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Method for differentiating dirty data in multilevel cache hierarchies using coherence states

IP.com Disclosure Number: IPCOM000128947D
Publication Date: 2005-Sep-21
Document File: 3 page(s) / 47K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for differentiating dirty data in multilevel cache hierarchies using coherence states. Benefits include improved functionality and improved performance.

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Method for differentiating dirty data in multilevel cache hierarchies using coherence states

Disclosed is a method for differentiating dirty data in multilevel cache hierarchies using coherence states. Benefits include improved functionality and improved performance.

Background

              Conventional cache coherence protocols are unable to correctly determine whether a line marked dirty, typically denoted as modified (M) in a higher-level cache has been more recently modified in a lower-level cache. This situation typically occurs when a dirty cache line is replaced in a higher-level cache. The issue is resolved by back-snooping (inquiring backward) the lower-level cache to determine if the lower-level cache has a dirty copy. The process of back-snooping consumes snoop bandwidth in the lower-level cache and delays the pending writeback of the evicted line in the higher-level cache.

              Conventional coherence protocols do not explicitly indicate whether a more-recently modified lower-level copy of a line exists.

General description

              The disclosed method differentiates dirty data in multilevel cache hierarchies using coherence states. The method defines a reserved (R) state that indicates that a dirty line in a higher-level cache is more recently modified in a lower-level cache.

Advantages

              The disclosed method provides advantages, including:
•             Improved functionality due to tracking the right-to-modify status of dirty lines
•             Improved performance due to reducing the number of snoop events issued

Detailed description

              The disclosed method includes a coherent state, R, that explicitly indicates that a dirty line in a higher-level cache has been recently modified in a lower-level cache. The R state reserves a location in higher-level cache for lines that are dirty in a lower-level cache. The reserved state is defined to have the following properties:

•             Lower-level cache has a copy of the line in a right-to-modify dirty state

•             Returns a cache hit value when snooped

•             Is cleared by the following actions:

              Subsequent writeback of the dirty line tracked by the lower-level cache

              Invalidating external snoop event

-         Replacement

The R state reserves a location in the higher-level cache while yielding the write-to-modify to the lower-level cache. Because R-state hits return A cache...