Getting the users in a large group times out/never finishes

[turt2live]

Description

2017-11-09 05:32:49,413 - synapse.access.http.8008 - 91 - INFO - GET-2705- 136.243.62.149 - 8008 - {@_discord_bot:t2bot.io} Processed request: 272080ms (1588ms, 163ms) (18576ms/15360) 0B 200 "GET /_matrix/client/r0/groups/%2Bdiscord%3At2bot.io/users?access_token=<redacted> HTTP/1.0" "None"
2017-11-09 05:32:54,790 - synapse.access.http.8008 - 91 - INFO - GET-2706- 136.243.62.149 - 8008 - {@_discord_bot:t2bot.io} Processed request: 277164ms (1615ms, 188ms) (19876ms/15360) 0B 200 "GET /_matrix/client/r0/groups/%2Bdiscord%3At2bot.io/users?access_token=<redacted> HTTP/1.0" "None"
2017-11-09 05:32:55,584 - synapse.access.http.8008 - 91 - INFO - GET-2707- 136.243.62.149 - 8008 - {@_discord_bot:t2bot.io} Processed request: 277647ms (1520ms, 195ms) (16390ms/15360) 0B 200 "GET /_matrix/client/r0/groups/%2Bdiscord%3At2bot.io/users?access_token=<redacted> HTTP/1.0" "None"

These are some of the fastest requests. At the time of reporting, the group (+discord:t2bot.io) has 7600 users in it. The slow part appears to be the profile fetching, even after the caches have had plenty time to warm up.

In a test, I commented out the profile lines ( https://github.com/matrix-org/synapse/blob/develop/synapse/groups/groups_server.py#L433-L434 ) and the requests immediately completed in under a second.

Version information

• Homeserver: t2bot.io
• Version: 0.24.1
• Install method: pip
• Platform: container

[turt2live]

Related: #2966

[clokep]

Communities/groups are being removed, see #11584.