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diff --git a/doc/queues.html b/doc/queues.html index 45ce1bd1..75b70fbf 100644 --- a/doc/queues.html +++ b/doc/queues.html @@ -336,6 +336,33 @@ in this regard - it was just not requested so far. So if you need more fine-grained control, let us know and we'll probably implement it. There are two configuration directives, both should be used together or results are unpredictable:" <i>$<object>QueueDequeueTimeBegin <hour></i>" and "<i>$<object>QueueDequeueTimeEnd <hour></i>". The hour parameter must be specified in 24-hour format (so 10pm is 22). A use case for this parameter can be found in the <a href="http://wiki.rsyslog.com/index.php/OffPeakHours">rsyslog wiki</a>. </p> +<h2>Performance</h2> +<p>The locking involved with maintaining the queue has a potentially large +performance impact. How large this is, and if it exists at all, depends much on +the configuration and actual use case. However, the queue is able to work on +so-called "batches" when dequeueing data elements. With batches, +multiple data elements are dequeued at once (with a single locking call). +The queue dequeues all available elements up to a configured upper +limit (<i><object>DequeueBatchSize <number></i>). It is important +to note that the actual upper limit is dictated by availability. The queue engine +will never wait for a batch to fill. So even if a high upper limit is configured, +batches may consist of fewer elements, even just one, if there are no more elements +waiting in the queue. +<p>Batching +can improve performance considerably. Note, however, that it affects the +order in which messages are passed to the queue worker threads, as each worker +now receive as batch of messages. Also, the larger the batch size and the higher +the maximum number of permitted worker threads, the more main memory is needed. +For a busy server, large batch sizes (around 1,000 or even more elements) may be useful. +Please note that with batching, the main memory must hold BatchSize * NumOfWorkers +objects in memory (worst-case scenario), even if running in disk-only mode. So if you +use the default 5 workers at the main message queue and set the batch size to 1,000, you need +to be prepared that the main message queue holds up to 5,000 messages in main memory +<b>in addition</b> to the configured queue size limits! +<p>The queue object's default maximum batch size +is eight, but there exists different defaults for the actual parts of +rsyslog processing that utilize queues. So you need to check these object's +defaults. <h2>Terminating Queues</h2> <p>Terminating a process sounds easy, but can be complex. Terminating a running queue is in fact the most complex operation a queue |