Special Environment Considerations

AFS

The HTCondor daemons do not run authenticated to AFS; they do not possess AFS tokens. Therefore, no child process of HTCondor will be AFS authenticated. The implication of this is that you must set file permissions so that your job can access any necessary files residing on an AFS volume without relying on having your AFS permissions.

If a job you submit to HTCondor needs to access files residing in AFS, you have the following choices:

  1. Copy the needed files from AFS to either a local hard disk where HTCondor can access them using remote system calls (if this is a standard universe job), or copy them to an NFS volume.

  2. If the files must be kept on AFS, then set a host ACL (using the AFS fs setacl command) on the subdirectory to serve as the current working directory for the job. If this is a standard universe job, then the host ACL needs to give read/write permission to any process on the submit machine. If this is a vanilla universe job, then set the ACL such that any host in the pool can access the files without being authenticated. If you do not know how to use an AFS host ACL, ask the person at your site responsible for the AFS configuration.

The Center for High Throughput Computing hopes to improve upon how HTCondor deals with AFS authentication in a subsequent release.

Please see the Using HTCondor with AFS section for further discussion of this problem.

NFS

If the current working directory when a job is submitted is accessed via an NFS automounter, HTCondor may have problems if the automounter later decides to unmount the volume before the job has completed. This is because condor_submit likely has stored the dynamic mount point as the job’s initial current working directory, and this mount point could become automatically unmounted by the automounter.

There is a simple work around. When submitting the job, use the submit command initialdir to point to the stable access point. For example, suppose the NFS automounter is configured to mount a volume at mount point /a/myserver.company.com/vol1/johndoe whenever the directory /home/johndoe is accessed. Adding the following line to the submit description file solves the problem.

initialdir = /home/johndoe

HTCondor attempts to flush the NFS cache on a submit machine in order to refresh a job’s initial working directory. This allows files written by the job into an NFS mounted initial working directory to be immediately visible on the submit machine. Since the flush operation can require multiple round trips to the NFS server, it is expensive. Therefore, a job may disable the flushing by setting

+IwdFlushNFSCache = False

in the job’s submit description file. See the Job ClassAd Attributes page for a definition of the job ClassAd attribute.

HTCondor Daemons That Do Not Run as root

HTCondor is normally installed such that the HTCondor daemons have root permission. This allows HTCondor to run the condor_shadow daemon and the job with the submitting user’s UID and file access rights. When HTCondor is started as root, HTCondor jobs can access whatever files the user that submits the jobs can.

However, it is possible that the HTCondor installation does not have root access, or has decided not to run the daemons as root. That is unfortunate, since HTCondor is designed to be run as root. To see if HTCondor is running as root on a specific machine, use the command

condor_status -master -l <machine-name>

where <machine-name> is the name of the specified machine. This command displays the full condor_master ClassAd; if the attribute RealUid equals zero, then the HTCondor daemons are indeed running with root access. If the RealUid attribute is not zero, then the HTCondor daemons do not have root access.

NOTE: The Unix program ps is not an effective method of determining if HTCondor is running with root access. When using ps, it may often appear that the daemons are running as the condor user instead of root. However, note that the ps command shows the current effective owner of the process, not the real owner. (See the getuid (2) and geteuid (2) Unix man pages for details.) In Unix, a process running under the real UID of root may switch its effective UID. (See the seteuid (2) man page.) For security reasons, the daemons only set the effective UID to root when absolutely necessary, as it will be to perform a privileged operation.

If daemons are not running with root access, make any and all files and/or directories that the job will touch readable and/or writable by the UID (user id) specified by the RealUid attribute. Often this may mean using the Unix command chmod 777 on the directory from which the HTCondor job is submitted.

Job Leases

A job lease specifies how long a given job will attempt to run on a remote resource, even if that resource loses contact with the submitting machine. Similarly, it is the length of time the submitting machine will spend trying to reconnect to the (now disconnected) execution host, before the submitting machine gives up and tries to claim another resource to run the job. The goal aims at run only once semantics, so that the condor_schedd daemon does not allow the same job to run on multiple sites simultaneously.

If the submitting machine is alive, it periodically renews the job lease, and all is well. If the submitting machine is dead, or the network goes down, the job lease will no longer be renewed. Eventually the lease expires. While the lease has not expired, the execute host continues to try to run the job, in the hope that the submit machine will come back to life and reconnect. If the job completes and the lease has not expired, yet the submitting machine is still dead, the condor_starter daemon will wait for a condor_shadow daemon to reconnect, before sending final information on the job, and its output files. Should the lease expire, the condor_startd daemon kills off the condor_starter daemon and user job.

