Where to run

At Milan, tier3 used by the ATLAS group is called proof. It is similar to lxplus at CERN. For some tasks, it may be more convenient to use your laptop.

Remote machines and SSH

A common way to interact with remote Linux machines is ssh. A ssh-server must run on the remote machine (by default on the machines we are using) and you need a ssh-client on the machine you use to connect (e.g. your laptop.)

If you have OpenSSH (default on Linux) you can connect to a remote machine with:

ssh username@hostname

Try to connect to the hostname proof-10.mi.infn.it, you have to use the username provided by INFN and the relative password which will be asked. Answer yes to the questions (you trust the remote domain). You will get a new shell into a Linux machine. The commands you execute are executed on the remote machine. Proof and lxplus are made by several machines, so each time you will be connected to a different one. Nevertheless, all machines share the same filesystem, so you should not notice any difference.

You have very limited software installed after the login. For example, you can use emacs or vi to edit text files.

To close the session you can use exit or Ctrl+d

If you want to copy a file to the remote machine you can use scp from your computer:

scp -R mydirectory username@proof-10.mi.infn.it:

this will copy the local directory mydirectory to proof. Since you haven't specified a path after : it will be copied in your $HOME.

On the contrary

scp username@proof-10.mi.infn.it:myfile.txt .

will copy the remote file myfile.txt into the current directory (.).

Terminal multiplexer

One problem with ssh is that the programs running in a session are usually terminated if the session is ended. For example, you would like to open a ssh session, run a command, close the session and then see the output of the command. To this, you have to tell the system to don't halt the processes when the SSH session is over. There are many ways to do that (e.g. nohup) but using a terminal multiplexer has many other advantages other than detach/attaching sessions.

for i in $( ls /etc ); do
    echo $i
    sleep 1
done

tmux is a younger project that gives you advanced possibilities. You have to install it manually (you can use this script to change the version to the latest one).

Proof and lxplus

Proof (the name comes from this technology that was popular in 2010) is the main local computing resource dedicated to the ATLAS group in Milan provided by INFN. You have to connect to a specific machine, the most powerful is proof-10

ssh username@proof-10.mi.infn.it

lxplus is similar to proof but it is provided by CERN. It is available at lxplus.cern.ch. You have to use your username/password provided by CERN.

The setups are very similar, for example, both provide cvmfs.

Using container (ssh to old machine)

It is possible to connect to a host and to run a container automatically. Presently the operating system installed in proof is centos7, which is old and in some cases it is considered as deprecated (e.g. vscode). You can change your ssh configuration (~/.ssh/config) creating a new hostname, for example

Host el9-proof-10
     RemoteCommand bash -c 'export ATLAS_LOCAL_ROOT_BASE=/cvmfs/atlas.cern.ch/repo/ATLASLocalRootBase; source ${ATLAS_LOCAL_ROOT_BASE}/user/atlasLocalSetup.sh -c el9 -m /storage_tmp:/storage_tmp -m /cnfs/homes_fs/home/your-username:/cnfs/homes_fs/home/your-username -m /users2:/users2'
     HostName proof-10.mi.infn.it
     User your-username
     RequestTTY yes

You have to modify the above setup with the mounting points you need (e.g. your home, i.e. /cnfs/homes_fs/home/your-username). You can connect to it with ssh el9-proof-10.

Your laptop

Your laptop can be convenient to develop and test the code. If you want to use the software that is available on the removal machine there are three ways:

• install the software on your machine, for example, you can install ROOT or Python packages in virtualenv using pip. You can also consider the package manager of your system.
• mount cmvfs (use CVMFS_HTTP_PROXY=DIRECT even if not optimal) and source that software as you do on remote machines
• user docker on your laptop

Batch system: HT condor

When you connect to proof or lxplus you get an interactive terminal. Batch systems work in different ways. Usually they are a set of machines where you can submit your jobs. The jobs are executed following specific rules, that take into account your priority and the resource availability. Batch systems are used for heavy jobs. Since they provide many cpus they are well suited when the task can be splitted in many jobs that can run in parallel.

Both lxplus and proof implement the HT condor batch system](https://research.cs.wisc.edu/htcondor/). A complete guide is provided by CERN: https://batchdocs.web.cern.ch/. The description of the Milano cluster is here: https://calcolo.mi.infn.it/?page_id=4582.