diff --git a/CHANGELOG.md b/CHANGELOG.md
index d8ebdd0ed..153be11fa 100644
--- a/CHANGELOG.md
+++ b/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
diff --git a/docs/maintenance-synapse.md b/docs/maintenance-synapse.md
index ed1dee7c2..93c150022 100644
--- a/docs/maintenance-synapse.md
+++ b/docs/maintenance-synapse.md
@@ -74,8 +74,32 @@ Synapse's presence feature which tracks which users are online and which are off
 
 If you have enough compute resources (CPU & RAM), you can make Synapse better use of them by [enabling load-balancing with workers](configuring-playbook-synapse.md#load-balancing-with-workers).
 
-Tuning Synapse's cache factor can help reduce RAM usage. [See the upstream documentation](https://github.com/element-hq/synapse#help-synapse-is-slow-and-eats-all-my-ram-cpu) for more information on what value to set the cache factor to. Use the variable `matrix_synapse_caches_global_factor` to set the cache factor.
-
 [Tuning your PostgreSQL database](maintenance-postgres.md#tuning-postgresql) could also improve Synapse performance. The playbook tunes the integrated Postgres database automatically, but based on your needs you may wish to adjust tuning variables manually. If you're using an [external Postgres database](configuring-playbook-external-postgres.md), you will aslo need to tune Postgres manually.
 
+### Tuning caches and cache autotuning
+
+Tuning Synapse's cache factor is useful for performance increases but also as part of controlling Synapse's memory use. Use the variable `matrix_synapse_caches_global_factor` to set the cache factor as part of this process.
+
+**The playbook defaults the global cache factor to a large value** (e.g. `10`). A smaller value (e.g. `0.5`) will decrease the amount used for caches, but will [not necessarily decrease RAM usage as a whole](https://github.com/matrix-org/synapse/issues/3939).
+
+Tuning the cache factor is useful only to a limited degree (as its crude to do in isolation) and therefore users who are tuning their cache factor should likely look into tuning autotune variables as well (see below).
+
+Cache autotuning is **enabled by default** and controlled 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`
+
+You can **learn more about cache-autotuning and the global cache factor settings** in the [Synapse's documentation on caches and associated values](https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#caches-and-associated-values).
+
+To **disable cache auto-tuning**, unset all values:
+
+```yml
+matrix_synapse_cache_autotuning_max_cache_memory_usage: ''
+matrix_synapse_cache_autotuning_target_cache_memory_usage: ''
+matrix_synapse_cache_autotuning_min_cache_ttl: ''
+```
+
+Users who wish to lower Synapse's RAM footprint should look into lowering the global cache factor and tweaking the autotune variables (or disabling auto-tuning). If your cache factor is too low for a given auto tune setting your caches will not reach autotune thresholds and autotune won't be able to do its job. Therefore, when auto-tuning is enabled (which it is by default), it's recommended to have your cache factor be large.
+
 See also [How do I optimize this setup for a low-power server?](faq.md#how-do-i-optimize-this-setup-for-a-low-power-server).
diff --git a/roles/custom/matrix-synapse/defaults/main.yml b/roles/custom/matrix-synapse/defaults/main.yml
index 13e16b360..9b9f068fd 100644
--- a/roles/custom/matrix-synapse/defaults/main.yml
+++ b/roles/custom/matrix-synapse/defaults/main.yml
@@ -547,8 +547,23 @@ matrix_synapse_event_cache_size: "100K"
 
 # Controls cache sizes for Synapse.
 # Raise this to increase cache sizes or lower it to potentially lower memory use.
-# To learn more, see: https://github.com/matrix-org/synapse/issues/3939
-matrix_synapse_caches_global_factor: 0.5
+# To learn more, see:
+# - https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#caching
+# - https://github.com/matrix-org/synapse/issues/3939
+# Defaults for timings of caches is from https://tcpipuk.github.io/synapse/deployment/synapse.html
+# The idea with the timings used is that you get to evict soon but also you keep stuff around for a long time when its not forced out.
+# Long cache lifetimes together with the low minimum TTL allows autotune to be the primary eviction method assuming size of cache is hit before we hit other caps.
+matrix_synapse_caches_global_factor: 10
+matrix_synapse_caches_expire_caches: true
+matrix_synapse_caches_cache_entry_ttl: "1080m"
+matrix_synapse_caches_sync_response_cache_duration: "2m"
+matrix_synapse_cache_autotuning_min_cache_ttl: "30s"
+# The Cache tune math used here is a derivative of the same math used to autotune sizes for postgres.
+# The memtotal variable can in theory be overiden to make Synapse think it has less ram to work with.
+# But if your at the point of considering that just override the math or put static values in.
+matrix_synapse_memtotal_kb: "{{ ansible_memtotal_mb*1024|int }}"
+matrix_synapse_cache_autotuning_max_cache_memory_usage: "{{ 2097152 if (matrix_synapse_memtotal_kb|int/8)/1024 >= 2048 else matrix_synapse_memtotal_kb|int/8 }}"
+matrix_synapse_cache_autotuning_target_cache_memory_usage: "{{ 1048576 if (matrix_synapse_memtotal_kb|int/16)/1024 >= 1024 else matrix_synapse_memtotal_kb|int/16 }}"
 
