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<h1>Understanding rsyslog Queues</h1>
<p>Rsyslog uses queues whenever two activities need to be loosely coupled. With a
queue, one part of the system "produces" something while another part "consumes"
this something. The "something" is most often syslog messages, but queues may
also be used for other purposes.</p>
<p>This document provides a good insight into technical details, operation modes
and implications. In addition to it, an
<a href="queues_analogy.html">rsyslog queue concepts overview</a> document
exists which tries to explain queues with the help of some analogies. This may
probably be a better place to start reading about queues. I assume that once you
have understood that document, the material here will be much easier to grasp
and look much more natural.
<p>The most prominent example is the main message queue. Whenever rsyslog
receives a message (e.g. locally, via UDP, TCP or in whatever else way), it
places these messages into the main message queue. Later, it is dequeued by the
rule processor, which then evaluates which actions are to be carried out. In
front of each action, there is also a queue, which potentially de-couples the
filter processing from the actual action (e.g. writing to file, database or
forwarding to another host).</p>
<h1>Where are Queues Used?</h1>
<p>Currently, queues are used for the main message queue and for the
actions.</p>
<p>There is a single main message queue inside rsyslog. Each input module
delivers messages to it. The main message queue worker filters messages based on
rules specified in rsyslog.conf and dispatches them to the individual action
queues. Once a message is in an action queue, it is deleted from the main
message queue.</p>
<p>There are multiple action queues, one for each configured action. By default,
these queues operate in direct (non-queueing) mode. Action queues are fully
configurable and thus can be changed to whatever is best for the given use case.</p>
<p>Future versions of rsyslog will most probably utilize queues at other places,
too.</p>
<p> Wherever "<i><object></i>" is used in the config file
statements, substitute "<i><object></i>" with either "MainMsg" or "Action". The
former will set main message queue
parameters, the later parameters for the next action that will be
created. Action queue parameters can not be modified once the action has been
specified. For example, to tell the main message queue to save its content on
shutdown, use <i>$MainMsgQueueSaveOnShutdown on</i>".</p>
<p>If the same parameter is specified multiple times before a queue is created,
the last one specified takes precedence. The main message queue is created after
parsing the config file and all of its potential includes. An action queue is
created each time an action selector is specified. Action queue parameters are
reset to default after an action queue has been created (to provide a clean
environment for the next action).</p>
<p>Not all queues necessarily support the full set of queue configuration
parameters, because not all are applicable. For example, in current output
module design, actions do not support multi-threading. Consequently, the number
of worker threads is fixed to one for action queues and can not be changed.</p>
<h1>Queue Modes</h1>
<p>Rsyslog supports different queue modes, some with submodes. Each of them has
specific advantages and disadvantages. Selecting the right queue mode is quite
important when tuning rsyslogd. The queue mode (aka "type") is set via the "<i>$<object>QueueType</i>"
config directive.</p>
<h2>Direct Queues</h2>
<p>Direct queues are <b>non</b>-queuing queues. A queue in direct mode does
neither queue nor buffer any of the queue elements but rather passes the element
directly (and immediately) from the producer to the consumer. This sounds strange,
but there is a good reason for this queue type.</p>
<p>Direct mode queues allow to use queues generically, even in places where
queuing is not always desired. A good example is the queue in front of output
actions. While it makes perfect sense to buffer forwarding actions or database
writes, it makes only limited sense to build up a queue in front of simple local
file writes. Yet, rsyslog still has a queue in front of every action. So for
file writes, the queue mode can simply be set to "direct", in which case no
queuing happens.</p>
<p>Please note that a direct queue also is the only queue type that passes back
the execution return code (success/failure) from the consumer to the producer.
