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Client multiplexers

Tuning multiplexers for optimal performance

The load of batching, conflating and merging messages being sent from publishers to outbound clients is spread across client multiplexers. The number of configured client multiplexers must take into account the expected message load and concurrent client connections. The more clients are assigned to a multiplexer the more load it must contend with.

By default, the number of client multiplexers is equal to the number of cores on the host system of the Diffusion™ server

A client multiplexer processes all client messages into the client queue. Clients are added to the multiplexers according to a round-robin load balancing policy.

Publishers either broadcast on a topic to all subscribed clients or send clients direct messages. When broadcasting all multiplexers are notified and go on to find all subscribed clients which are assigned to the particular multiplexer. When a message is sent to a particular client only that client's multiplexer is notified. It is more efficient to broadcast than it is to send the same message to a large number of clients by iterating over them.

Client multiplexers are non-blocking, high priority threads so having too many can be detrimental, as they are competing for the same resource (CPU). As a rule of thumb, the number of multiplexers must not exceed the number of available logical cores. If a client multiplexer becomes over-subscribed, message latency can increase. For maximum throughput, the number of multiplexers can be set to the number of available cores, but this configuration is only recommended in the case where other threads are assumed to be mostly idle (for example, little inbound traffic, low publisher overhead).

Client multiplexers performance is influenced by the use of merge and conflation policies as those are executed in the multiplexer thread. It is recommended that conflation policy changes and in particular changes to merge conflation logic be profiled and written with performance in mind. In particular the use of locks or any other blocking code is highly discouraged.

Each multiplexer uses a different buffer for each output buffer size that is specified to any connector. If there were three connectors with different output buffer sizes specified, each multiplexer assigns three different buffers. Each multiplexer might also assign an extra buffer for HTTP use. A larger output buffer enables more efficient batching of messages per write, as large writes are generally more efficient but care must be taken to not overwhelm client connections regularly and causing them to be blocked for any period of time.

When a multiplexer is unable to write a message to a client because the buffer has become full, a selector thread is notified. The selector thread is responsible for watching the client and notifying the multiplexer when it becomes writable. The multiplexer remains responsible for writing the message.