SWEL: Hardware cache coherence protocols to map shared data onto shared caches

Abstract

Snooping and directory-based coherence protocols have become the de facto standard in chip multi-processors, but neither design is without drawbacks. Snooping protocols are not scalable, while directory protocols incur directory storage overhead, frequent indirections, and are more prone to design bugs. In this paper, we propose a novel coherence protocol that greatly reduces the number of coherence operations and falls back on a simple broadcast-based snooping protocol when infrequent coherence is required. This new protocol is based on the premise that most blocks are either private to a core or read-only, and hence, do not require coherence. This will be especially true for future large-scale multi-core machines that will be used to execute message-passing workloads in the HPC domain, or multiple virtual machines for servers. In such systems, it is expected that a very small fraction of blocks will be both shared and frequently written, hence the need to optimize coherence protocols for a new common case. In our new protocol, dubbed SWEL (protocol states are Shared, Written, Exclusivity Level), the L1 cache attempts to store only private or read-only blocks, while shared and written blocks must reside at the shared L2 level. These determinations are made at runtime without software assistance. While accesses to blocks banished from the L1 become more expensive, SWEL can improve throughput because directory indirection is removed for many common write-sharing patterns. Compared to a MESI based directory implementation, we see up to 15% increased performance, a maximum degradation of 2%, and an average performance increase of 2.5% using SWEL and its derivatives. Other advantages of this strategy are reduced protocol complexity (achieved by reducing transient states) and significantly less storage overhead than traditional directory protocols.