Decoupled zero-compressed memory

For each computer system generation, there are always applications or workloads for which the main memory size is the major limitation. On the other hand, in many cases, one could free a very significant portion of the memory space by storing data in a compressed form. Therefore, a hardware compressed memory is an attractive way to artificially increase the amount of data accessible in a reasonable delay. Among the data that are highly compressible are null data blocks. Previous work has shown that, on many applications null blocks represent a significant fraction of the working set resident in main memory. We propose to leverage this property through the use of a hardware compressed memory that only targets null data blocks, the decoupled zero-compressed memory, or DZC memory. Main memory is managed as a decoupled sectored cache with physical pages treated as sectors and 64-byte memory blocks treated as subblock. Null memory blocks are represented through a single bit, thus freeing physical memory space for the applications. Our experiments show that for many applications, the DZC memory allows to artificially enlarge the main memory, i.e. it reduces the effective physical memory size needed to accommodate the working set of an application without excessive page swapping. Moreover, through caching null memory blocks in the memory controller, the DZC memory also decreases the average access time to the main memory for many applications.