High CPU usage with large number of groups with large number of users

A Lotus Domino 8.5 server exhibits high CPU usage which deads to slowdowns if the Directory contains a large number of groups there is a large number of users in those groups.

If a user opens and closes many sessions to the Domino server, high CPU usage can occur due to the building of the group list each time the session with the server is opened. This occurs for the groups that the user belongs to. If the user belongs to many groups, building the group list will be a very expensive operation.

Because the default setting for the group cache is only 4 MB, it is not large enough to contain these groups. With many users opening sessions in rapid succession and the groups for which they belong are not in the group cache, the BuildGroupList operation occurs many times which then leads to contention causing a slowdown and high CPU usage.

In this particular case, increasing the default Group Cache size from 4096 (4MB) to 12288 (12MB) resolved this issue. Do this by adding the following parameter to the Domino Server's notes.ini file:

GROUP_CACHE_SIZE=12288