This, for example, is needed for the backup action logic. Consequently, backup
actions require the to-be-checked action to use a "direct" mode queue.</p>
<p>To create a direct queue, use the "<i>$<object>QueueType Direct</i>" config
directive.</p>
<h2>Disk Queues</h2>
<p>Disk queues use disk drives for buffering. The important fact is that the
always use the disk and do not buffer anything in memory. Thus, the queue is
ultra-reliable, but by far the slowest mode. For regular use cases, this queue
mode is not recommended. It is useful if log data is so important that it must
not be lost, even in extreme cases.</p>
<p>When a disk queue is written, it is done in chunks. Each chunk receives its
individual file. Files are named with a prefix (set via the "<i>$<object>QueueFilename</i>"
config directive) and followed by a 7-digit number (starting at one and
incremented for each file). Chunks are 10mb by default, a different size can be
set via the"<i>$<object>QueueMaxFileSize</i>" config directive. Note that
the size limit is not a sharp one: rsyslog always writes one complete queue
entry, even if it violates the size limit. So chunks are actually a little but
(usually less than 1k) larger then the configured size. Each chunk also has a
different size for the same reason. If you observe different chunk sizes, you
can relax: this is not a problem.</p>
<p>Writing in chunks is used so that processed data can quickly be deleted and
is free for other uses - while at the same time keeping no artificial upper
limit on disk space used. If a disk quota is set (instructions further below),
be sure that the quota/chunk size allows at least two chunks to be written.
Rsyslog currently does not check that and will fail miserably if a single chunk
is over the quota.</p>
<p>Creating new chunks costs performance but provides quicker ability to free
disk space. The 10mb default is considered a good compromise between these two.
However, it may make sense to adapt these settings to local policies. For
example, if a disk queue is written on a dedicated 200gb disk, it may make sense
to use a 2gb (or even larger) chunk size.</p>
<p>Please note, however, that the disk queue by default does not update its
housekeeping structures every time it writes to disk. This is for performance
reasons. In the event of failure, data will still be lost (except when manually
is mangled with the file structures). However, disk queues can be set to write
bookkeeping information on checkpoints (every n records), so that this can be
made ultra-reliable, too. If the checkpoint interval is set to one, no data can
be lost, but the queue is exceptionally slow.</p>
<p>Each queue can be placed on a different disk for best performance and/or
isolation. This is currently selected by specifying different <i>$WorkDirectory</i>
config directives before the queue creation statement.</p>
<p>To create a disk queue, use the "<i>$<object>QueueType Disk</i>" config
directive. Checkpoint intervals can be specified via "<i>$<object>QueueCheckpointInterval</i>",
with 0 meaning no checkpoints. Note that disk-based queues can be made very reliable
by issuing a (f)sync after each write operation. Starting with version 4.3.2, this can
be requested via "<i><object>QueueSyncQueueFiles on/off</i> with the
default being off. Activating this option has a performance penalty, so it should
not be turned on without reason.</p>
<h2>In-Memory Queues</h2>
<p>In-memory queue mode is what most people have on their mind when they think
about computing queues. Here, the enqueued data elements are held in memory.
Consequently, in-memory queues are very fast. But of course, they do not survive
any program or operating system abort (what usually is tolerable and unlikely).
Be sure to use an UPS if you use in-memory mode and your log data is important
to you. Note that even in-memory queues may hold data for an infinite amount of
time when e.g. an output destination system is down and there is no reason to move
the data out of memory (lying around in memory for an extended period of time is
NOT a reason). Pure in-memory queues can't even store queue elements anywhere
else than in core memory. </p>
<p>There exist two different in-memory queue modes: LinkedList and FixedArray.
