The memory system is a major performance determinant for server processors. Ever-growing core counts and datasets demand higher bandwidth and capacity as well as lower latency from the memory system. To keep up with growing demands, DDR--the dominant processor interface to memory over the past two decades--has offered higher bandwidth with every generation. However, because each parallel DDR interface requires a large number of on-chip pins, the processor's memory bandwidth is ultimately restrained by its pin-count, which is a scarce resource. With limited bandwidth, multiple memory requests typically contend for each memory channel, resulting in significant queuing delays that often overshadow DRAM's service time and degrade performance. We present CoaXiaL, a server design that overcomes memory bandwidth limitations by replacing \textit{all} DDR interfaces to the processor with the more pin-efficient CXL interface. The widespread adoption and industrial momentum of CXL makes such a transition possible, offering $4\times$ higher bandwidth per pin compared to DDR at a modest latency overhead. We demonstrate that, for a broad range of workloads, CXL's latency premium is more than offset by its higher bandwidth. As CoaXiaL distributes memory requests across more channels, it drastically reduces queuing delays and thereby both the average value and variance of memory access latency. Our evaluation with a variety of workloads shows that CoaXiaL improves the performance of manycore throughput-oriented servers by $1.52\times$ on average and by up to $3\times$.

翻译：内存系统是服务器处理器性能的主要决定因素。不断增长的核心数量和数据集要求内存系统提供更高的带宽、容量以及更低的延迟。为了满足日益增长的需求，过去二十年中处理器与内存之间的主导接口DDR每一代都提供了更高的带宽。然而，由于每个并行DDR接口需要大量片上引脚，处理器的内存带宽最终受到其引脚数量（一种稀缺资源）的限制。在带宽有限的情况下，多个内存请求通常会争用同一内存通道，导致显著的排队延迟，这常常超过DRAM的服务时间并降低性能。我们提出了CoaXiaL，一种通过用引脚效率更高的CXL接口替换处理器上的所有DDR接口来克服内存带宽限制的服务器设计。CXL的广泛采用和工业推动使得这种转变成为可能，相比DDR，每引脚带宽提高了4倍，同时延迟开销适中。我们证明，对于广泛的工作负载，CXL的延迟代价被其更高的带宽所抵消。由于CoaXiaL将内存请求分配到更多通道，它大幅减少了排队延迟，从而降低了内存访问延迟的平均值和方差。我们对多种工作负载的评估显示，CoaXiaL使多核吞吐量导向型服务器的性能平均提高了1.52倍，最高可达3倍。