Modern DRAM cells are periodically refreshed to prevent data loss due to
leakage. Commodity DDR DRAM refreshes cells at the rank level. This degrades
performance significantly because it prevents an entire rank from serving
memory requests while being refreshed. DRAM designed for mobile platforms,
LPDDR DRAM, supports an enhanced mode, called per-bank refresh, that refreshes
cells at the bank level. This enables a bank to be accessed while another in
the same rank is being refreshed, alleviating part of the negative performance
impact of refreshes. However, there are two shortcomings of per-bank refresh.
First, the per-bank refresh scheduling scheme does not exploit the full
potential of overlapping refreshes with accesses across banks because it
restricts the banks to be refreshed in a sequential round-robin order. Second,
accesses to a bank that is being refreshed have to wait.
To mitigate the negative performance impact of DRAM refresh, we propose two
complementary mechanisms, DARP (Dynamic Access Refresh Parallelization) and
SARP (Subarray Access Refresh Parallelization). The goal is to address the
drawbacks of per-bank refresh by building more efficient techniques to
parallelize refreshes and accesses within DRAM. First, instead of issuing
per-bank refreshes in a round-robin order, DARP issues per-bank refreshes to
idle banks in an out-of-order manner. Furthermore, DARP schedules refreshes
during intervals when a batch of writes are draining to DRAM. Second, SARP
exploits the existence of mostly-independent subarrays within a bank. With
minor modifications to DRAM organization, it allows a bank to serve memory
accesses to an idle subarray while another subarray is being refreshed.
Extensive evaluations show that our mechanisms improve system performance and
energy efficiency compared to state-of-the-art refresh policies and the benefit
increases as DRAM density increases.Comment: The original paper published in the International Symposium on
High-Performance Computer Architecture (HPCA) contains an error. The arxiv
version has an erratum that describes the error and the fix for i