Both are quite similar from the user's point of view, but utilize different
algorithms. </p>
<p>A FixedArray queue uses a fixed, pre-allocated array that holds pointers to
queue elements. The majority of space is taken up by the actual user data
elements, to which the pointers in the array point. The pointer array itself is
comparatively small. However, it has a certain memory footprint even if the
queue is empty. As there is no need to dynamically allocate any housekeeping
structures, FixedArray offers the best run time performance (uses the least CPU
cycle). FixedArray is best if there is a relatively low number of queue elements
expected and performance is desired. It is the default mode for the main message
queue (with a limit of 10,000 elements).</p>
<p>A LinkedList queue is quite the opposite. All housekeeping structures are
dynamically allocated (in a linked list, as its name implies). This requires
somewhat more runtime processing overhead, but ensures that memory is only
allocated in cases where it is needed. LinkedList queues are especially
well-suited for queues where only occasionally a than-high number of elements
need to be queued. A use case may be occasional message burst. Memory
permitting, it could be limited to e.g. 200,000 elements which would take up
only memory if in use. A FixedArray queue may have a too large static memory
footprint in such cases.</p>
<p><b>In general, it is advised to use LinkedList mode if in doubt</b>. The
processing overhead compared to FixedArray is low and may be
outweigh by the reduction in memory use. Paging in most-often-unused
pointer array pages can be much slower than dynamically allocating them.</p>
<p>To create an in-memory queue, use the "<i>$<object>QueueType LinkedList</i>"
or "<i>$<object>QueueType FixedArray</i>" config directive.</p>
<h3>Disk-Assisted Memory Queues</h3>
<p>If a disk queue name is defined for in-memory queues (via <i>
$<object>QueueFileName</i>), they automatically
become "disk-assisted" (DA). In that mode, data is written to disk (and read
back) on an as-needed basis.</p>
<p>Actually, the regular memory queue (called the
"primary queue") and a disk queue (called the "DA queue") work in tandem in this
mode. Most importantly, the disk queue is activated if the primary queue is full
or needs to be persisted on shutdown. Disk-assisted queues combine the
advantages of pure memory queues with those of pure disk queues. Under normal
operations, they are very fast and messages will never touch the disk. But if
there is need to, an unlimited amount of messages can be buffered (actually
limited by free disk space only) and data can be persisted between rsyslogd runs.</p>
<p>With a DA-queue, both disk-specific and in-memory specific configuration
parameters can be set. From the user's point of view, think of a DA queue like a
"super-queue" which does all within a single queue [from the code perspective,
there is some specific handling for this case, so it is actually much like a
single object].</p>
<p>DA queues are typically used to de-couple potentially long-running and
unreliable actions (to make them reliable). For example, it is recommended to
use a disk-assisted linked list in-memory queue in front of each database and
"send via tcp" action. Doing so makes these actions reliable and de-couples
their potential low execution speed from the rest of your rules (e.g. the local
file writes). There is a howto on <a href="rsyslog_high_database_rate.html">
massive database inserts</a> which nicely describes this use case. It may even
be a good read if you do not intend to use databases.</p>
<p>With DA queues, we do not simply write out everything to disk and then run as
a disk queue once the in-memory queue is full. A much smarter algorithm is used,
which involves a "high watermark" and a "low watermark". Both specify numbers of
queued items. If the queue size reaches high watermark elements, the queue
begins to write data elements to disk. It does so until it reaches the low water
mark elements. At this point, it stops writing until either high water mark is
reached again or the on-disk queue becomes empty, in which case the queue
reverts back to in-memory mode, only. While holding at the low watermark, new
elements are actually enqueued in memory. They are eventually written to disk,
but only if the high water mark is ever reached again. If it isn't, these items
never touch the disk. So even when a queue runs disk-assisted, there is
in-memory data present (this is a big difference to pure disk queues!).</p>
<p>This algorithm prevents unnecessary disk writes, but also leaves some
additional buffer space for message bursts. Remember that creating disk files
and writing to them is a lengthy operation. It is too lengthy to e.g. block
receiving UDP messages. Doing so would result in message loss. Thus, the queue
initiates DA mode, but still is able to receive messages and enqueue them - as
long as the maximum queue size is not reached. The number of elements between
the high water mark and the maximum queue size serves as this "emergency
buffer". Size it according to your needs, if traffic is very bursty you will
probably need a large buffer here. Keep in mind, though, that under normal
