Increase Synapse caches and enable cache-autotuning by default (#3017)

* Modify Synapse Cache Factor to use Auto Tune

Synapse has the ability to as it calls in its config auto tune caches.

This ability lets us set very high cache factors and then instead limit our resource use.

Defaults for this commit are 1/10th of what Element apparently runs for EMS stuff and matrix.org on Cache Factor and upstream documentation defaults for auto tune.

* Add vars to Synapse main.yml to control cache related config

This commit adds various cache related vars to main.yml for Synapse.

Some are auto tune and some are just adding explicit ways to control upstream vars.

* Updated Auto Tune figures

Autotuned figures have been bumped in consultation with other community members as to a reasonable level. Please note these defaults are more on the one of each workers side than they are on the monolith Side.

* Fix YML Error

The playbook is not happy with the previous state of this patch so this commit hopefully fixes it

* Add to_json to various Synapse tuning related configs

* Fix incorrect indication in homeserver.yaml.j2

* Minor cleanups

* Synapse Cache Autotuning Documentation

* Upgrade Synapse Cache Autotune to auto configure memory use

* Update Synapse Tuning docs to reflect automatic memory use configuration

* Fix Linting errors in synapses main.yml

* Rename variables for consistency (matrix_synapse_caches_autotuning_* -> matrix_synapse_cache_autotuning_*)

* Remove FIX ME comment about Synapse's `cache_autotuning`

`docs/maintenance-synapse.md` and `roles/custom/matrix-synapse/defaults/main.yml`
already contains documentation about these variables and the default values we set.

* Improve "Tuning caches and cache autotuning" documentation for Synapse

* Announce larger Synapse caches and cache auto-tuning

---------

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

CHANGELOG.md

@@ -1,3 +1,23 @@
+# 2024-02-14
+
+## Much larger Synapse caches and cache auto-tuning enabled by default
+
+Thanks to [FSG-Cat](https://github.com/FSG-Cat), the playbook now uses much larger caches and enables Synapse's [cache auto-tuning functionality](https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#caches-and-associated-values).
+This work and the default values used by the playbook are inspired by [Tom Foster](https://github.com/tcpipuk)'s [Synapse homeserver guide](https://tcpipuk.github.io/synapse/deployment/synapse.html).
+
+The playbook has always used a very conservative cache factor (`matrix_synapse_caches_global_factor`) value of `0.5`, which may be OK for small and underactive deployments, but is not ideal for larger servers. Paradoxically, a small global cache factor value [does not necessarily decrease RAM usage as a whole](https://github.com/matrix-org/synapse/issues/3939).
+
+The playbook now uses **a 20x larger cache factor** (currently `10`), adjusts a few other cache-related variables, and **enables cache auto-tuning** via the following variables:
+
+- `matrix_synapse_cache_autotuning_max_cache_memory_usage` - defaults to 1/8 of total RAM with a cap of 2GB; values are specified in bytes
+- `matrix_synapse_cache_autotuning_target_cache_memory_usage` - defaults to 1/16 of total RAM with a cap of 1GB; values are specified in bytes
+- `matrix_synapse_cache_autotuning_min_cache_ttl` - defaults to `30s`
+
+These values should be good defaults for most servers, but may change over time as we experiment further.
+
+Refer to our new [Tuning caches and cache autotuning](docs/maintenance-synapse.md#tuning-caches-and-cache-autotuning) documentation section for more details.
+
+
 # 2024-01-31
 
 ## (Backward-compatibility break) Minor changes necessary for some people serving a static website at the base domain