Analyzing Cache Coherence Protocols for Server Consolidation

Server consolidation is commonly used today to make the most out of all the cores of a chip multiprocessor by running several virtual machines (VMs) on it. Cache coherence protocols can be adapted to take advantage of such an scenario. In this line, Virtual Hierarchies (VHs) use two levels of cache coherence in a consolidated server. They isolate the coherence actions of each VM and improve performance by maximizing the number of memory accesses serviced by caches within the VM. In this paper we show how hierarchical protocols with no single ordering point for the requests, such as VHs in the form currently proposed, are prone to deadlocks. Besides, when memory deduplication is used, VHs cannot take advantage of memory deduplication at the cache level, both because deduplicated data is reduplicated in cache, and because accesses to deduplicated data often require the access to the cache tiles used by a different VM by means of broadcast. We analyze all these problems and we propose solutions for them, showing the actual performance of these protocols, and giving some insights for the future development of coherence protocols optimized for server consolidation.