We had an incident recently where some pages on our SharePoint application would just not show. We would just get a timeout error. These pages were functioning a few hours ago but all of a sudden they stopped.
What was common with all these pages is that they all had one or more instance of the Content Query Web Part (CQWP). I ruled out database and network issues quite fast. After spending hours trying to understand why these pages were timing out I finally tried one last hunch. The CQWP uses cross list queries that in turn use the object cache for optimal performance.In a previous post I talk about the importance of the object cache with regards to Content Query Web Part performance.
Since the CQWP is so tightly coupled to the object cache I decided to go to Site Settings, Modify All Site Settings, Site Collection Object Cache and flush and clear the cache across all servers. All the problem pages started working. I guess sometimes the object cache gets corrupted and then all pages that have a CQWP or some form of a cross list query will perform poorly or just stop working.
I have yet to determine the cause of the so called cache corruption.