operations these queue elements will probably never be used. Setting the high
water mark too low will cause disk-assistance to be turned on more often than
actually needed.</p>
<p>The water marks can be set via the "<i>$<object>QueueHighWatermark</i>" and
"<i>$<object>QueueHighWatermark</i>" configuration file directives. Note that
these are actual numbers, not precentages. Be sure they make sense (also in
respect to "<i>$<object>QueueSize</i>"), as rsyslodg does currently not perform
any checks on the numbers provided. It is easy to screw up the system here (yes,
a feature enhancement request is filed ;)).</p>
<h1>Limiting the Queue Size</h1>
<p>All queues, including disk queues, have a limit of the number of elements
they can enqueue. This is set via the "<i>$<object>QueueSize</i>" config
parameter. Note that the size is specified in number of enqueued elements, not
their actual memory size. Memory size limits can not be set. A conservative
assumption is that a single syslog messages takes up 512 bytes on average
(in-memory, NOT on the wire, this *is* a difference).</p>
<p>Disk assisted queues are special in that they do <b>not</b> have any size
limit. The enqueue an unlimited amount of elements. To prevent running out of
space, disk and disk-assisted queues can be size-limited via the "<i>$<object>QueueMaxDiskSpace</i>"
configuration parameter. If it is not set, the limit is only available free
space (and reaching this limit is currently not very gracefully handled, so
avoid running into it!). If a limit is set, the queue can not grow larger than
it. Note, however, that the limit is approximate. The engine always writes
complete records. As such, it is possible that slightly more than the set limit
is used (usually less than 1k, given the average message size). Keeping strictly
on the limit would be a performance hurt, and thus the design decision was to
favour performance. If you don't like that policy, simply specify a slightly
lower limit (e.g. 999,999K instead of 1G).</p>
<p>In general, it is a good idea to limit the pysical disk space even if you
dedicate a whole disk to rsyslog. That way, you prevent it from running out of
space (future version will have an auto-size-limit logic, that then kicks in in
such situations).</p>
<h1>Worker Thread Pools</h1>
<p>Each queue (except in "direct" mode) has an associated pool of worker
threads. Worker threads carry out the action to be performed on the data
elements enqueued. As an actual sample, the main message queue's worker task is
to apply filter logic to each incoming message and enqueue them to the relevant
output queues (actions).</p>
<p>Worker threads are started and stopped on an as-needed basis. On a system
without activity, there may be no worker at all running. One is automatically
started when a message comes in. Similarily, additional workers are started if
the queue grows above a specific size. The "<i>$<object>QueueWorkerThreadMinimumMessages</i>"
config parameter controls worker startup. If it is set to the minimum number of
elements that must be enqueued in order to justify a new worker startup. For
example, let's assume it is set to 100. As long as no more than 100 messages are
in the queue, a single worker will be used. When more than 100 messages arrive,
a new worker thread is automatically started. Similarily, a third worker will be
started when there are at least 300 messages, a forth when reaching 400 and so
on.</p>
<p>It, however, does not make sense to have too many worker threads running in
parall. Thus, the upper limit ca be set via "<i>$<object>QueueWorkerThreads</i>".
If it, for example, is set to four, no more than four workers will ever be
started, no matter how many elements are enqueued. </p>
<p>Worker threads that have been started are kept running until an inactivity
timeout happens. The timeout can be set via "<i>$<object>QueueWorkerTimeoutThreadShutdown</i>"
and is specified in milliseconds. If you do not like to keep the workers
running, simply set it to 0, which means immediate timeout and thus immediate
shutdown. But consider that creating threads involves some overhead, and this is
why we keep them running. If you would like to never shutdown any worker
threads, specify -1 for this parameter.</p>
<h2>Discarding Messages</h2>
<p>If the queue reaches the so called "discard watermark" (a number of queued
elements), less important messages can automatically be discarded. This is in an
effort to save queue space for more important messages, which you even less like
to loose. Please note that whenever there are more than "discard watermark"
messages, both newly incoming as well as already enqueued low-priority messages
are discarded. The algorithm discards messages newly coming in and those at the
front of the queue.</p>
<p>The discard watermark is a last resort setting. It should be set sufficiently
high, but low enough to allow for large message burst. Please note that it take
effect immediately and thus shows effect promptly - but that doesn't help if the
burst mainly consist of high-priority messages...</p>
<p>The discard watermark is set via the "<i>$<object>QueueDiscardMark</i>"
directive. The priority of messages to be discarded is set via "<i>$<object>QueueDiscardSeverity</i>".