 # Controls whether Synapse will federate at all.
 # Disable this to completely isolate your server from the rest of the Matrix network.
diff --git a/roles/custom/matrix-synapse/tasks/validate_config.yml b/roles/custom/matrix-synapse/tasks/validate_config.yml
index 2f1a5e1c0..530ce4be6 100644
--- a/roles/custom/matrix-synapse/tasks/validate_config.yml
+++ b/roles/custom/matrix-synapse/tasks/validate_config.yml
@@ -89,6 +89,9 @@
     - {'old': 'matrix_synapse_send_federation', 'new': '<unnecessary - Synapse relies on federation_sender_instances now>'}
     - {'old': 'matrix_synapse_start_pushers', 'new': '<unnecessary - Synapse relies on pusher_instances now>'}
     - {'old': 'matrix_synapse_spam_checker', 'new': '<superseded by matrix_synapse_modules>'}
+    - {'old': 'matrix_synapse_caches_autotuning_max_cache_memory_usage', 'new': 'matrix_synapse_cache_autotuning_max_cache_memory_usage'}
+    - {'old': 'matrix_synapse_caches_autotuning_target_cache_memory_usage', 'new': 'matrix_synapse_cache_autotuning_target_cache_memory_usage'}
+    - {'old': 'matrix_synapse_caches_autotuning_min_cache_ttl', 'new': 'matrix_synapse_cache_autotuning_min_cache_ttl'}
 
 - name: (Deprecation) Catch and report renamed settings in matrix_synapse_configuration_extension_yaml
   ansible.builtin.fail:
diff --git a/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2 b/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
index b6cc6428b..5206dce3f 100644
--- a/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
+++ b/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
@@ -760,49 +760,48 @@ federation_domain_whitelist: {{ matrix_synapse_federation_domain_whitelist|to_js
 # The number of events to cache in memory. Not affected by
 # caches.global_factor.
 #
-event_cache_size: "{{ matrix_synapse_event_cache_size }}"
+event_cache_size: {{ matrix_synapse_event_cache_size | to_json }}
 
 caches:
-   # Controls the global cache factor, which is the default cache factor
-   # for all caches if a specific factor for that cache is not otherwise
-   # set.
-   #
-   # This can also be set by the "SYNAPSE_CACHE_FACTOR" environment
-   # variable. Setting by environment variable takes priority over
-   # setting through the config file.
-   #
-   # Defaults to 0.5, which will half the size of all caches.
-   #
-   global_factor: {{ matrix_synapse_caches_global_factor }}
-
-   # A dictionary of cache name to cache factor for that individual
-   # cache. Overrides the global cache factor for a given cache.
-   #
-   # These can also be set through environment variables comprised
-   # of "SYNAPSE_CACHE_FACTOR_" + the name of the cache in capital
-   # letters and underscores. Setting by environment variable
-   # takes priority over setting through the config file.
-   # Ex. SYNAPSE_CACHE_FACTOR_GET_USERS_WHO_SHARE_ROOM_WITH_USER=2.0
-   #
-   # Some caches have '*' and other characters that are not
-   # alphanumeric or underscores. These caches can be named with or
-   # without the special characters stripped. For example, to specify
-   # the cache factor for `*stateGroupCache*` via an environment
-   # variable would be `SYNAPSE_CACHE_FACTOR_STATEGROUPCACHE=2.0`.
-   #
-   per_cache_factors:
-     #get_users_who_share_room_with_user: 2.0
+  # Controls the global cache factor, which is the default cache factor
+  # for all caches if a specific factor for that cache is not otherwise
+  # set.
+  #
+  # This can also be set by the "SYNAPSE_CACHE_FACTOR" environment
+  # variable. Setting by environment variable takes priority over
+  # setting through the config file.
+  #
+  # Defaults to 0.5, which will half the size of all caches.
+  #
+  global_factor: {{ matrix_synapse_caches_global_factor | to_json }}
+  # A dictionary of cache name to cache factor for that individual
+  # cache. Overrides the global cache factor for a given cache.
+  #
+  # These can also be set through environment variables comprised
+  # of "SYNAPSE_CACHE_FACTOR_" + the name of the cache in capital
+  # letters and underscores. Setting by environment variable
+  # takes priority over setting through the config file.
+  # Ex. SYNAPSE_CACHE_FACTOR_GET_USERS_WHO_SHARE_ROOM_WITH_USER=2.0
+  #
+  # Some caches have '*' and other characters that are not
+  # alphanumeric or underscores. These caches can be named with or
+  # without the special characters stripped. For example, to specify
+  # the cache factor for `*stateGroupCache*` via an environment
+  # variable would be `SYNAPSE_CACHE_FACTOR_STATEGROUPCACHE=2.0`.
+  #
+  per_cache_factors:
+    #get_users_who_share_room_with_user: 2.0
 
   # Controls whether cache entries are evicted after a specified time
   # period. Defaults to true. Uncomment to disable this feature.
   #
-  #expire_caches: false
+  expire_caches: {{ matrix_synapse_caches_expire_caches | to_json }}
 
   # If expire_caches is enabled, this flag controls how long an entry can
   # be in a cache without having been accessed before being evicted.
   # Defaults to 30m. Uncomment to set a different time to live for cache entries.
   #
-  #cache_entry_ttl: 30m
+  cache_entry_ttl: {{ matrix_synapse_caches_cache_entry_ttl | to_json }}
 
   # Controls how long the results of a /sync request are cached for after
   # a successful response is returned. A higher duration can help clients with
@@ -811,7 +810,12 @@ caches:
   # By default, this is zero, which means that sync responses are not cached
   # at all.
   #
-  #sync_response_cache_duration: 2m
+  sync_response_cache_duration: {{ matrix_synapse_caches_sync_response_cache_duration | to_json }}
+
+  cache_autotuning:
+    max_cache_memory_usage: {{ matrix_synapse_cache_autotuning_max_cache_memory_usage | to_json }}
+    target_cache_memory_usage: {{ matrix_synapse_cache_autotuning_target_cache_memory_usage | to_json }}
+    min_cache_ttl: {{ matrix_synapse_cache_autotuning_min_cache_ttl | to_json }}
 
 
 ## Database ##