17.1. Multithreading concepts of syslog-ng OSE

This section is a brief overview on how syslog-ng OSE works in multithreaded mode. It is mainly for illustration purposes: the concept has been somewhat simplified and may not completely match reality.


The way syslog-ng OSE uses multithreading may change in future releases. The current documentation applies to version 3.12.

syslog-ng OSE always uses multiple threads:

  • A main thread that is always running

  • A number of worker threads that process the messages. You can influence the behavior of worker threads using the threaded() option and the --worker-threads command-line option.

  • Some other, special threads for internal functionalities. For example, certain destinations run in a separate thread, independently of the multithreading (threaded()) and --worker-threads settings of syslog-ng OSE.

The maximum number of worker threads syslog-ng OSE uses is the number of CPUs or cores in the host running syslog-ng OSE (up to 64). You can limit this value using the --worker-threads command-line option that sets the maximum total number of threads syslog-ng OSE can use, including the main syslog-ng OSE thread. However, the --worker-threads option does not affect the supervisor of syslog-ng OSE. The supervisor is a separate process (see syslog-ng(8)), but certain operating systems might display it as a thread. In addition, certain destinations always run in a separate thread, independently of the multithreading (threaded()) and --worker-threads settings of syslog-ng OSE.

When an event requiring a new thread occurs (for example, syslog-ng OSE receives new messages, or a destination becomes available), syslog-ng OSE tries to start a new thread. If there are no free threads, the task waits until a thread finishes its task and becomes available. There are two types of worker threads:

  • Reader threads read messages from a source (as many as possible, but limited by the log-fetch-limit() and log-iw-size() options. The thread then processes these messages, that is, performs filtering, rewriting and other tasks as necessary, and puts the log message into the queue of the destination. If the destination does not have a queue (for example, usertty), the reader thread sends the message to the destination, without the interaction of a separate writer thread.

  • Writer threads take the messages from the queue of the destination and send them to the destination, that is, write the messages into a file, or send them to the syslog server over the network. The writer thread starts to process messages from the queue only if the destination is writable, and there are enough messages in the queue, as set in the flush-lines() and the flush-timeout() options. Writer threads stop processing messages when the destination becomes unavailable, or there are no more messages in the queue.

Sources and destinations affected by multithreading. The following list describes which sources and destinations can use multiple threads. Changing the --worker-threads command-line option changes the number of threads available to these sources and destinations.

  • The tcp and syslog(tcp) sources can process independent connections in separate threads. The number of independent connections is limited by the max-connections() option of the source. Separate sources are processed by separate thread, for example, if you have two separate tcp sources defined that receive messages on different IP addresses or port, syslog-ng OSE will use separate threads for these sources even if they both have only a single active connection.

  • The udp, file, and pipe sources use a single thread for every source statement.

  • The tcp, syslog, and pipe destinations use a single thread for every destination.

  • The file destination uses a single thread for writing the destination file, but may use a separate thread for each destination file if the filename includes macros.

Sources and destinations not affected by multithreading. The following list describes sources and destinations that use a separate thread even if you disable multithreading in syslog-ng OSE, in addition to the limit set in the --worker-threads command-line option.

  • Every sql destination uses its own thread. These threads are independent from the setting of the --worker-threads command-line option.

  • The java destinations use one thread, even if there are multiple Java-based destinations configured. This thread is independent from the setting of the --worker-threads command-line option.