SILC-FM: Subblocked InterLeaved Cache-Like Flat Memory Organization

With current DRAM technology reaching its limit, emerging heterogeneous memory systems have become attractive to continue scaling memory performance. This paper argues for using a small, fast memory closer to the processor as part of a flat address space where the memory system is composed of two or more memory types. OS-transparent management of such memory has been proposed in prior works such as CAMEO and Part of Memory (PoM). Data migration is typically handled either at coarse granularity with high bandwidth overheads (as in PoM) or at fine granularity with low hit rate (as in CAMEO). Prior work uses restricted address mapping from only congruence groups in order to simplify the mapping. At any time, only one page (block) from a congruence group is resident in the fast memory. In this paper, we present a flat address space organization called SILC-FM that uses large granularity but allows subblocks from two pages to coexist in an interleaved fashion in fast memory. Data movement is done at subblocked granularity, avoiding fetching of useless subblocks and consuming less bandwidth compared to migrating the entire large block. SILC-FM can achieve more spatial locality hits than CAMEO and PoM due to page-level operation and interleaving blocks respectively. The interleaved subblock placement improves performance by 55% on average over a static placement scheme without data migration. We also selectively lock hot blocks to prevent them from being involved in hardware swapping operations. Additional features such as locking, associativity and bandwidth balancing improve performance by 11%, 8%, and 8% respectively, resulting in a total of 82% performance improvement over a no migration static placement scheme. Compared to the best state-of-the-art scheme, SILC-FM gets performance improvement of 36% with 13% energy savings.