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3.8 The Checkpoint Server

A Checkpoint Server maintains a repository for checkpoint files. Within Condor, checkpoints may be produced only for standard universe jobs. Using checkpoint servers reduces the disk requirements of submitting machines in the pool, since the submitting machines no longer need to store checkpoint files locally. Checkpoint server machines should have a large amount of disk space available, and they should have a fast connection to machines in the Condor pool.

If the spool directories are on a network file system, then checkpoint files will make two trips over the network: one between the submitting machine and the execution machine, and a second between the submitting machine and the network file server. A checkpoint server configured to use the server's local disk means that the checkpoint file will travel only once over the network, between the execution machine and the checkpoint server. The pool may also obtain checkpointing network performance benefits by using multiple checkpoint servers, as discussed below.

Note that it is a good idea to pick very stable machines for the checkpoint servers. If individual checkpoint servers crash, the Condor system will continue to operate, although poorly. While the Condor system will recover from a checkpoint server crash as best it can, there are two problems that can and will occur:

1. A checkpoint cannot be sent to a checkpoint server that is not functioning. Jobs will keep trying to contact the checkpoint server, backing off exponentially in the time they wait between attempts. Normally, jobs only have a limited time to checkpoint before they are kicked off the machine. So, if the checkpoint server is down for a long period of time, chances are that a lot of work will be lost by jobs being killed without writing a checkpoint.

2. If a checkpoint is not available from the checkpoint server, a job cannot be retrieved, and it will either have to be restarted from the beginning, or the job will wait for the server to come back on line. This behavior is controlled with the MAX_DISCARDED_RUN_TIME configuration variable. This variable represents the maximum amount of CPU time the job is willing to discard, by starting a job over from its beginning if the checkpoint server is not responding to requests.

3.8.1 Preparing to Install a Checkpoint Server

The location of checkpoint files changes upon the installation of a checkpoint server. A configuration change will cause currently queued jobs with checkpoints to not be able to find their checkpoints. This results in the jobs with checkpoints remaining indefinitely queued, due to the lack of finding their checkpoints. It is therefore best to either remove jobs from the queues or let them complete before installing a checkpoint server. It is advisable to shut the pool down before doing any maintenance on the checkpoint server. See section 3.10 on page  for details on shutting down the pool.

A graduated installation of the checkpoint server may be accomplished by configuring submit machines as their queues empty.

3.8.2 Installing the Checkpoint Server Module

The files relevant to a checkpoint server are

        sbin/condor_ckpt_server
        etc/examples/condor_config.local.ckpt.server

condor_ckpt_server is the checkpoint server binary. condor_condor_config.local.ckpt.server is an example configuration for a checkpoint server. The settings embodied in this file must be customized with site-specific information.

There are three steps necessary towards running a checkpoint server:

1. Configure the checkpoint server.
2. Start the checkpoint server.
3. Configure the pool to use the checkpoint server.

Configure the Checkpoint Server

Place settings in the local configuration file of the checkpoint server. The file etc/examples/condor_config.local.ckpt.server contains a template for the needed configuration. Insert these into the local configuration file of the checkpoint server machine.

The value of CKPT_SERVER_DIR must be customized. This variable defines the location of checkpoint files. It is better if this location is within a very fast local file system, and preferably a RAID. The speed of this file system will have a direct impact on the speed at which checkpoint files can be retrieved from the remote machines.

The other optional variables are:

DAEMON_LIST

Described in section 3.3.9. To have the checkpoint server managed by the condor_master, the DAEMON_LIST variable's value must list both MASTER and CKPT_SERVER. Also add STARTD to allow jobs to run on the checkpoint server machine. Similarly, add SCHEDD to permit the submission of jobs from the checkpoint server machine.

The remainder of these variables are the checkpoint server-specific versions of the Condor logging entries, as described in section 3.3.4 on page .

CKPT_SERVER_LOG

The location of the checkpoint server log.

MAX_CKPT_SERVER_LOG

Sets the maximum size of the checkpoint server log, before it is saved and the log file restarted.

CKPT_SERVER_DEBUG

Regulates the amount of information printed in the log file. Currently, the only debug level supported is D_ALWAYS.

Start the Checkpoint Server

To start the newly configured checkpoint server, restart Condor on that host to enable the condor_master to notice the new configuration. Do this by sending a condor_restart command from any machine with administrator access to the pool. See section 3.6.9 on page  for full details about IP/host-based security in Condor.

Note that when the condor_ckpt_server starts up, it will immediately inspect any checkpoint files in the location described by the CKPT_SERVER_DIR variable, and determine if any of them are stale. Stale checkpoint files will be removed.

Configure the Pool to Use the Checkpoint Server

After the checkpoint server is running, modify a few configuration variables to let the other machines in the pool know about the new server:

USE_CKPT_SERVER

A boolean value that should be set to True to enable the use of the checkpoint server.

CKPT_SERVER_HOST

Provides the full host name of the machine that is now running the checkpoint server.

It is most convenient to set these variables in the pool's global configuration file, so that they affect all submission machines. However, it is permitted to configure each submission machine separately (using local configuration files), for example if it is desired that not all submission machines begin using the checkpoint server at one time. If the variable USE_CKPT_SERVER is set to False, the submission machine will not use a checkpoint server.

