Communication Notification Through User-Level Interrupts for the BXI Network

To reduce the cost of communications in highperformance computing, it is possible to overlap communications with computations. Some communication protocols, such as rendez-vous, multi-chunk messages, and collectives, may require a completion notification to be processed before they can further progress. With active polling or passive waiting, completions are not processed while the application is busy with computation, and thus communication does not progress. However, with an eventbased method like interrupts, it is expected to be much more reactive. Nevertheless, using interrupts usually involves system calls, which are avoided with high-performance networks. The Intel Sapphire Rapids processors introduced user-level interrupts (UINTR), hardware interrupts designed to be used directly in user space, without going through the kernel. However, their current implementation is limited to inter-process communication. They cannot be triggered from a device. In this paper, we propose new mechanisms to extend the scope of user-level interrupts, so as to be able to trigger them from a device and not only from a CPU. We have implemented these mechanisms in the BXI network from Eviden. We have evaluated their performance: we obtain a latency only 2.4 times higher than active polling (v.s. 6 times higher for interrupts with system calls). We have assessed their ability to make communication progress when overlapped with computation; we observe a near-perfect computation/communication overlap.