====== Login and compute servers at the Department of Mathematics ======

===== Login servers =====
  * login1.math.uni-hamburg.de and login2.math.uni-hamburg.de
  * are available from outside math.uni-hamburg.de via ssh (see [[en:user:server:fingerprints]]),
  * have limited resources, what makes them suitable for
    * logging into compute servers,
    * starting print jobs,
    * scp or rsync,
    * compiling LaTeX documents and similar tasks,
  * may be restarted without prior announcement for example after security updates.

===== Compute servers =====
  * <del>compu1.math.uni-hamburg.de with 24 CPU cores and 40 GB RAM running Ubuntu 18.04 (sporadic problems known),</del>
  * <del>compu2.math.uni-hamburg.de with 24 CPU cores and 48 GB RAM running Ubuntu 18.04 (sporadic problems known),</del>
  * compu3.math.uni-hamburg.de with 20 CPU cores and 384 GB RAM running Ubuntu 22.04,
  * compu4.math.uni-hamburg.de  with 24 CPU cores and 512 GB RAM running Ubuntu 22.04, and
  * compu5.math.uni-hamburg.de  with 64 CPU cores and 1504 GB RAM running Ubuntu 22.04
  * are only available from inside the IP-area math.uni-hamburg.de via SSH (see [[en:user:server:fingerprints]]), for example from login servers or desktop computers,
  * will usually only be restarted after prior announcement on the department blog,
  * are suitable for computation jobs.

===== Computations independent of the ssh connection =====
For login servers, desktop computers etc. to be rebooted after updates, long running computations should always be done disconnected from the ssh connection to the compute server. This is possible using terminal multiplexors like tmux or GNU Screen. Here is an example for the use of tmux:

==== Starting a tmux session ====


From the first ssh session, you should become independent of the kerberos/AFS credentials, which will be destroyed upon closing the ssh session:\\
''kpagsh''

You needs to obtain new credentials [[en:user:server:kpagsh|(Explanation)]]:\\
''%%kinit -l 2w --renewable -r 3month%%''

Now you have got 2 weeks access to files on AFS and are entitled to renew these credentials for 3 months without reentering your password. With these credentials, you can start a tmux session, in which you will run your computations:\\
''tmux''

You can detach from the tmux session, which will continue to run in the background:\\
''<Ctrl>+<B> <D>''

After that, you can close the kpagsh and the ssh connection:\\
''exit''\\
''exit''

==== Restoring a tmux session ====
After logging back in via ssh, you can reconnect to your running tmux session:\\
''tmux attach''

==== Renewing credentials ====
If you have to renew your credentials while a computation is running, you can create a new window in tmux:\\
''<Ctrl>+<B> <C>''

From here, you can renew your kerberos/AFS credentials (in this example for 2 weeks):\\
''kinit -R -l 2w''


Now you can return to the original window 0 (and possibly later back to the new one with 1 instead of 0):\\
''<Ctrl>+<B> <0>''

Alternatively, you can close the window:\\
''exit''

==== Closing the tmux session ====
Before you close the tmux session, you should destroy the kerberos/AFS credentials, which kpagsh made independent of the login:\\
''kdestroy''

You close the tmux session by closing the last window:\\
''exit''

(jan henrik, 2012-02-24)