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Eric Eastwood
ba3ab1876a
### Background On `matrix.org`, since 2026-08-15, we are seeing the database CPU being saturated more than usual ([grafana](https://grafana.matrix.org/d/rYdddlPWk/node-exporter?orgId=1&from=2026-08-10T23:23:14.237Z&to=2026-08-17T23:23:14.237Z&timezone=browser&var-DS_PROMETHEUS=default&var-job=machine&var-node=matrix-db-01.matrix.org:9100&var-diskdevices=%5Ba-z%5D%2B%7Cnvme%5B0-9%5D%2Bn%5B0-9%5D%2B&refresh=1m&viewPanel=panel-77)) <img width="919" height="265" alt="CPU of database server" src="https://github.com/user-attachments/assets/9a43cf3c-bf9a-4ade-b70e-0bf0fb53f0ef" /> @reivilibre [found](https://matrix.to/#/!yHWhpxlXVaLcsgDUKb:matrix.org/$fUMf60IbFocpEuJNbmbEzFiCyKCFnAl4I4XpB27DUQs?via=banzan.uk&via=element.io&via=matrix.org) `mark_as_sent_devices_by_remote` spiking in the `DB transactions by total txn time` graph ([grafana](https://grafana.matrix.org/d/000000012/synapse?var-bucket_size=$__auto&orgId=1&from=2026-08-10T22:26:07.336Z&to=2026-08-17T22:26:07.336Z&timezone=browser&var-datasource=default&var-instance=matrix.org&var-job=synapse_federation_sender&var-index=$__all&showCategory=Thresholds&viewPanel=panel-11)) <img width="919" height="269" alt="'mark_as_sent_devices_by_remote' spiking in the 'DB transactions by total txn time' graph" src="https://github.com/user-attachments/assets/9336ce29-c045-4940-9c46-9e4906300f2d" /> And we indeed see a bunch of time being spent on `mark_as_sent_devices_by_remote` by looking at `pg_stat_statements`. The top two queries we're spending CPU on are the `SELECT` and `DELETE` statements in [`mark_as_sent_devices_by_remote`](https://github.com/element-hq/synapse/blob/94a5f2afb36a4afdc36813fd1c24b9a1c4aec252/synapse/storage/databases/main/devices.py#L922-L950). We also did some related work in this area recently with https://github.com/element-hq/synapse/pull/20098 although it was tackling a different bottle-neck. ### This PR This PRs combines the two separate `SELECT` and `DELETE` queries which touch the same data into one `DELETE ... RETURNING ...` query. this means we get to save the cost of one of those statements (less CPU on the database) and fewer statements per transaction (less round-trips) means connections turn over faster, and can move on to process the next thing. This is a micro-optimization I spotted while reading [`_mark_as_sent_devices_by_remote_txn`](https://github.com/element-hq/synapse/blob/develop/synapse/storage/databases/main/devices.py#L922-L950) rather than a structural fix. I'm sure there are even better things to do to where we could even avoid this kind of work altogether but this seemed like a quick win especially given how much this particular transaction is saturating the database. Doing things faster doesn't necessarily mean we solve the saturated/starved CPU problem but it does mean the same task costs less CPU. From the `pg_stat_statements` samples above, we can expect to be up to ~27% more efficient with database CPU on this code path (derived from the `cpus_busy` numbers above `22.932 / (22.932 + 61.956)`). We're removing the `SELECT` (`61.956` `cpus_busy`) but I still expect the `DELETE` (`22.932` `cpus_busy`) to take on a similar cost as I'm sure it's a warm cache situation between them.
