X-PPR: Post Package Repair for CXL Memory

CXL is an emerging interface that can cost-efficiently expand the capacity and bandwidth of servers, recycling DRAM modules from retired servers. Such DRAM modules, however, will likely have many uncorrectable faulty words due to years of strenuous use in datacenters. To repair faulty words in the field, a few solutions based on Post Package Repair (PPR) and memory offlining have been proposed. Nonetheless, they are either unable to fix thousands of faulty words or prone to causing severe memory fragmentation, as they operate at the granularity of DRAM row and memory page addresses, respectively. In this work, for cost-efficient use of recycled DRAM modules with thousands of faulty words, we propose CXL-PPR (X-PPR), exploiting the CXL's support for near-memory processing and variable memory access latency. We demonstrate that X-PPR implemented in a commercial CXL device with DDR4 DRAM modules can handle a faulty bit probability that is $3.3 \times 10^{4}$ higher than ECC for a 512GB DRAM module. Meanwhile, X-PPR negligibly degrades the performance of popular memory-intensive benchmarks, which is achieved through two mechanisms designed in X-PPR to minimize the performance impact of additional DRAM accesses required for repairing faulty words.