This directive accepts both the usual textual severity as well as a
numerical one. To understand it, you must be aware of the numerical
severity values. They are defined in RFC 3164:</p>
<pre> Numerical Severity<br> Code<br><br> 0 Emergency: system is unusable<br> 1 Alert: action must be taken immediately<br> 2 Critical: critical conditions<br> 3 Error: error conditions<br> 4 Warning: warning conditions<br> 5 Notice: normal but significant condition<br> 6 Informational: informational messages<br> 7 Debug: debug-level messages</pre>
<p>Anything of the specified severity and (numerically) above it is
discarded. To turn message discarding off, simply specify the discard
watermark to be higher than the queue size. An alternative is to
specify the numerical value 8 as DiscardSeverity. This is also the
default setting to prevent unintentional message loss. So if you would
like to use message discarding, you need to set" <i>$<object>QueueDiscardSeverity</i>" to an actual value.</p>
<p>An interesting application is with disk-assisted queues: if the discard
watermark is set lower than the high watermark, message discarding will start
before the queue becomes disk-assisted. This may be a good thing if you would
like to switch to disk-assisted mode only in cases where it is absolutely
unavoidable and you prefer to discard less important messages first.</p>
<h1>Filled-Up Queues</h1>
<p>If the queue has either reached its configured maximum number of entries or
disk space, it is finally full. If so, rsyslogd throttles the data element
submitter. If that, for example, is a reliable input (TCP, local log socket),
that will slow down the message originator which is a good
resolution for this scenario.</p>
<p>During throtteling, a disk-assisted queue continues to write to disk and
messages are also discarded based on severity as well as regular dequeuing and
processing continues. So chances are good the situation will be resolved by
simply throttling. Note, though, that throtteling is highly undesirable for
unreliable sources, like UDP message reception. So it is not a good thing to run
into throtteling mode at all.</p>
<p>We can not hold processing
infinitely, not even when throtteling. For example, throtteling the local
log socket too long would cause the system at whole come to a standstill. To
prevent this, rsyslogd times out after a configured period ("<i>$<object>QueueTimeoutEnqueue</i>",
specified in milliseconds) if no space becomes available. As a last resort, it
then discards the newly arrived message.</p>
<p>If you do not like throtteling, set the timeout to 0 - the message will then
immediately be discarded. If you use a high timeout, be sure you know what you
do. If a high main message queue enqueue timeout is set, it can lead to
something like a complete hang of the system. The same problem does not apply to
action queues.</p>
<h2>Rate Limiting</h2>
<p>Rate limiting provides a way to prevent rsyslogd from processing things too
fast. It can, for example, prevent overruning a receiver system.</p>
<p>Currently, there are only limited rate-limiting features available. The "<i>$<object>QueueDequeueSlowdown</i>"
directive allows to specify how long (in microseconds) dequeueing should be
delayed. While simple, it still is powerful. For example, using a
DequeueSlowdown delay of 1,000 microseconds on a UDP send action ensures that no
more than 1,000 messages can be sent within a second (actually less, as there is
also some time needed for the processing itself).</p><h2>Processing Timeframes</h2><p>Queues
can be set to dequeue (process) messages only during certain
timeframes. This is useful if you, for example, would like to transfer
the bulk of messages only during off-peak hours, e.g. when you have
only limited bandwidth on the network path the the central server.</p><p>Currently,
only a single timeframe is supported and, even worse, it can only be
specified by the hour. It is not hard to extend rsyslog's capabilities
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
object can perform. You don't see that from a user's point of view, but its
quite hard work for the developer to do everything in the right order.</p>
<p>The complexity arises when the queue has still data enqueued when it
finishes. Rsyslog tries to preserve as much of it as possible. As a first
measure, there is a regular queue time out ("<i>$<object>QueueTimeoutShutdown</i>",
specified in milliseconds): the queue workers are given that time period to
finish processing the queue.</p>
<p>If after that period there is still data in the queue, workers are instructed
to finish the current data element and then terminate. This essentially means
any other data is lost. There is another timeout ("<i>$<object>QueueTimeoutActionCompletion</i>",
also specified in milliseconds) that specifies how long the workers have to
finish the current element. If that timeout expires, any remaining workers are
cancelled and the queue is brought down.</p>
<p>If you do not like to lose data on shutdown, the "<i>$<object>QueueSaveOnShutdown</i>"
parameter can be set to "on". This requires either a disk or disk-assisted
queue. If set, rsyslogd ensures that any queue elements are saved to disk before
it terminates. This includes data elements there were begun being processed by
workers that needed to be cancelled due to too-long processing. For a large
queue, this operation may be lengthy. No timeout applies to a required shutdown
save.</p>
[<a href="manual.html">manual index</a>]
[<a href="rsyslog_conf.html">rsyslog.conf</a>]
[<a href="http://www.rsyslog.com/">rsyslog site</a>]</p>
<p><font size="2">This documentation is part of the
<a href="http://www.rsyslog.com/">rsyslog</a> project.<br>
Copyright © 2008, 2009 by <a href="http://www.gerhards.net/rainer">Rainer Gerhards</a> and
<a href="http://www.adiscon.com/">Adiscon</a>. Released under the GNU GPL
version 3 or higher.</font></p>
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