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The Virtual Write Queue: Coordinating DRAM and Last-Level Cache Policies



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The Virtual Write Queue Coordinating DRAM and Last Level Cache Policies Jeffrey Stuecheli1 2 Dimitris Kaseridis1 David Daly3 Hillery C Hunter3 Lizy K John1 1 ECE Department The University of Texas at Austin Austin TX USA 2 IBM Corp Austin TX USA 3 IBM Thomas J Watson Research Center Yorktown Heights NY USA ABSTRACT In computer architecture caches have primarily been viewed as a means to hide memory latency from the CPU Cache policies have focused on anticipating the CPU s data needs and are mostly oblivious to the main memory In this paper we demonstrate that the era of many core architectures has created new main memory bottlenecks and mandates a new approach coordination of cache policy with main memory characteristics Using the cache for memory optimization purposes we propose a Virtual Write Queue which dramatically expands the memory controller s visibility of processor behavior at low implementation overhead Through memory centric modification of existing policies such as scheduled writebacks this paper demonstrates that performancelimiting effects of highly threaded architectures can be overcome We show that through awareness of the physical main memory layout and by focusing on writes both read and write average latency can be shortened memory power reduced and overall system performance improved Through full system cycle accurate simulations of SPEC cpu2006 we demonstrate that the proposed Virtual Write Queue achieves an average 10 9 system level throughput improvement on memory intensive workloads along with an overall reduction of 8 7 in memory power across the whole suite Categories and Subject Descriptors B 3 1 Memory Structures Semiconductor Memories Dynamic memory DRAM B 3 2 Memory Structures Design styles cache memories Primary Memories Shared Memory Interleaved Memories General Terms Design Performance 1 INTRODUCTION It is now well understood that in the nanometer era technology scaling will continue to provide transistor density improvements



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