Snoop filters can also attempt to track the set of blocks that will return negative responses to snoop requests. While the portion of memory \emph{not} present in the cache is many times larger than the portion present in the cache, it is sometimes possible to identify a small set of addresses that are likely to be snooped in the near future, or perhaps to identify large areas of memory that will never be present in the cache.
A simple example of an exclusive filter is the exclude-Jetty which is a set-associative table containing the most recently snooped addresses that returned negative responses. This approach takes advantage of temporal locality in snoop addresses, filtering out most unnecessary snoops for highly contended blocks. To increase the reach of this filter, Moshovos et al. also propose a vector exclude-Jetty that combines an address tag with a bit-vector to represent multiple consecutive blocks. This allows the filter to track many addresses that have high spatial locality with minimal overhead. The Blue Gene/P system includes such a vector exclude Jetty as part of its snoop filter.
Moshovos, A., Memik, G., Choudhary, A., and Falsafi, B. 2001. JETTY: Filtering Snoops for Reduced Energy Consumption in SMP Servers. In Proceedings of the 7th international Symposium on High-Performance Computer Architecture (January 20 - 24, 2001). HPCA. IEEE Computer Society, Washington, DC, 85.
Valentina Salapura, Matthias A. Blumrich, Alan Gara: Design and implementation of the blue gene/P snoop filter. HPCA 2008
The Blue Gene/P includes another type of exclusive filter called a range filter. This filter indentifies a large range of addresses and specifies either that addresses outside that range, or possibly inside that range, do not need to perform tag lookups on snoops. The range filter is controlled by software and works well when multiple processors are known to be using completely distinct and continuous portions of physical memory.
Valentina Salapura, Matthias A. Blumrich, Alan Gara: Design and implementation of the blue gene/P snoop filter. HPCA 2008