Computing Elements

Direct access to the site computing clusters is done by sending pilot jobs in a similar way as it is done for the grid sites. The pilot jobs are sent by a specialized agent called SiteDirector.

The SiteDirector is part of the agents of the Workload Management System, and can’t work alone. Please refer to documentation of the WMS for info about the other WMS components.

The SiteDirector is usually serving one or several sites and can run as part of the central service installation or as an on-site component. At the initialization phase it gets description of the site’s capacity and then runs in a loop performing the following operations:

  • Check if there are tasks in the DIRAC TaskQueue eligible for running on the site;
  • If there are tasks to run, check the site current occupancy in terms of numbers of already running or waiting pilot jobs;
  • If there is a spare capacity on the site, submit a number of pilot jobs corresponding to the number of user jobs in the TaskQueue and the number of slots in the site computing cluster;
  • Monitor the status of submitted pilot jobs, update the PilotAgentsDB accordingly;
  • Retrieve the standard output/error of the pilot jobs.

SiteDirector is submitting pilot jobs with credentials of a user entitled to run generic pilots for the given user community. The generic pilots are called so as they are capable of executing jobs on behalf of community users.

SiteDirector Configuration

The SiteDirector configuration is defined in the standard way as for any DIRAC agent. It belongs to the WorkloadManagement System and its configuration section is:

/Systems/WorkloadManagement/<instance>/Agents/SiteDirector

For detailed information on the CS configuration of the SiteDirector, please refer to the WMS Code Documentation.

Computing Elements

DIRAC can use different computing resources via specialized clients called ComputingElements. Each computing resource is accessed using an appropriate ComputingElement class derived from a common base class.

The ComputingElements should be properly described to be useful. The configuration of the ComputingElement is located in the inside the corresponding site section in the /Resources section. An example of a site description is given below:

Resources
{
  Sites
  {
    # Site administrative domain
    LCG
    {
      # Site section
      LCG.CNAF.it
      {
        # Site name
        Name = CNAF

        # List of valid CEs on the site
        CE = ce01.infn.it, ce02.infn.it

        # Section describing each CE
        CEs
        {
          # Specific CE description section
          ce01.infn.it
          {
            # Type of the CE
            CEType = CREAM

            # Submission mode should be "direct" in order to work with SiteDirector
            # Otherwise the CE will be eligible for the use with third party broker, e.g.
            # gLite WMS
            SubmissionMode = direct

            # Section to describe various queue in the CE
            Queues
            {
              long
              {
                ...
              }
            }
          }
        }
      }
    }
  }
}

This is the general structure in which specific CE descriptions are inserted. The CE configuration is part of the general DIRAC configuration It can be placed in the general Configuration Service or in the local configuration of the DIRAC installation.

Additional info can be found here.

Some CE parameters are confidential, e.g. password of the account used for the SSH tunnel access to a site. The confidential parameters should be stored in the local configuration in protected files.

The SiteDirector is getting the CE descriptions from the configuration and uses them according to their specified capabilities and preferences. Configuration options specific for different types of CEs are describe in the subsections below

CREAM Computing Element

A commented example follows:

# Section placed in the */Resources/Sites/<domain>/<site>/CEs* directory
ce01.infn.it
{
  CEType = CREAM
  SubmissionMode = direct


  Queues
  {
    # The queue section name should be the same as in the BDII description
    long
    {
      # Max CPU time in HEP'06 unit secs
      CPUTime = 10000
      # Max total number of jobs in the queue
      MaxTotalJobs = 5
      # Max number of waiting jobs in the queue
      MaxWaitingJobs = 2
    }
  }
}

SSH Computing Element

The SSHComputingElement is used to submit pilots through an SSH tunnel to computing clusters with various batch systems. A commented example of its configuration follows

