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<h2 id="understanding-synapse-through-grafana-graphs"><a class="header" href="#understanding-synapse-through-grafana-graphs">Understanding Synapse through Grafana graphs</a></h2>
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<p>It is possible to monitor much of the internal state of Synapse using <a href="https://prometheus.io">Prometheus</a>
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metrics and <a href="https://grafana.com/">Grafana</a>.
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A guide for configuring Synapse to provide metrics is available <a href="../../metrics-howto.html">here</a>
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and information on setting up Grafana is <a href="https://github.com/matrix-org/synapse/tree/master/contrib/grafana">here</a>.
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In this setup, Prometheus will periodically scrape the information Synapse provides and
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store a record of it over time. Grafana is then used as an interface to query and
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present this information through a series of pretty graphs.</p>
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<p>Once you have grafana set up, and assuming you're using <a href="https://github.com/matrix-org/synapse/blob/master/contrib/grafana/synapse.json">our grafana dashboard template</a>, look for the following graphs when debugging a slow/overloaded Synapse:</p>
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<h2 id="message-event-send-time"><a class="header" href="#message-event-send-time">Message Event Send Time</a></h2>
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<p><img src="https://user-images.githubusercontent.com/1342360/82239409-a1c8e900-9930-11ea-8081-e4614e0c63f4.png" alt="image" /></p>
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<p>This, along with the CPU and Memory graphs, is a good way to check the general health of your Synapse instance. It represents how long it takes for a user on your homeserver to send a message.</p>
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<h2 id="transaction-count-and-transaction-duration"><a class="header" href="#transaction-count-and-transaction-duration">Transaction Count and Transaction Duration</a></h2>
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<p><img src="https://user-images.githubusercontent.com/1342360/82239985-8d392080-9931-11ea-80d0-843ab2f22e1e.png" alt="image" /></p>
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<p><img src="https://user-images.githubusercontent.com/1342360/82240050-ab068580-9931-11ea-98f1-f94671cbac9a.png" alt="image" /></p>
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<p>These graphs show the database transactions that are occurring the most frequently, as well as those are that are taking the most amount of time to execute.</p>
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<p><img src="https://user-images.githubusercontent.com/1342360/82240192-e86b1300-9931-11ea-9aac-3e2c9bfa6fdc.png" alt="image" /></p>
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<p>In the first graph, we can see obvious spikes corresponding to lots of <code>get_user_by_id</code> transactions. This would be useful information to figure out which part of the Synapse codebase is potentially creating a heavy load on the system. However, be sure to cross-reference this with Transaction Duration, which states that <code>get_users_by_id</code> is actually a very quick database transaction and isn't causing as much load as others, like <code>persist_events</code>:</p>
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<p><img src="https://user-images.githubusercontent.com/1342360/82240467-62030100-9932-11ea-8db9-917f2d977fe1.png" alt="image" /></p>
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<p>Still, it's probably worth investigating why we're getting users from the database that often, and whether it's possible to reduce the amount of queries we make by adjusting our cache factor(s).</p>
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<p>The <code>persist_events</code> transaction is responsible for saving new room events to the Synapse database, so can often show a high transaction duration.</p>
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<h2 id="federation"><a class="header" href="#federation">Federation</a></h2>
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<p>The charts in the "Federation" section show information about incoming and outgoing federation requests. Federation data can be divided into two basic types:</p>
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<ul>
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<li>PDU (Persistent Data Unit) - room events: messages, state events (join/leave), etc. These are permanently stored in the database.</li>
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<li>EDU (Ephemeral Data Unit) - other data, which need not be stored permanently, such as read receipts, typing notifications.</li>
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</ul>
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<p>The "Outgoing EDUs by type" chart shows the EDUs within outgoing federation requests by type: <code>m.device_list_update</code>, <code>m.direct_to_device</code>, <code>m.presence</code>, <code>m.receipt</code>, <code>m.typing</code>.</p>
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<p>If you see a large number of <code>m.presence</code> EDUs and are having trouble with too much CPU load, you can disable <code>presence</code> in the Synapse config. See also <a href="https://github.com/matrix-org/synapse/issues/3971">#3971</a>.</p>
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<h2 id="caches"><a class="header" href="#caches">Caches</a></h2>
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<p><img src="https://user-images.githubusercontent.com/1342360/82240572-8b239180-9932-11ea-96ff-6b5f0e57ebe5.png" alt="image" /></p>
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<p><img src="https://user-images.githubusercontent.com/1342360/82240666-b8703f80-9932-11ea-86af-9f663988d8da.png" alt="image" /></p>
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<p>This is quite a useful graph. It shows how many times Synapse attempts to retrieve a piece of data from a cache which the cache did not contain, thus resulting in a call to the database. We can see here that the <code>_get_joined_profile_from_event_id</code> cache is being requested a lot, and often the data we're after is not cached.</p>
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<p>Cross-referencing this with the Eviction Rate graph, which shows that entries are being evicted from <code>_get_joined_profile_from_event_id</code> quite often:</p>
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<p><img src="https://user-images.githubusercontent.com/1342360/82240766-de95df80-9932-11ea-8c15-5acfc57c48da.png" alt="image" /></p>
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<p>we should probably consider raising the size of that cache by raising its cache factor (a multiplier value for the size of an individual cache). Information on doing so is available <a href="https://github.com/matrix-org/synapse/blob/ee421e524478c1ad8d43741c27379499c2f6135c/docs/sample_config.yaml#L608-L642">here</a> (note that the configuration of individual cache factors through the configuration file is available in Synapse v1.14.0+, whereas doing so through environment variables has been supported for a very long time). Note that this will increase Synapse's overall memory usage.</p>
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<h2 id="forward-extremities"><a class="header" href="#forward-extremities">Forward Extremities</a></h2>
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<p><img src="https://user-images.githubusercontent.com/1342360/82241440-13566680-9934-11ea-8b88-ba468db937ed.png" alt="image" /></p>
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<p>Forward extremities are the leaf events at the end of a DAG in a room, aka events that have no children. The more that exist in a room, the more <a href="https://spec.matrix.org/v1.1/server-server-api/#room-state-resolution">state resolution</a> that Synapse needs to perform (hint: it's an expensive operation). While Synapse has code to prevent too many of these existing at one time in a room, bugs can sometimes make them crop up again.</p>
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<p>If a room has >10 forward extremities, it's worth checking which room is the culprit and potentially removing them using the SQL queries mentioned in <a href="https://github.com/matrix-org/synapse/issues/1760">#1760</a>.</p>
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<h2 id="garbage-collection"><a class="header" href="#garbage-collection">Garbage Collection</a></h2>
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<p><img src="https://user-images.githubusercontent.com/1342360/82241911-da6ac180-9934-11ea-9a0d-a311fe22acd0.png" alt="image" /></p>
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<p>Large spikes in garbage collection times (bigger than shown here, I'm talking in the
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multiple seconds range), can cause lots of problems in Synapse performance. It's more an
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indicator of problems, and a symptom of other problems though, so check other graphs for what might be causing it.</p>
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<h2 id="final-thoughts"><a class="header" href="#final-thoughts">Final Thoughts</a></h2>
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<p>If you're still having performance problems with your Synapse instance and you've
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tried everything you can, it may just be a lack of system resources. Consider adding
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more CPU and RAM, and make use of <a href="../../workers.html">worker mode</a>
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to make use of multiple CPU cores / multiple machines for your homeserver.</p>
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