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?
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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.
David Boike
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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.
Udi Dahan
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