Advanced Job Scheduling

This page documents some of the more advanced things that the scheduler is capable of.

Metahost entries (“Run on any…”)

Jobs can be scheduled to run against any host with a particular label. This is used through the frontend with the “Run on any…” box (for example, “run on any x86”). Such entries are called metahost entries. Metahost entries will be assigned to eligible and ready hosts dynamically by the scheduler.

“Only if needed” labels

If a label is marked only if needed, any host with that label is not eligible for assignment to metahost entries unless

  • the job’s dependency labels includes that label, or
  • the metahost is against that particular label.

Note that such hosts can still be used by any job if selected explicitly (i.e. not through a metahost).

Atomic Groups

An atomic group is a group of machines that must be scheduled together for a job. Regular jobs cannot be scheduled against hosts within these groups; they must be used together.

Atomic groups are created in the admin interface to specify classes of atomic groups of machines (for example, “x86-64 rack” might be an atomic group). Labels can then be marked as instances of a particular atomic group; in this case, a label would include all machines for a particular group (for instance, “x86-64 rack #1”). Finally, machines can be added to these labels to form the actual groups.


As an example, assume you have twenty hosts, ten x86-64 and ten i386. You wish to run a test that requires a rack of five machines. You might do the following:

  • Create two atomic groups, “x86-64 rack” and “i386 rack”.
  • Create four labels: “x86-64 rack #1” and “x86-64 rack #2” are both labels with atomic group type “x86-64 rack”, and likewise for i386.
  • Assign five x86-64 hosts to the “x86-64 rack #1” label, and the remaining five to the “x86-64 rack #2” label. Likewise for i386.

Now, you could run a job with synch count = 5, and specify that you want to run against one atomic group of type “x86-64 rack” and one of type “i386 rack”. The scheduler will dynamically pick a rack of each type that is ready to run the job. Users trying to schedule regular jobs against hosts within these groups will be unable to do so; they will remain reserved for jobs intended for the entire group.

Variable host counts

Some tests can run against a variable number of machines, and you may with to run such a test against all the ready machines within an atomic group, within some bounds. The scheduler can do this for you – at job run time, it will verify all machines in the group and use all the ones that are ready. The following constraints are available:

  • The “max number of machines” attribute on the Atomic Group specifies the maximum number of machines to use at once.
  • The job’s “synch count” attribute specifies the minimum number of machines to use from the group. If fewer than this number are ready, the job will be unable to run. Note that this behavior is unique to jobs run against atomic groups – normally, synch count specifies the exact number of machines to use, but with an atomic group, the scheduler will use as many machines as are ready (up to the maximum).