dram的刷新详解

In typical modern DRAM systems, thememory controller periodically issues an auto-refresh (auto-refreshis sometimes called CAS-before-RAS refresh)command to the DRAM.The DRAM chip thenchooses which rows to refresh using an
internal counter, and refreshes a numberof rows based on the device capacity. During normal temperature operation (below 85’C), the average time between auto-refresh commands(called tREFI ) is 7.8us . In the extended temperature range (between 85 _C and 95
_C), the temperature range in whichdense server environments operate [10] and 3D-stacked DRAMs are expected tooperate [1], the time between

auto-refresh commands is halved to 3.9 us [15]. An auto-refresh operation occupies all banks on the ranksimultaneously (preventing the rank from servicing any requests) for a lengthof time tRFC, where tRFC depends on the
number of rows beingrefreshed .

Some devices support per-bank refreshcommands, which refresh several rows at a single bank [16], allowing forbank-level parallelism at a rank during refreshes. However, this feature is notavailable in most DRAM devices.

在典型的现代dram系统中，MC周期性的向dram发送 auto-refresh (有时候这这种刷新方式称为CAS-before-RAS刷新)命令。Dram芯片使用一个内部计数器，来选择需要刷新哪些行，每一次刷新的行数和Dram的容量有关。在正常温度下(低于85‘C)，auto-refresh命令的时间间隔(称为 tREFI )为7.8us，在高温下(介于85’C和95‘C，通常密集的服务器环境和3D
dram处于这种温度下)，时间间隔为3.9us.一个auto-refresh操作同时占有一个rank的所有的bank(使rank不能相应任何内存请求)的时间为 tRFC (注意：tRFC为刷新所占用的时间，而tREFI为两次刷新的时间间隔)，而tRFC取决于刷新的行数。（注：这种方式最为常见）

         以前的Dram也允许MC通过一行一行的打开每一行的方式执行刷新操作(称为RAS-only刷新)，但是由于需要额外的功耗来向总线传送行地址，所以这种方式被废弃了。

         有的Dram也支持每个bank刷新的命令，每次同时刷新一个bank的多个行，在一个rank刷新的时候允许bank-level 并行(注：这指的是刷新rank中的一个bank的时候，可以访问这个rank的另外一个bank)

参考资料：RAIDR: Retention-Aware Intelligent DRAM Refresh,Jamie Liu Ben Jaiyen Richard Veras Onur Mutlu 

Carnegie Mellon University 

{jamiel,bjaiyen,rveras,onur}@cmu.edu