SSH session automatically disconnects after parallel programming

[yaotianran]

I want to know how much free memory on each node simultaneously. The code I run as following:

get.remote.free.mem <- function(ssh.session) {
      out <- ssh_exec_internal(ssh.session, command = c('cat /proc/meminfo'));
      mem = floor(as.numeric(stringr::str_extract(sys::as_text(out$stdout)[3], '\\d+'))/1024/1024);
      return(mem)
   }

# SSH.SESSION is a list, every item of which is a ssh session
free.mem.list = parallel::mclapply(SSH.SESSION,  get.remote.free.mem, mc.cores = length(SSH.SESSION))

I found that for this code mclapply can run normally only once. After I get the the free memory, re-running the last state (parallel::mclapply) will raise "Error: libssh failure at 'ssh_channel_open_session': Socket error: disconnected".