Zero-content augmented caches

• Authors: Julien Dusser;Thomas Piquet;André Seznec
• Venue: Proceedings of the 23rd international conference on Supercomputing
• Year: 2009

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.