.. image:: https://github.com/element-hq/synapse/raw/develop/docs/element_logo_white_bg.svg :height: 60px **Element Synapse - Matrix homeserver implementation** |support| |development| |documentation| |license| |pypi| |python| Synapse is an open source `Matrix <https://matrix.org>`__ homeserver implementation, written and maintained by `Element <https://element.io>`_. `Matrix <https://github.com/matrix-org>`__ is the open standard for secure and interoperable real-time communications. You can directly run and manage the source code in this repository, available under an AGPL license (or alternatively under a commercial license from Element). There is no support provided by Element unless you have a subscription from Element. 🚀 Getting started ================== This component is developed and maintained by `Element <https://element.io>`_. It gets shipped as part of the **Element Server Suite (ESS)** which provides the official means of deployment. ESS is a Matrix distribution from Element with focus on quality and ease of use. It ships a full Matrix stack tailored to the respective use case. There are three editions of ESS: - `ESS Community <https://github.com/element-hq/ess-helm>`_ - the free Matrix distribution from Element tailored to small-/mid-scale, non-commercial community use cases - `ESS Pro <https://element.io/server-suite>`_ - the commercial Matrix distribution from Element for professional use - `ESS TI-M <https://element.io/server-suite/ti-messenger>`_ - a special version of ESS Pro focused on the requirements of TI-Messenger Pro and ePA as specified by the German National Digital Health Agency Gematik 🛠️ Standalone installation and configuration ============================================ The Synapse documentation describes `options for installing Synapse standalone <https://element-hq.github.io/synapse/latest/setup/installation.html>`_. See below for more useful documentation links. - `Synapse configuration options <https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html>`_ - `Synapse configuration for federation <https://element-hq.github.io/synapse/latest/federate.html>`_ - `Using a reverse proxy with Synapse <https://element-hq.github.io/synapse/latest/reverse_proxy.html>`_ - `Upgrading Synapse <https://element-hq.github.io/synapse/develop/upgrade.html>`_ 🎯 Troubleshooting and support ============================== 🚀 Professional support ----------------------- Enterprise quality support for Synapse including SLAs is available as part of an `Element Server Suite (ESS) <https://element.io/pricing>`_ subscription. If you are an existing ESS subscriber then you can raise a `support request <https://ems.element.io/support>`_ and access the `Element product documentation <https://docs.element.io>`_. 🤝 Community support -------------------- The `Admin FAQ <https://element-hq.github.io/synapse/latest/usage/administration/admin_faq.html>`_ includes tips on dealing with some common problems. For more details, see `Synapse's wider documentation <https://element-hq.github.io/synapse/latest/>`_. For additional support installing or managing Synapse, please ask in the community support room |room|_ (from a matrix.org account if necessary). We do not use GitHub issues for support requests, only for bug reports and feature requests. .. |room| replace:: ``#synapse:matrix.org`` .. _room: https://matrix.to/#/#synapse:matrix.org .. |docs| replace:: ``docs`` .. _docs: docs 🛠️ Development ============== We welcome contributions to Synapse from the community! The best place to get started is our `guide for contributors <https://element-hq.github.io/synapse/latest/development/contributing_guide.html>`_. This is part of our broader `documentation <https://element-hq.github.io/synapse/latest>`_, which includes information for Synapse developers as well as Synapse administrators. Developers might be particularly interested in: * `Synapse's database schema <https://element-hq.github.io/synapse/latest/development/database_schema.html>`_, * `notes on Synapse's implementation details <https://element-hq.github.io/synapse/latest/development/internal_documentation/index.html>`_, and * `how we use git <https://element-hq.github.io/synapse/latest/development/git.html>`_. Alongside all that, join our developer community on Matrix: `#synapse-dev:matrix.org <https://matrix.to/#/#synapse-dev:matrix.org>`_, featuring real humans! Copyright and Licensing ======================= | Copyright 2014–2017 OpenMarket Ltd | Copyright 2017 Vector Creations Ltd | Copyright 2017–2025 New Vector Ltd | Copyright 2025 Element Creations Ltd This software is dual-licensed by Element Creations Ltd (Element). It can be used either: (1) for free under the terms of the GNU Affero General Public License (as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version); OR (2) under the terms of a paid-for Element Commercial License agreement between you and Element (the terms of which may vary depending on what you and Element have agreed to). Unless required by applicable law or agreed to in writing, software distributed under the Licenses is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the Licenses for the specific language governing permissions and limitations under the Licenses. Please contact `licensing@element.io <mailto:licensing@element.io>`_ to purchase an Element commercial license for this software. .. |support| image:: https://img.shields.io/badge/matrix-community%20support-success :alt: (get community support in #synapse:matrix.org) :target: https://matrix.to/#/#synapse:matrix.org .. |development| image:: https://img.shields.io/matrix/synapse-dev:matrix.org?label=development&logo=matrix :alt: (discuss development on #synapse-dev:matrix.org) :target: https://matrix.to/#/#synapse-dev:matrix.org .. |documentation| image:: https://img.shields.io/badge/documentation-%E2%9C%93-success :alt: (Rendered documentation on GitHub Pages) :target: https://element-hq.github.io/synapse/latest/ .. |license| image:: https://img.shields.io/github/license/element-hq/synapse :alt: (check license in LICENSE file) :target: LICENSE .. |pypi| image:: https://img.shields.io/pypi/v/matrix-synapse :alt: (latest version released on PyPi) :target: https://pypi.org/project/matrix-synapse .. |python| image:: https://img.shields.io/pypi/pyversions/matrix-synapse :alt: (supported python versions) :target: https://pypi.org/project/matrix-synapse