[chore/performance] Make sender multiplier configurable (#1750)

docs/configuration/advanced.md

@@ -79,4 +79,36 @@ advanced-rate-limit-requests: 300
 # Examples: [8, 4, 9, 0]
 # Default: 8
 advanced-throttling-multiplier: 8
+
+# Int. CPU multiplier for the amount of goroutines to spawn in order to send messages via ActivityPub.
+# Messages will be batched so that at most multiplier * CPU count messages will be sent out at once.
+# This can be tuned to limit concurrent POSTing to remote inboxes, preventing your instance CPU
+# usage from skyrocketing when an account with many followers posts a new status.
+#
+# Messages are split among available senders, and each sender processes its assigned messages in serial.
+# For example, say a user with 1000 followers is on an instance with 2 CPUs. With the default multiplier
+# of 2, this means 4 senders would be in process at once on this instance. When the user creates a new post,
+# each sender would end up iterating through about 250 Create messages + delivering them to remote instances.
+#
+# If you set this to 0 or less, only 1 sender will be used regardless of CPU count. This may be
+# useful in cases where you are working with very tight network or CPU constraints.
+#
+# Example values for multiplier 2 (default):
+#
+# 1 cpu = 2 concurrent senders
+# 2 cpu = 4 concurrent senders
+# 4 cpu = 8 concurrent senders
+#
+# Example values for multiplier 4:
+#
+# 1 cpu = 4 concurrent senders
+# 2 cpu = 8 concurrent senders
+# 4 cpu = 16 concurrent senders
+#
+# Example values for multiplier <1:
+#
+# 1 cpu = 1 concurrent sender
+# 2 cpu = 1 concurrent sender
+# 4 cpu = 1 concurrent sender
+advanced-sender-multiplier: 2
 ```