Basic Concepts

This section introduces the basic concepts for XPU management used by dpctl.

Note

For SYCL-level details, refer to a more topical SYCL reference, such as the SYCL 2020 spec.

• Heterogeneous computing

  Refers to using multiple devices in a program.

• Host

  Every program starts by running on a host, and most of the lines of code in a program, in particular lines of code implementing the Python interpreter itself, are usually for the host. Hosts are customarily CPUs.

• Device

  A device is an XPU connected to a host that is programmable with a specific device driver. Different types of devices can have different architectures (CPUs, GPUs, FPGA, ASICs, DSP) but are programmable using the same oneAPI programming model.

• Platform

  A device driver installed on the system is called the platform. As multiple devices of the same type can share the same device driver, a platform may contain multiple devices. The same physical hardware (for example, GPU) may be reflected as two separate devices if they can be programmed by more than one platform. For example, the same GPU hardware can be listed as an OpenCL* GPU device and a Level-Zero* GPU device.

• Context

  Holds the runtime information needed to operate on a device or a group of devices from the same platform. Contexts are relatively expensive to create and should be reused as much as possible.

• Queue

  A queue is needed to schedule the execution of any computation or data copying on the device. Queue construction requires specifying a device and a context targeting that device as well as additional properties, such as whether profiling information should be collected or submitted tasks are executed in the order in which they were submitted.

• Event

  An event holds information related to computation/data movement operation scheduled for execution on a queue, such as its execution status as well as profiling information if the queue the task was submitted to allowed for collection of such information. Events can be used to specify task dependencies as well as to synchronize host and devices.

• USM

  Unified Shared Memory (USM) refers to pointer-based device memory management. USM allocations are bound to context. It means, a pointer representing USM allocation can be unambiguously mapped to the data it represents only if the associated context is known. USM allocations are accessible by computational kernels that are executed on a device, provided that the allocation is bound to the same context that is used to construct the queue where the kernel is scheduled for execution.

  Depending on the capability of the device, USM allocations can be:

Name	Allocation accessible	Access
Device allocation	No	Refers to an allocation in host memory that is accessible from a device.
Shared allocation	Yes	Accessible by both the host and device.
Host allocation	Yes	Accessible by both the host and device.

Runtime manages synchronization of the host’s and device’s view into shared allocations. The initial placement of the shared allocations is not defined.

• Backend

  Refers to the implementation of oneAPI programming model exposed by the underlying runtime.