UCX  1.8
Unified Communication X

The UCX framework consists of the three main components: UC-Services (UCS), UC-Transports (UCT), and UC-Protocols (UCP). Each one of these components exports a public API, and can be used as a stand-alone library.

UCX Framework Architecture


UCS is a service layer that provides the necessary functionality for implementing portable and efficient utilities. This layer includes the following services:

  • an abstraction for accessing platform specific functionality (atomic operations, thread safety, etc.),
  • tools for efficient memory management (memory pools, memory allocators, and memory allocators hooks),
  • commonly used data structures (hashes, trees, lists).


UCT is a transport layer that abstracts the differences across various hardware architectures and provides a low-level API that enables the implementation of communication protocols. The primary goal of the layer is to provide direct and efficient access to hardware network functionality. For this purpose, UCT relies on vendor provided low-level drivers such as InfiniBand Verbs, Cray's uGNI, libfabrics, etc. In addition, the layer provides constructs for communication context management (thread-based and application level), and allocation and management of device-specific memories including those found in accelerators. In terms of communication APIs, UCT defines interfaces for immediate (short), buffered copy-and-send (bcopy), and zero-copy (zcopy) communication operations.

Short: This type of operation is optimized for small messages that can be posted and completed in place.

Bcopy: This type of operation is optimized for medium size messages that are typically sent through a so-called bouncing-buffer. This auxiliary buffer is typically allocated given network constraints and ready for immediate utilization by the hardware. Since a custom data packing routine could be provided, this method can be used for non-contiguos i/o.

Zcopy: This type of operation exposes zero-copy memory-to-memory communication semantics, which means that message is sent directly from user buffer, or received directly to user buffer, without being copied between the network layers.


UCP implements higher-level protocols that are typically used by message passing (MPI) and PGAS programming models by using lower-level capabilities exposed through the UCT layer. UCP is provides the following functionality: ability to select different transports for communication, message fragmentation, multi-rail communication, and initializing and finalizing the library. Currently, the API has the following classes of interfaces: Initialization, Remote Memory Access (RMA) communication, Atomic Memory Operations (AMO), Active Message, Tag-Matching, and Collectives.

Initialization: This subset of interfaces defines the communication context setup, queries the network capabilities, and initializes the local communication endpoints. The context represented by the UCX context is an abstraction of the network transport resources. The communication endpoint setup interfaces initialize the UCP endpoint, which is an abstraction of all the necessary resources associated with a particular connection. The communication endpoints are used as input to all communication operations to describe the source and destination of the communication.

RMA: This subset of interfaces defines one-sided communication operations such as PUT and GET, required for implementing low overhead, direct memory access communications constructs needed by both distributed and shared memory programming models. UCP includes a separate set of interfaces for communicating non-contiguous data. This functionality was included to support various programming models' communication requirements and leverage the scatter/gather capabilities of modern network hardware.

AMO: This subset of interfaces provides support for atomically performing operations on the remote memory, an important class of operations for PGAS programming models, particularly OpenSHMEM.

Tag Matching: This interface supports tag-matching for send-receive semantics which is a key communication semantic defined by the MPI specification.

Stream : The API provides order and reliable communication semantics. Data is treated as an ordered sequence of bytes pushed through the connection. In contrast of tag-matching interface, the size of each individual send does not necessarily have to match the size of each individual receive, as long as the total number of bytes is the same. This API is designed to match widely used BSD-socket based programming models.

Active Message: A subset of functionality where the incoming packet invokes a sender-specified callback in order to be processed by the receiving process. As an example, the two-sided MPI interface can easily be implemented on top of such a concept (TBD: cite openmpi ). However, these interfaces are more general and suited for other programming paradigms where the receiver process does not prepost receives, but expects to react to incoming packets directly. Like RMA and tag-matching interfaces, the active message interface provides separate APIs for different message types and non-contiguous data.

Collectives: This subset of interfaces defines group communication and synchronization operations. The collective operations include barrier, all-to-one, all-to-all, and reduction operations. When possible, we will take advantage of hardware acceleration for collectives (e.g., InfiniBand Switch collective acceleration).