Addressing Service Interruptions In Memory With Thread-To-Rank Assignment

In future memory systems, some regions of memory will be periodically unavailable to the processor. In DRAM systems, this may happen because a rank is busy performing refresh. In non-volatile memory systems, this may happen because a rank is busy draining long-latency writes. Unfortunately, such service interruptions can introduce stalls in all running threads. This is because the operating system spreads the pages of a thread across all memory ranks. Therefore, the probability of a thread accessing data in an unavailable rank is high. This is a performance artifact that has previously not been carefully analyzed. To reduce these stalls, we propose a simple page coloring mechanism that tries to minimize the number of ranks over which a thread's pages are spread. This approach ensures that a service interruption in a single rank only stalls a subset of threads; non-stalled threads even have the potential to run faster at this time because of reduced bus contention. Our analysis shows that this approach is more effective than recent hardware-based mechanisms to deal with such service interruptions. For example, when dealing with service interruptions because of DRAM refresh, the proposed page coloring approach yields an execution time that is 15% lower than the best competing hardware approach.