Issue #32: Fix scaling issue for large classes with high concurrent users

[pauldamiani]

No description provided.

[brendanheywood]

@deraadt any chance you could review this please? See #32

This patch is quite simple but has quite big impact on performance, eg 4 seconds of processing on a large page:

[alex-rowe]

There was a new release recently but this was not included. Was there a reason for skipping this simple change as it causes issues on sites with lots of users?

[brendanheywood]

We'd seen clients with ~10 instances of this block on one course page and the performance is horrendous without this patch, ie 30+ seconds

I still think there can be even more work done to reuse some lookups when there are multiple block instances on the same page but we'll sort another pr for that

[alex-rowe]

Looking at our Percona Monitoring and Management application, this morning this single query is 0.11% of all total queries but is causing 5.21% of our database load.

Having 1000 users logging in and accessing a course with this block enabled, at the same time, is putting unnecessary load on our DB server.

[DegrangeM]

@jonof Is tagged as "Lead maintainer" on the moodle plugin page, so maybe he can check this.

[brendanheywood]

The repos links are wrong in the plugin dir, I've logged #61 to sort that out

[jonof]

Seems it's difficult to get a reproduction using a synthetically generated medium-size course and several hundred exclusions to be quite as bad as the profile shot above suggests, but I'll believe you. A formal release will come soon.

[brendanheywood]

thanks heaps @jonof