Once these variables are in place, send the command condor_reconfig to all machines in the pool, so the changes take effect. This is described in section 3.10.3 on page .

3.8.3 Configuring the Pool to Use Multiple Checkpoint Servers

A Condor pool may use multiple checkpoint servers. The deployment of checkpoint servers across the network improves the performance of checkpoint production. In this case, Condor machines are configured to send checkpoints to the nearest checkpoint server. There are two main performance benefits to deploying multiple checkpoint servers:

• Checkpoint-related network traffic is localized by intelligent placement of checkpoint servers.
• Better performance implies that jobs spend less time dealing with checkpoints, and more time doing useful work, leading to jobs having a higher success rate before returning a machine to its owner, and workstation owners see Condor jobs leave their machines quicker.

With multiple checkpoint servers running in the pool, the following configuration changes are required to make them active.

Set USE_CKPT_SERVER to True (the default) on all submitting machines where Condor jobs should use a checkpoint server. Additionally, variable STARTER_CHOOSES_CKPT_SERVER should be set to True (the default) on these submitting machines. When True, this variable specifies that the checkpoint server specified by the machine running the job should be used instead of the checkpoint server specified by the submitting machine. See section 3.3.8 on page  for more details. This allows the job to use the checkpoint server closest to the machine on which it is running, instead of the server closest to the submitting machine. For convenience, set these parameters in the global configuration file.

Second, set CKPT_SERVER_HOST on each machine. This identifies the full host name of the checkpoint server machine, and should be the host name of the nearest server to the machine. In the case of multiple checkpoint servers, set this in the local configuration file.

Third, send a condor_reconfig command to all machines in the pool, so that the changes take effect. This is described in section 3.10.3 on page .

After completing these three steps, the jobs in the pool will send their checkpoints to the nearest checkpoint server. On restart, a job will remember where its checkpoint was stored and retrieve it from the appropriate server. After a job successfully writes a checkpoint to a new server, it will remove any previous checkpoints left on other servers.

Note that if the configured checkpoint server is unavailable, the job will keep trying to contact that server. It will not use alternate checkpoint servers. This may change in future versions of Condor.

3.8.4 Checkpoint Server Domains

The configuration described in the previous section ensures that jobs will always write checkpoints to their nearest checkpoint server. In some circumstances, it is also useful to configure Condor to localize checkpoint read transfers, which occur when the job restarts from its last checkpoint on a new machine. To localize these transfers, it is desired to schedule the job on a machine which is near the checkpoint server on which the job's checkpoint is stored.

In terminology, all of the machines configured to use checkpoint server A are in checkpoint server domain A. To localize checkpoint transfers, jobs which run on machines in a given checkpoint server domain should continue running on machines in that domain, thereby transferring checkpoint files in a single local area of the network. There are two possible configurations which specify what a job should do when there are no available machines in its checkpoint server domain:

• The job can remain idle until a workstation in its checkpoint server domain becomes available.
• The job can try to immediately begin executing on a machine in another checkpoint server domain. In this case, the job transfers to a new checkpoint server domain.

These two configurations are described below.

The first step in implementing checkpoint server domains is to include the name of the nearest checkpoint server in the machine ClassAd, so this information can be used in job scheduling decisions. To do this, add the following configuration to each machine:

  CkptServer = "$(CKPT_SERVER_HOST)"
  STARTD_ATTRS = $(STARTD_ATTRS), CkptServer

For convenience, set these variables in the global configuration file. Note that this example assumes that STARTD_ATTRS is previously defined in the configuration. If not, then use the following configuration instead:

  CkptServer = "$(CKPT_SERVER_HOST)"
  STARTD_ATTRS = CkptServer

With this configuration, all machine ClassAds will include a CkptServer attribute, which is the name of the checkpoint server closest to this machine. So, the CkptServer attribute defines the checkpoint server domain of each machine.

To restrict jobs to one checkpoint server domain, modify the jobs' Requirements expression as follows:

  Requirements = ((LastCkptServer == TARGET.CkptServer) || (LastCkptServer =?= UNDEFINED))

This Requirements expression uses the LastCkptServer attribute in the job's ClassAd, which specifies where the job last wrote a checkpoint, and the CkptServer attribute in the machine ClassAd, which specifies the checkpoint server domain. If the job has not yet written a checkpoint, the LastCkptServer attribute will be Undefined, and the job will be able to execute in any checkpoint server domain. However, once the job performs a checkpoint, LastCkptServer will be defined and the job will be restricted to the checkpoint server domain where it started running.

To instead allow jobs to transfer to other checkpoint server domains when there are no available machines in the current checkpoint server domain, modify the jobs' Rank expression as follows:

  Rank = ((LastCkptServer == TARGET.CkptServer) || (LastCkptServer =?= UNDEFINED))

This Rank expression will evaluate to 1 for machines in the job's checkpoint server domain and 0 for other machines. So, the job will prefer to run on machines in its checkpoint server domain, but if no such machines are available, the job will run in a new checkpoint server domain.

The checkpoint server domain Requirements or Rank expressions can be automatically appended to all standard universe jobs submitted in the pool using the configuration variables APPEND_REQ_STANDARD or APPEND_RANK_STANDARD. See section 3.3.14 on page  for more details.