Message Queue Priority

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The STREAMS scheduler operates strictly in a first-in-first-out (FIFO) manner so that each QUEUE service procedure receives control in the order it was scheduled.

When a service procedure receives control, it may encounter multiple messages on its message queue. This buildup can occur if there is a long interval between the time a message is queued by a put procedure and the time that the STREAMS scheduler calls the associated service procedure. In this interval, there can be multiple calls to the put procedure causing multiple messages. The service procedure processes all messages on its message queue unless prevented by flow control. Each message must pass through all the modules connecting its origin and destination in the stream.

If service procedures were used in all QUEUEs and there was no message priority, the most recently scheduled message would be processed after all the other scheduled messages on all streams had been processed. In certain cases, message types containing urgent information (such as a break or alarm condition) must pass through the stream quickly. To accommodate these cases, STREAMS assigns priorities to the messages. These priorities order the messages on the queue. Each message has a priority band associated with it. Ordinary messages have a priority of 0. Message priorities range from 0 (ordinary) to 255 (highest). This provides up to 256 bands of message flow within a stream. (See Figure 1.)

Figure 1. Message Ordering on a Queue
This diagram shows the head of the continuum of message ordering on the right and the tail on the left. At the head, are high-priority messages, followed by priority band n messages. The next box of dots represent all bands between n and 2. Following (to the left) are priority band 2 messages and priority band 1 messages. On the tail (left) end are normal band 0 messages.

High-priority messages are not affected by flow control. Their priority band is ignored. The putq utility places high priority messages at the head of the message queue, followed by priority band messages and ordinary messages. STREAMS prevents high-priority messages from being blocked by flow control and causes a service procedure to process them ahead of all other messages on the procedure queue. This procedure results in the high-priority message moving through each module with minimal delay.

Message queues are generally not present in a QUEUE unless that QUEUE contains a service procedure. When a message is passed to the putq utility to schedule the message for service procedure processing, the putq utility places the message on the message queue in order of priority. High-priority messages are placed ahead of all ordinary messages, but behind any other high-priority messages on the queue. Other messages are placed after messages of the same priority that are already on the queue. STREAMS utilities deliver the messages to the processing service procedure in a FIFO manner within each priority band. The service procedure is unaware of the message priority and receives the next message.

Message priority is defined by the message type; after a message is created, its priority cannot be changed. Certain message types come in equivalent high/ordinary priority pairs (for example, M_PCPROTO and M_PROTO), so that a module or device driver can choose between the two priorities when sending information.