First off, if you have not begun to use a compiled configuration, then begin doing so. This will noticeably reduce the startup time of PMDF processing jobs (and on OpenVMS, the startup time for PMDF MAIL) as well as reduce the time spent waiting for a response the first time your users use a PMDF handled address in their user agent, such as send a message from Pine (or on OpenVMS, send to an IN% address in VMS MAIL). See Section 8.1 for instructions on how to generate a compiled configuration.

Consider establishing processing queues (OpenVMS) or Job Controller queues (UNIX and NT) for specific channels which you want to ensure always have processing slots. For instance, set up a separate queue for your pager channel so that delivery jobs for urgent pages do not get held up waiting in the MAIL$BATCH queue (OpenVMS) or DEFAULT queue (UNIX and NT). Then use the queue keyword to direct particular channels to run in particular queues; see Section 2.3.4.18 for more details on the queue keyword, and Section 32.4 for a further discussion of directing channels to run in specific queues.

Busy channels --- channels which usually have immediate jobs processing in the queues --- are likely to achieve greater overall throughput in exchange for slightly increased latency by use of the after channel keyword with a (typically small) delta time value. By specifying a delta time value that, while not introducing too much of a delay for new messages, does allow PMDF to "collect" multiple messages to be handled by one channel job, the overhead of image activation or expensive protocol connections can be reduced. For a busy channel, this can lead to a substantial increase in overall throughput. Channels such as multithreaded TCP/IP channels, or on OpenVMS also the L channel or MR channels, are often candidates. Multithreaded TCP/IP channels, for instance, sort messages to different hosts into different threads; when given multiple messages to deliver to a single host, those messages can then be delivered during a single SMTP connection session. See Section 2.3.4.18 for more details on the after keyword.

If your system has the memory to spare, increasing the size of message that processing jobs can buffer internally can reduce use of temporary buffer files on disk when receiving or processing large messages. See the discussion of the MAX_INTERNAL_BLOCKS PMDF option in Section 7.3.5. On OpenVMS, make sure that the account under which PMDF jobs are operating (normally the SYSTEM account) has sufficient memory quotas; PGFLQUOTA, WSDEF, WSQUO, and WSEXTENT are particularly relevant.

The PMDF Dispatcher controls the creation and use of multithreaded SMTP server processes. If, as is typical, incoming SMTP over TCP/IP messages are a major component of e-mail traffic at your site, monitor how many simultaneous incoming SMTP connections you tend to have, and the pacing at which such connections come in. Tuning of Dispatcher configuration options controlling the number of SMTP server processes, the number of connections each can handle, the threshold at which new server processes are created, etc., can be beneficial if your site's incoming SMTP over TCP/IP traffic is unusually high or low.

For typical SMTP over TCP/IP channels, used to send to multiple different remote systems, the PMDF multithreaded TCP/IP channel's default behavior of sorting messages to different destinations into different threads and then handling all messages to a single host in a single thread is desirable for performance. However, for a daemon TCP/IP channel, one dedicated to sending to a specific system, if the receiving system supports multiple simultaneous connections it can be preferable to force PMDF to split the outgoing messages into separate threads, by using a combination of the threaddepth keyword set for some appropriate value and the MAX_CLIENT_THREADS channel option; see Section 2.3.4.29 and Section 21.1.2.2.

To make better utilization of your CPU resources, consider making MAIL$BATCH a generic queue feeding specific queues across your cluster. If you do this, keep in mind that those channels which use software available on only a few systems must do their processing on those systems. For instance, if you only run Jnet on one system, then you must process your bit_ channels on that system. Use the queue channel keyword to designate which queues a channel should use for its processing. By default, channels will use MAIL$BATCH; thus, you need only specify this keyword on those channels which should use a separate queue. See 2.3.4.18 for information on the queue keyword.

On OpenVMS, if you are not already using the Process Symbiont, then consider using it. By default, MAIL$BATCH is a batch queue. Thus, PMDF's processing jobs are by default batch jobs: each processing job must be created and go through the LOGINOUT procedure. The Process Symbiont reduces this overhead by instead using a pool of detached processes for PMDF processing. When PMDF needs to launch a processing job, one of the idle detached processes is used. This avoids the overhead of creating and logging in a new process for each processing task. Use of Process Symbiont queues also reduces the creation of unnecessary log files. The Process Symbiont is a multi-threaded server symbiont. You can control how many detached processes it runs and how long they are allowed to remain idle before being deleted. As with batch jobs, you can create a generic Process Symbiont queue which feeds specific queues spread across your cluster. See Section 9.1 for instructions on how to configure the Process Symbiont.

On OpenVMS, installing as known images channel programs for often used channels will reduce processing job overhead. For local delivery, you do not need to do anything: the requisite images, SYS$SYSTEM:mail.exe and PMDF_SHARE_LIBRARY, are already installed. Sites with large volumes of outgoing SMTP messages should consider installing the PMDF_EXE:tcp_smtp_client.exe image (using the DCL INSTALL utility's /OPEN/HEADER/SHARED qualifiers).