# Section placed in the */Resources/Sites/<domain>/<site>/CEs* directory
pc.farm.ch
{
  CEType = SSH
  # Type of the local batch system. Available batch system implementations are:
  # Torque, Condor, GE, LSF, OAR, SLURM
  BatchSystem = Torque
  SubmissionMode = direct
  SSHHost = pc.domain.ch
  # SSH connection details to be defined in the local configuration
  # of the corresponding SiteDirector
  SSHUser = dirac_ssh
  SSHPassword = XXXXXXX
  # Alternatively, the private key location can be specified instead
  # of the SSHPassword
  SSHKey = /path/to/the/key
  # SSH port if not standard one
  SSHPort = 222
  # Sometimes we need an extra tunnel where the batch system is on accessible
  # directly from the site gateway host
  SSHTunnel = ssh pcbatch.domain.ch
  # SSH type: ssh (default) or gsissh
  SSHType = ssh
  # Options to SSH command
  SSHOptions = -o option1=something -o option2=somethingelse
  # Queues section contining queue definitions
  Queues
  {
    # The queue section name should be the same as the name of the actual batch queue
    long
    {
      # Max CPU time in HEP'06 unit secs
      CPUTime = 10000
      # Max total number of jobs in the queue
      MaxTotalJobs = 5
      # Max number of waitin jobs in the queue
      MaxWaitingJobs = 2
      # Flag to include pilot proxy in the payload sent to the batch system
      BundleProxy = True
      # Directory on the CE site where the pilot standard output stream will be stored
      BatchOutput = /home/dirac_ssh/localsite/output
      # Directory on the CE site where the pilot standard output stream will be stored
      BatchError = /home/dirac_ssh/localsite/error
      # Directory where the payload executable will be stored temporarily before
      # submission to the batch system
      ExecutableArea = /home/dirac_ssh/localsite/submission
      # Extra options to be passed to the qsub job submission command
      SubmitOptions =
      # Flag to remove the pilot output after it was retrieved
      RemoveOutput = True
    }
  }
}

SSHBatch Computing Element

This is an extension of the SSHComputingElement capable of submitting several jobs on one host.

Like all SSH Computing Elements, it’s defined like the following:

# Section placed in the */Resources/Sites/<domain>/<site>/CEs* directory
pc.farm.ch
{
  CEType = SSHBatch
  SubmissionMode = direct

  # Parameters of the SSH conection to the site. The /2 indicates how many cores can be used on that host.
  # It's equivalent to the number of jobs that can run in parallel.
  SSHHost = pc.domain.ch/2
  SSHUser = dirac_ssh
  # if SSH password is not given, the public key connection is assumed.
  # Do not put this in the CS, put it in the local dirac.cfg of the host.
  # You don't want external people to see the password.
  SSHPassword = XXXXXXXXX
  # If no password, specify the key path
  SSHKey = /path/to/key.pub
  # In case your SSH connection requires specific attributes (see below) available in late v6r10 versions (TBD).
  SSHOptions = -o option1=something -o option2=somethingelse

  Queues
  {
    # Similar to the corresponding SSHComputingElement section
  }
}

New in version >: v6r10 The SSHOptions option.

The SSHOptions is needed when for example the user used to run the agent isn’t local and requires access to afs. As the way the agents are started isn’t a login, they does not have access to afs (as they have no token), so no access to the HOME directory. Even if the HOME environment variable is replaced, ssh still looks up the original home directory. If the ssh key and/or the known_hosts file is hosted on afs, the ssh connection is likely to fail. The solution is to pass explicitely the options to ssh with the SSHOptions option. For example:

SSHOptions = -o UserKnownHostsFile=/local/path/to/known_hosts

allows to have a local copy of the known_hosts file, independent of the HOME directory.

InProcessComputingElement

The InProcessComputingElement is usually invoked by a JobAgent to execute user jobs in the same process as the one of the JobAgent. Its configuration options are usually defined in the local configuration /Resources/Computing/CEDefaults section

Resources
{
  Computing
  {
    CEDefaults
    {
      NumberOfProcessors = 2
      Tag = MultiProcessor
      RequiredTag = MultiProcessor
    }
  }
}

PoolComputingElement

The Pool Computing Element is used on multi-processor nodes, e.g. cloud VMs and can execute several user payloads in parallel using an internal ProcessPool. Its configuration is also defined by pilots locally in the /Resources/Computing/CEDefaults section

Resources
{
  Computing
  {
    CEDefaults
    {
      NumberOfProcessors = 2
      Tag = MultiProcessor
      RequiredTag = MultiProcessor
      # The MultiProcessorStrategy flag defines if the Pool Computing Element
      # will generate several descriptions to present possibly several queries
      # to the Matcher in each cycle trying to select multi-processor jobs first
      # and, if no match found, simple jobs finally
      MultiProcessorStrategy = True
    }
  }
}