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Abstract-As the size and speed of DRAM devices increase, the performance and energy overheads due to refresh become more significant. To reduce refresh penalty we propose techniques referred collectively as "Coordinated Refresh", in which scheduling of low power modes and refresh commands are coordinated so that most of the required refreshes are issued when the DRAM device is in the deepest low power Self Refresh (SR) mode. Our approach saves DRAM background power because the peripheral circuitry and clocks are turned off in the SR mode. Our proposed solutions improve DRAM energy efficiency by 10% as compared to baseline, averaged across all the SPEC CPU 2006 benchmarks.

DRAM cells require periodic refreshing to preserve data. In JEDEC DDRx devices, a refresh operation is performed via an auto-refresh command, which refreshes multiple rows in multiple banks simultaneously. The internal implementation of auto-refresh is completely opaque outside the DRAMall the memory controller can do is to instruct the DRAM to refresh itself -the DRAM handles all else, in particular determining which rows in which banks are to be refreshed. This is in conflict with a large body of research on reducing the refresh overhead, in which the memory controller needs fine-grained control over which regions of the memory are refreshed. For example, prior works exploit the fact that a subset of DRAM rows can be refreshed at a slower rate than other rows due to access rate or retention period variations. However, such row-granularity approaches cannot use the standard auto-refresh command, which refreshes an entire batch of rows at once and does not permit skipping of rows. Consequently, prior schemes are forced to use explicit sequences of activate (ACT) and precharge (PRE) operations to mimic row-level refreshing. The drawback is that, compared to using JEDEC's auto-refresh mechanism, using explicit ACT and PRE commands is inefficient, both in terms of performance and power.In this paper, we show that even when skipping a high percentage of refresh operations, existing row-granurality refresh techniques are mostly ineffective due to the inherent efficiency disparity between ACT/PRE and the JEDEC autorefresh mechanism. We propose a modification to the DRAM that extends its existing control-register access protocol to include the DRAM's internal refresh counter. We also introduce a new "dummy refresh" command that skips refresh operations and simply increments the internal counter. We show that these modifications allow a memory controller to reduce as many refreshes as in prior work,