Zero Inclusion Victim: Isolating Core Caches from Inclusive Last-level Cache Evictions

Abstract

The most widely used last-level cache (LLC) architecture in the microprocessors has been the inclusive LLC design. The popularity of the inclusive design stems from the bandwidth optimization and simplification it offers to the implementation of the cache coherence protocols. However, inclusive LLCs have always been associated with the curse of inclusion victims. An inclusion victim is a block that must be forcefully replaced from the inner levels of the cache hierarchy when the copy of the block is replaced from the inclusive LLC. This tight coupling between the LLC victims and the inner-level cache contents leads to three major drawbacks. First, live inclusion victims can lead to severe performance degradation depending on the LLC replacement policies. Second, a process can victimize the blocks of another process in an LLC shared by multiple cores and this can be exploited to leak information through well-known eviction-based timing side-channels. An inclusive LLC makes these channels much less noisy due to the presence of inclusion victims which allow the malicious processes to control the contents of the percore private caches through LLC evictions. Third, to reduce the impact of the aforementioned two drawbacks, the inner-level caches, particularly the mid-level cache in a three-level inclusive cache hierarchy, must be kept small even if a larger mid-level cache could have been beneficial in the absence of inclusion victims.We observe that inclusion victims are not fundamental to the inclusion property, but arise due to the way the contents of an inclusive LLC are managed. Motivated by this observation, we introduce a fundamentally new inclusive LLC design named the Zero Inclusion Victim (ZIV) LLC that guarantees freedom from inclusion victims while retaining all advantages of an inclusive LLC. This is the first inclusive LLC design proposal to offer such a guarantee, thereby completely isolating the core caches from LLC evictions. We observe that the root cause of inclusion victims is the constraint that an LLC victim must be chosen from the set pointed to by the set indexing function. The ZIV LLC relaxes this constraint only when necessary by efficiently and minimally enabling a global victim selection scheme in the inclusive LLC to avoid generation of inclusion victims. Detailed simulations conducted with a chip-multiprocessor model using multi-programmed and multi-threaded workloads show that the ZIV LLC gracefully supports large mid-level caches (e.g., half the size of the LLC) and delivers performance close to a non-inclusive LLC for different classes of LLC replacement policies. We also show that the ZIV LLC comfortably outperforms the existing related proposals and its performance lead grows with increasing mid-level cache capacity.