A default value equal to 40 minutes exists for a job’s ClassAd attribute JobLeaseDuration, or this attribute may be set in the submit description file, using job_lease_duration , to keep a job running in the case that the submit side no longer renews the lease. There is a trade off in setting the value of job_lease_duration . Too small a value, and the job might get killed before the submitting machine has a chance to recover. Forward progress on the job will be lost. Too large a value, and an execute resource will be tied up waiting for the job lease to expire. The value should be chosen based on how long the user is willing to tie up the execute machines, how quickly submit machines come back up, and how much work would be lost if the lease expires, the job is killed, and the job must start over from its beginning.

As a special case, a submit description file setting of

job_lease_duration = 0

as well as utilizing submission other than condor_submit that do not set JobLeaseDuration (such as using the web services interface) results in the corresponding job ClassAd attribute to be explicitly undefined. This has the further effect of changing the duration of a claim lease, the amount of time that the execution machine waits before dropping a claim due to missing keep alive messages.

Heterogeneous Submit: Execution on Differing Architectures

If executables are available for the different platforms of machines in the HTCondor pool, HTCondor can be allowed the choice of a larger number of machines when allocating a machine for a job. Modifications to the submit description file allow this choice of platforms.

A simplified example is a cross submission. An executable is available for one platform, but the submission is done from a different platform. Given the correct executable, the requirements command in the submit description file specifies the target architecture. For example, an executable compiled for a 32-bit Intel processor running Windows Vista, submitted from an Intel architecture running Linux would add the requirement

requirements = Arch == "INTEL" && OpSys == "WINDOWS"

Without this requirement, condor_submit will assume that the program is to be executed on a machine with the same platform as the machine where the job is submitted.

Vanilla Universe Example for Execution on Differing Architectures

A more complex example of a heterogeneous submission occurs when a job may be executed on many different architectures to gain full use of a diverse architecture and operating system pool. If the executables are available for the different architectures, then a modification to the submit description file will allow HTCondor to choose an executable after an available machine is chosen.

A special-purpose Machine Ad substitution macro can be used in string attributes in the submit description file. The macro has the form

$$(MachineAdAttribute)

The $$() informs HTCondor to substitute the requested MachineAdAttribute from the machine where the job will be executed.

An example of the heterogeneous job submission has executables available for two platforms: RHEL 3 on both 32-bit and 64-bit Intel processors. This example uses povray to render images using a popular free rendering engine.

The substitution macro chooses a specific executable after a platform for running the job is chosen. These executables must therefore be named based on the machine attributes that describe a platform. The executables named

povray.LINUX.INTEL
povray.LINUX.X86_64

will work correctly for the macro

povray.$$(OpSys).$$(Arch)

The executables or links to executables with this name are placed into the initial working directory so that they may be found by HTCondor. A submit description file that queues three jobs for this example:

####################
#
# Example of heterogeneous submission
#
####################

universe     = vanilla
Executable   = povray.$$(OpSys).$$(Arch)
Log          = povray.log
Output       = povray.out.$(Process)
Error        = povray.err.$(Process)

Requirements = (Arch == "INTEL" && OpSys == "LINUX") || \
               (Arch == "X86_64" && OpSys =="LINUX")

Arguments    = +W1024 +H768 +Iimage1.pov
Queue

Arguments    = +W1024 +H768 +Iimage2.pov
Queue

Arguments    = +W1024 +H768 +Iimage3.pov
Queue

These jobs are submitted to the vanilla universe to assure that once a job is started on a specific platform, it will finish running on that platform. Switching platforms in the middle of job execution cannot work correctly.

There are two common errors made with the substitution macro. The first is the use of a non-existent MachineAdAttribute. If the specified MachineAdAttribute does not exist in the machine’s ClassAd, then HTCondor will place the job in the held state until the problem is resolved.

The second common error occurs due to an incomplete job set up. For example, the submit description file given above specifies three available executables. If one is missing, HTCondor reports back that an executable is missing when it happens to match the job with a resource that requires the missing binary.

Vanilla Universe Example for Execution on Differing Operating Systems

The addition of several related OpSys attributes assists in selection of specific operating systems and versions in heterogeneous pools.

####################
#
# Example targeting only RedHat platforms
#
####################

universe     = vanilla
Executable   = /bin/date
Log          = distro.log
Output       = distro.out
Error        = distro.err

Requirements = (OpSysName == "RedHat")

Queue
####################
#
# Example targeting RedHat 6 platforms in a heterogeneous Linux pool
#
####################

universe     = vanilla
Executable   = /bin/date
Log          = distro.log
Output       = distro.out
Error        = distro.err

Requirements = ( OpSysName == "RedHat" && OpSysMajorVer == 6)

Queue

Here is a more compact way to specify a RedHat 6 platform.

####################
#
# Example targeting RedHat 6 platforms in a heterogeneous Linux pool
#
####################

universe     = vanilla
Executable   = /bin/date
Log          = distro.log
Output       = distro.out
Error        = distro.err

Requirements = ( OpSysAndVer == "RedHat6")

Queue