We noticed that CPU usage went up from 5% TO 50% after adding NServicebus to our ASP.net MVC app. This was on a server that was not under any load. We noticed the same behavior on another server that hosted a WCF app. After trying out different things, we figured out that if we configured the bus as send only, the CPU usage dropped to 5%. Does anybody know why the cpu usage was so high when the bus is not configured as send only?
High CPU usage with NServicebus when IIS hosted (Asp.net and WCF)?
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I've experienced this before.
What happened to me was I set up an application pool, and it started out running as Network Service. Before I had the chance to set the application pool identity to a domain-level user (for access to file shares, etc.) the pages had already been hit, and so the NServiceBus installers had already created a queue with the Network Service credentials.
When I set the application pool user, all of a sudden it didn't have the proper permissions to the queue.
Normally NServiceBus checks for messages with a timeout if none are available to be received, but in this instance, it goes into a very tight loop of "Are there messages? I don't have permission. Are there messages? I don't have permission." and so you get the very high CPU.
I fixed the problem by deleting the queue and allowing NServiceBus to recreate it with the proper permissions.
It's possible that the cause of the high CPU was the NServiceBus code that looks for a message in the queue, though I find that a bit hard to believe. Send-only mode prevents NServiceBus from looking for messages in the queue.