diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb406b802..9e3bd14e5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,13 @@
+# 2018-09-27
+
+## Synapse Cache Factor made configurable
+
+The playbook now makes the Synapse cache factor configurable, through the playbook's `matrix_synapse_cache_factor` variable (having a default value of `0.5`).
+
+Changing that value allows you to potentially decrease RAM usage or to increase performance by caching more stuff.
+Some information on it is available here: https://github.com/matrix-org/synapse#help-synapse-eats-all-my-ram
+
+
 # 2018-09-26
 
 ## Disabling Docker container logging
diff --git a/roles/matrix-server/defaults/main.yml b/roles/matrix-server/defaults/main.yml
index 9b24ae400..ab19be38e 100644
--- a/roles/matrix-server/defaults/main.yml
+++ b/roles/matrix-server/defaults/main.yml
@@ -74,6 +74,13 @@ matrix_synapse_password_config_pepper: ""
 # Controls the number of events that Matrix Synapse caches in memory.
 matrix_synapse_event_cache_size: "100K"
 
+# Controls cache sizes for Matrix Synapse via the SYNAPSE_CACHE_FACTOR environment variable.
+# Raise this to increase cache sizes or lower it to potentially lower memory use.
+# To learn more, see:
+# - https://github.com/matrix-org/synapse#help-synapse-eats-all-my-ram
+# - https://github.com/matrix-org/synapse/issues/3939
+matrix_synapse_cache_factor: 0.5
+
 # A list of additional "volumes" to mount in the container.
 # This list gets populated dynamically based on Synapse extensions that have been enabled.
 # Contains definition objects like this: `{"src": "/outside", "dst": "/inside", "options": "rw|ro|slave|.."}
diff --git a/roles/matrix-server/tasks/setup_synapse_main.yml b/roles/matrix-server/tasks/setup_synapse_main.yml
index 9a5169d2c..ecc93b700 100644
--- a/roles/matrix-server/tasks/setup_synapse_main.yml
+++ b/roles/matrix-server/tasks/setup_synapse_main.yml
@@ -61,6 +61,13 @@
     dest: "{{ matrix_synapse_config_dir_path }}/{{ hostname_matrix }}.log.config"
     mode: 0644
 
+- name: Ensure Synapse environment variables file created
+  template:
+    src: "{{ role_path }}/templates/env/env-synapse.j2"
+    dest: "{{ matrix_environment_variables_data_path }}/synapse"
+    owner: root
+    mode: 0600
+
 - name: Ensure matrix-synapse.service installed
   template:
     src: "{{ role_path }}/templates/systemd/matrix-synapse.service.j2"
diff --git a/roles/matrix-server/templates/env/env-synapse.j2 b/roles/matrix-server/templates/env/env-synapse.j2
new file mode 100644
index 000000000..69f267009
--- /dev/null
+++ b/roles/matrix-server/templates/env/env-synapse.j2
@@ -0,0 +1,2 @@
+SYNAPSE_CONFIG_PATH=/data/homeserver.yaml
+SYNAPSE_CACHE_FACTOR={{ matrix_synapse_cache_factor }}
diff --git a/roles/matrix-server/templates/systemd/matrix-synapse.service.j2 b/roles/matrix-server/templates/systemd/matrix-synapse.service.j2
index 8b014a6db..829f73727 100644
--- a/roles/matrix-server/templates/systemd/matrix-synapse.service.j2
+++ b/roles/matrix-server/templates/systemd/matrix-synapse.service.j2
@@ -27,6 +27,7 @@ ExecStartPre=/bin/sleep 5
 {% endif %}
 ExecStart=/usr/bin/docker run --rm --name matrix-synapse \
 			--log-driver=none \
+			--env-file={{ matrix_environment_variables_data_path }}/synapse \
 			--network={{ matrix_docker_network }} \
 			-p 8448:8448 \
 			{% if not matrix_nginx_proxy_enabled %}
@@ -38,7 +39,6 @@ ExecStart=/usr/bin/docker run --rm --name matrix-synapse \
 			{% for volume in matrix_synapse_container_additional_volumes %}
 			-v {{ volume.src }}:{{ volume.dst }}:{{ volume.options }} \
 			{% endfor %}
-			-e SYNAPSE_CONFIG_PATH=/data/homeserver.yaml \
 			{{ matrix_docker_image_synapse }}
 ExecStop=-/usr/bin/docker kill matrix-synapse
 ExecStop=-/usr/bin/docker rm matrix-synapse