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Connor McDonald

Thanks for the question, Ingimundur.

Asked: November 11, 2024 - 11:07 am UTC

Last updated: July 09, 2025 - 6:51 am UTC

Version: 19

Viewed 1000+ times

You Asked

Hi Team,

I have tried to find good explanation for this wait event online, but not found anything that gives me a clear and concise answer.

Recently we have started getting "resmgr:cpu quantum" wait event on our production database. This is a database with 40 CPUs and is run on exadata.

I have tried all the scripts I could fin online to determine what is the problem, but I cannot see that we have too much happening on our machine at the time of the event. Are there maybe some other events I should try to find in conjunction to this one?

Hope you can help.

Ingimundur K. Gudmundsson

and Connor said...

"resmgr:cpu quantum' means we are deliberately throttling some sessions to avoid exceeding a CPU resource limit.

This might be overall server load being too much for the machine, or it might be artificially constrained by some setting, eg if you set cpu_count to 10, then after 10 cpu's "worth" of work, we will start to limit things. (Some material here https://www.oracle.com/technetwork/database/performance/instance-caging-wp-166854.pdf )

Start with your standard AWR reports. There is rule that says it is the sessions that are having this wait that are the issue - they might be just the victims of other sessions being too hard on the server.

Check your standard metrics - "DB Time", "Average Active Sessions" etc.

Also, check your alert log to ensure that you are not errantly in a resource manage plan that you did not intend to have active. (Note - there are some preconfigured ones even if you didnt create any). You could explore *temporarily* turning off resource manager (set the parameter to null) to see if it has an impact to help with root cause detection.


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Comments

Turn resource manager off

Mladen Gogala, July 01, 2025 - 9:15 pm UTC

Don't bother checking, resource manager plan is a complex monstrosity which can be turned off easily. Most of the DBAs configure this only once and never again. The documentation is hard to read and plan directives are complex and confusing. Turn off the resource manager and the wait event will go away.
Also, make sure that you turn off concurrent statistics collection because it will not work without the resource manager.
Connor McDonald
July 09, 2025 - 6:51 am UTC

Turn off the resource manager and the wait event will go away.

and there's a good chance something else way worse might pop up, because if you are truly short on server resources, then that's a nice way to have your instance start misbehaving or crash altogether due to CPU starvation

Cpu quantum: problem or symptom

Jonas S Rosenthal, October 07, 2025 - 2:46 am UTC

While quantum cpu shows highh counts, dont assume the database or resource manager is the problem. You need to look holisticallly starting with your awr reports.
The One key metric to observe from thhe AWR report. should start with the following .

1) Disk Latency. When time to read or write exceeds 5 to 10 ms, that's something to tahe a peak at. More importantly, when it exceeds 1 SECOND, that's a CATASTROPHE. In one use case where this happened, emc was called to check their netwok on the storage array. I was.not working properly which explained the system performance. Applications were blocked from disk, java pools surged in sessions and cpu spiked. First thing to check is disk latency. If you don't, you are chasing your tail. Ill be back with more comments tomorrow.
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