Assume that lots of users synchronize their mailboxes in a Microsoft Exchange Server 2010 environment by using Apple devices such as iPhone or iPad. These Apple devices are running iOS 5.0 or iOS 6.0, or a later version than iOS 5.0 or iOS 6.0. In this situation, high CPU usage (about 80 to 100 percent) occurs on the Client Access server (CAS). Then, the synchronizations fail, and the users cannot receive new email messages.
This issue occurs because of an inefficient throttling object pool that is used for synchronization requests. The locking scheme is used to allocate the buffer in the pool to the threads that are for individual users and that are required to wait for longer time under high load conditions. The length of the waiting queue keeps increasing, and therefore the end-user experience degrades.
To resolve this issue, install the following update rollup:
Description of Update Rollup 2 for Exchange Server 2010 SP3
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
The third-party products that this article discusses are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, about the performance or reliability of these products.