Job dispatching can be slowed down excessively by host loads query
Description
With a significant number of jobs (in history and/or running), job dispatching can become very slow, in excess of 3 minutes per dispatch cycle.
This is caused by the call:
SystemLoad systemLoad = getHostLoads(em, true);
for every job being dispatched. This is the query
On our 3.x production system, New Relic reports this query taking 94% of all database time. Response time can be up to 2s, and the query can be called hundreds of times per minute.
The job dispatching code needs refactoring to avoid this expensive query being called frequently.
Activity
Yeah I guess I'm the only person that can review that it works on MSSQL!
let's reopen here, I'll create a branch and file another PR.
looks like changing it to "GROUP BY job.processorServiceRegistration.hostRegistration.baseUrl" fixes things for MS SQL, should make it faster for MYSQL too as it was grouping by every field of job.processorServiceRegistration (mh_service_registration) previously.
I can reopen this ticket or create a new one, preference?
Looks like it's the other way around - you have to add it to the GROUP BY.
we could also add the host to the output of the select. It wouldn't be technically useful, but it might resolve the issue with MSSQL. If I made that change in a separate branch would you have time to test?