Zero-Content Augmented Caches

Julien Dusser 1 Thomas Piquet 1 André Seznec 1
1 ALF - Amdahl's Law is Forever
Inria Rennes – Bretagne Atlantique , IRISA-D3 - ARCHITECTURE
Abstract : It has been observed that some applications manipulate large amounts of null data. Moreover these zero data often exhibit high spatial locality. On some applications more than 20\% of the data accesses concern null data blocks. Representing a null block in a cache on a standard cache line appears as a waste of resources. In this paper, we propose the Zero-Content Augmented cache, the ZCA cache. A ZCA cache consists of a conventional cache augmented with a specialized cache for memorizing null blocks, the Zero-Content cache or ZC cache. In the ZC cache, the data block is represented by its address tag and a validity bit. Moreover, as null blocks generally exhibit high spatial locality, several null blocks can be associated with a single address tag in the ZC cache. For instance, a ZC cache mapping 32MB of zero 64-byte lines uses less than 80KB of storage. Decompression of a null block is very simple, therefore read access time on the ZCA cache is in the same range as the one of a conventional cache. On applications manipulating large amount of null data blocks, such a ZC cache allows to significantly reduce the miss rate and memory traffic, and therefore to increase performance for a small hardware overhead. In particular, the write-back traffic on null blocks is limited. For applications with a low null block rate, no performance loss is observed.
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Julien Dusser, Thomas Piquet, André Seznec. Zero-Content Augmented Caches. 23rd International Conference on Supercomputing, ACM, 2009, ⟨10.1145/1542275.1542288⟩. ⟨inria-00374524⟩

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