ssh_init();
std::mutex mutex;
std::vector<std::future<void>> tasks;
tasks.reserve(16);
for (list<Metric>::iterator it = metrics.begin(); it != metrics.end(); it++) {
    Metric metric = *it;
    tasks.emplace_back(std::async(std::launch::async, [&ip, &user, &pw, metric, i, &mutex]() {
        std::lock_guard<std::mutex> lock(mutex);
        try {
            ssh_session session = ssh_new();
            ssh_options_set(session, SSH_OPTIONS_HOST, IP);
            rc = ssh_userauth_password(session, USERNAME, PASSWORD);
            ssh_channel channel = ssh_channel_new(session);
            rc = ssh_channel_open_session(channel);
            rc = ssh_channel_request_exec(channel, COMMAND);
            // get output
            char buffer[32] = {0};
            int nbytes;
            nbytes = ssh_channel_read(channel, buffer, sizeof(buffer)-1, 0);
            ssh_channel_send_eof(channel);
            //get exit code
            int exitCode = -1;
            exitCode = ssh_channel_get_exit_status(channel);
            ssh_channel_close(channel);
            ssh_channel_free(channel);
            ssh_disconnect(session);
            ssh_free(session);
         }
         catch(...) {
              //something
         }
     }));
}
std::for_each(std::begin(tasks), std::end(tasks), [](auto& task) {
     task.get();
});
std::cout << "Done.\n";
ssh_finalize();