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Method for filtering internal snoop events using qualified coherence states

IP.com Disclosure Number: IPCOM000128944D
Publication Date: 2005-Sep-21
Document File: 4 page(s) / 63K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for filtering internal snoop events using qualified coherence states. Benefits include improved functionality and improved performance.

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Method for filtering internal snoop events using qualified coherence states

Disclosed is a method for filtering internal snoop events using qualified coherence states. Benefits include improved functionality and improved performance.

Background

              Virtually all microprocessors contain low-latency caches that are used to store data and code that is frequently used by the microprocessor. These caches are typically the lowest-level caches in the microprocessor memory hierarchy. Generally, most processors are cache coherent with memory. As such, these lower-level caches must support snooping to ensure correct state changes and invalidation due to both external and internal events.

              Some lower-level caches share a single read port between standard memory requests and snoop traffic. Others implement a dedicated snoop port that is independent of the request port, such as multiported caches. All snoop traffic, whether from an external probe of a bus request or from an internal event, arbitrate for the snoop port to the lower-level cache. By eliminating the requirement to issue a snoop event, the dependency on the lone snoop port to the lower-level cache is reduced.

General description

              The disclosed method reduces and/or removes external and internal snoop events, including tag inquiries (external probes) and lower-level cache state directives (internal confirms). The method provides an enhancement to the standard hardware-based coherence protocols, including the addition of a core-present (CP) qualifier applied to the higher-level cache coherence state.

Advantages

              The disclosed method provides advantages, including:
•             Improved functionality due to reducing and/or removing internal and external snoop events

•             Improved functionality due to adding the CP qualifier applied to the higher-level cache coherence state

•             Improved performance due to filtering probes and confirms intended for the lower-level cache hierarchy

•             Improved performance due to simplifying the management of explicit writebacks

•             Improved performance due to optimizing support of true (forced) inclusion between higher-level and lower-level cache hierarchies

Detailed description

              The disclosed method filters internal snoop events using qualified coherence states. When a lower-level cache requests a line from memory, the coherence state of the applicable line in the higher-level cache is tagged with an attribute. It indicates that a lower-level cache has a copy of the line; that it is present in the lower-level processor cache. This attribute is termed CP. It enables several protocol optimizations, including the following:

•             CP enables filtering probes and confirms intended for the lower-level cache hierarchy

              -             If a higher-level cache reports a hit and CP=0, no requirement exists to issue a probe to the lower-level cache.

              -             Based on the...