Network virtual memory

Abstract

This thesis describes the design and implementation of NetVM, which is a network interface that supports user-mode access, zero-copy transfer and sender-managed communication without pinning source or destination memory. To do this, the network interface maintains a shadow page table, which the host operating system updates whenever it maps or unmaps a page in host memory. The network interface uses this table to briefly lock and translate the virtual address of a page when it accesses that page for DMA transfer. The operating system is prevented from replacing a page in the short interval that the network interface has locked that page. If a destination page is not resident in memory, the network interface redirects the data to an intermediate system buffer, which the operating system uses to complete the transfer with a single host-to-host memory copy after fetching in the required page. A credit-based flow-control scheme prevents the system buffer from overflowing. Application-level DMA transfers only data. To support control transfers, NetVM implements a counter-based notification mechanism for applications to issue and detect notifications. The sending application increments an event counter by specifying its identifier in an RDMA write operation. The receiving application detects this event by busy waiting, block waiting or triggering a user-defined handler whenever the notifying write completes. This range of detection mechanisms allows the application to decide appropriate tradeoffs between reducing signaling latency and reducing processor overhead. NetVM enforces ordered notifications over an out-of-order delivery network by using a sequence window. The NetVM prototype is implemented in firmware for the Myrinet LANai-9.2 and integrated with the FreeBSD 4.6 virtual memory system. NetVM's memory-management overhead is low; it adds only less than 5.0% write latency compared to a static pinning approach and has a lower pinning cost compared to a dynamic pinning approach that has up to 94.5% hit rate in the pinned-page cache. Minimum write latency is 5.56μs and maximum throughput is 155.46MB/s, which is 97.2% of the link bandwidth. Transferring control through notification adds between 2.96μs and 17.49μs to the write operation, depending on the detection mechanism used. Compared to standard low-level atomic operations, NetVM adds only up to 18.2% and 12.6% to application latencies for high-level wait-queue and counting-semaphore operations respectively. (Abstract shortened by UMI.)