PMDF System Manager's Guide

Each time a message is placed in a channel queue for a channel that is marked as master and immediate,⁴ PMDF attempts to start a processing job to deliver the message. The job is placed in the queue specified by the queue keyword in the channel definition. If no queue is specified, the default queue is used.
On OpenVMS systems, the default queue is MAIL$BATCH. PMDF requires CMKRNL and SYSPRV privileges in order to submit jobs. Note that PMDF does not use the $CMKRNL system service directly in submitting jobs: the OpenVMS services PMDF calls to submit jobs require that the calling process have the CMKRNL privilege.

On UNIX and NT, the PMDF Job Controller implements PMDF's job queuing system. The default queue is the queue named DEFAULT in the Job Controller configuration file.

PMDF always checks to see if a processing job is already pending to process messages on the channel requesting service. If such a job is in fact pending, then PMDF does not bother to create an additional, superfluous job. This strategy prevents large numbers of incoming messages from creating inordinate numbers of superfluous delivery processing jobs.

With this scheme, messages typically get delivered very quickly. Unfortunately, a price must be paid in terms of overhead --- approximately one processing job is generated per message. If this overhead is unacceptable (either in terms of CPU overhead or in terms of the expense of making a connection), PMDF can, on a per-channel basis, be prevented from generating jobs on demand. See the discussions of the periodic and after channel keywords in Section 2.3 for details.