Zero Copy, OS Bypass and Application Bypass

In traditional system architectures, network packets arrive at the network interface card (NIC), are passed through one or more protocol layers in the operating system, and eventually copied into the address space of the application. As network bandwidth began to approach memory copy rates, reduction of memory copies became a critical concern. This concern lead to the development of zero-copy message passing protocols in which message copies are eliminated or pipelined to avoid the loss of bandwidth.

A typical zero-copy protocol has the NIC generate an interrupt for the CPU when a message arrives from the network. The interrupt handler then controls the transfer of the incoming message into the address space of the appropriate application. The interrupt latency, the time from the initiation of an interrupt until the interrupt handler is running, is fairly significant. To avoid this cost, some modern NICs have processors that can be programmed to implement part of a message passing protocol. Given a properly designed protocol, it is possible to program the NIC to control the transfer of incoming messages, without needing to interrupt the CPU. Because this strategy does not need to involve the OS on every message transfer, it is frequently called "OS Bypass." ST[12], VIA[3], FM[5], GM[9], and Portals are examples of OS Bypass protocols.

Many protocols that support OS Bypass still require that the application actively participate in the protocol to ensure progress. As an example, the long message protocol of PM requires that the application receive and reply to a request to put or get a long message. This complicates the runtime environment, requiring a thread to process incoming requests, and significantly increases the latency required to initiate a long message protocol. The Portals message passing protocol does not require activity on the part of the application to ensure progress. We use the term "Application Bypass" to refer to this aspect of the Portals protocol.