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Another facet of traditional networks is that arriving data is placed in a pre-allocated buffer and then copied to the user-specified final destination. Copying large messages can take a long time, and so eliminating this step is beneficial. Another classic development in computing systems is [[direct memory access]] (DMA), in which a device can access main memory directly while the CPU is free to perform other tasks.
In a network with "remote direct memory access" ([[Remote direct memory access|RDMA]]), the sending NIC uses DMA to read data in the user-specified buffer and transmit it as a self-contained message across the network. The receiving NIC then uses DMA to place the data into the user-specified buffer. There is no intermediary copying and all of these actions occur without involvement of the CPUs, which has an added benefit of lower CPU utilization.
For the NIC to actually access the data through DMA, the user's page must be in memory. In VIA, the user must "pin-down" its buffers before transmission, so as to prevent the OS from swapping the page out to the disk. This action—one of the few that involves the kernel—ties the page to physical memory. To ensure that only the process that owns the registered memory may access it, the VIA NICs require permission keys known as "protection tags" during communication.
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