Títol:
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Filtering directory lookups in CMPs
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Autor/a:
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Bosque, Ana; Viñals Yufera, Víctor; Ibáñez, Pablo; Llaberia Griñó, José M.
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Altres autors:
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Universitat Politècnica de Catalunya. Departament d'Arquitectura de Computadors; Universitat Politècnica de Catalunya. CAP - Grup de Computació d'Altes Prestacions |
Abstract:
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Coherence protocols consume an important fraction of power to determine which coherence action should take place. In this paper we focus on CMPs with a shared cache
and a directory-based coherence protocol implemented as a duplicate of local caches tags. We observe that a big fraction of directory lookups produce a miss since the block looked up is not cached in any local cache. We propose to add a filter before the directory lookup in order to reduce the number of lookups to this structure. The filter identifies whether the current block was last accessed as a data or as an instruction. With this information, looking up the whole directory can be avoided for most accesses. We evaluate the filter in a CMP with 8 in-order processors with 4 threads each and a memory hierarchy with a shared L2 cache.We show that a filter with a size of 3% of the tag array of the shared cache can avoid more than 70% of all comparisons performed by directory lookups with a performance loss of just 0.2% for SPLASH2 and 1.5% for Specweb2005. On average,
the number of 15-bit comparisons avoided per cycle is 54 out of 77 for SPLASH2 and 29 out of 41 for Specweb2005. In both cases, the filter requires less than one read of 1 bit per cycle. |
Matèries:
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-Àrees temàtiques de la UPC::Informàtica::Arquitectura de computadors -Multiprocessors -Cache storage -Multiprocessing systems -Protocols -Multiprocessadors |
Drets:
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Tipus de document:
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Article - Versió publicada Objecte de conferència |
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