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RDMA and zero-copy protocol latency might be affected by memory registration. |
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RDMA reduces the need for [[Protocol (computing)|protocol]] overhead, which can squeeze out the capacity to move data across a network, reducing [[Network performance|performance]], limiting how fast an application can get the data it needs, and restricting the size and scalability of a [[Computer cluster|cluster]].
However, one must be aware that there also may exists some overhead given the need for memory registration. [[zero-copy]] protocols indeed usually imply to make sure that
the memory area involved in the communications will be kept in main memory, at least during the duration of the transfer. One must for instance make sure that this memory will not be swapped out. Else, the [[DMA]] engine might use out-dated data, thus raising the risk of memory corruption. The usual way is to pin memory down so that it
will be kept in main memory, but this creates a somehow unexpected overhead since this memory registration is very expensive, thus increasing the latency linearly with the size of the data. In order to address that issue, there are several attitudes that were adopted :
* defering memory registration out of the critical path, thus somehow hidding the latency increase.
* using caching techniques to keep data pinned as long as possible so that the overhead could be reduced for application performing communications in the same memory area several times.
* somehow getting rid of the need for registration as [[Quadrics]] high-speed networks does.
RDMA’s acceptance is also limited by the need to install a different networking infrastructure. New standards enable [[Ethernet]] RDMA implementation at the physical layer and [[Transmission Control Protocol|TCP]]/[[Internet Protocol|IP]] as the transport, combining the performance and latency advantages of RDMA with a low-cost, standards-based solution. The RDMA Consortium and the DAT Collaborative<ref>[http://www.datcollaborative.org/ DAT Collaborative website.]</ref> have played key roles in the development of RDMA protocols and [[Application programming interface|APIs]] for consideration by standards groups such as the [[Internet Engineering Task Force]] and the Interconnect Software Consortium.<ref>[http://www.opengroup.org/icsc/ The Interconnect Software Consortium website.]</ref> Software vendors such as [[Oracle Corporation]] support these APIs in their latest products, and network adapters that implement RDMA over Ethernet are being developed.
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