Intel® oneAPI Programming Guide

Programming Guide

Contents

Device Selection

Offloading code to a device (such as a CPU, GPU, or FPGA) is available for both DPC++ and OpenMP* applications.

DPC++ Device Selection in the Host Code

Host code can explicitly select a device type. To do select a device, select a queue and initialize its device with one of the following:
  • default_selector
  • cpu_selector
  • gpu_selector
  • accelerator_selector
If
default_selector
 is used, the kernel runs based on a heuristic that chooses from available compute devices (all, or a subset based on the value of the
SYCL_DEVICE_FILTER
 environment variable).
If a specific device type (such as
cpu_selector
 or
gpu_selector
) is used, then it is expected that the specified device type is available in the platform or included in the filter specified by
SYCL_DEVICE_FILTER
. If such a device is not available, then the runtime system throws an exception indicating that the requested device is not available. This error can be thrown in the situation where an ahead-of-time (AOT) compiled binary is run in a platform that does not contain the specified device type.
While
DPC++
 applications can run on any supported target hardware, tuning is required to derive the best performance advantage on a given target architecture. For example, code tuned for a CPU likely will not run as fast on a GPU accelerator without modification.
SYCL_DEVICE_FILTER
 is a complex environment variable that allows you to limit the runtimes, compute device types, and compute device IDs that may be used by the DPC++ runtime to a subset of all available combinations. The compute device IDs correspond to those returned by the SYCL API,
clinfo
, or
sycl-ls
 (with the numbering starting at 0). They have no relation to whether the device with that ID is of a certain type or supports a specific runtime. Using a programmatic special selector (like
gpu_selector
) to request a filtered out device will cause an exception to be thrown. Refer to the environment variable description in GitHub for details on use and example values: https://github.com/intel/llvm/blob/sycl/sycl/doc/EnvironmentVariables.md.
The
sycl-ls
 tool enumerates a list of devices available in the system. It is strongly recommended to run this tool before running any SYCL or DPC++ programs to make sure the system is configured properly. As a part of enumeration,
sycl-ls
 prints the
SYCL_DEVICE_FILTER
 string as a prefix of each device listing. The format of the
sycl-ls
 output is
[SYCL_DEVICE_FILTER] Platform_name, Device_name, Device_version [driver_version]
. In the following example, the string enclosed in the bracket ([ ]) at the beginning of each line is the
SYCL_DEVICE_FILTER
 string used to designate the specific device on which the program will run. 
$ sycl-ls
[opencl:acc:0] Intel(R) FPGA Emulation Platform for OpenCL(TM), Intel(R) FPGA Emulation Device 1.2 [2021.12.9.0.24_005321]
[opencl:gpu:1] Intel(R) OpenCL HD Graphics, Intel(R) UHD Graphics 630 [0x3e92] 3.0 [21.37.20939]
[opencl:cpu:2] Intel(R) OpenCL, Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz 3.0 [2021.12.9.0.24_005321]
[level_zero:gpu:0] Intel(R) Level-Zero, Intel(R) UHD Graphics 630 [0x3e92] 1.1 [1.2.20939]
[host:host:0] SYCL host platform, SYCL host device 1.2 [1.2]
Additional information about device selection is available from the DPC++ Language Guide and API Reference.

OpenMP* Device Query and Selection in the Host Code

OpenMP provided a set of APIs for programmers to query and set device for running code on the device. Host code can explicitly select and set a device num. For each offloading region, a programmer can also use a
device
 clause to specify the target device that is to be used for executing the offloading region.
  • int omp_get_num_procs (void)
     routine returns the number of processors available to the device
  • void omp_set_default_device(int device_num)
     routine controls the default target device
  • int omp_get_default_device(void)
     routine returns the default target device
  • int omp_get_num_devices(void)
     routine returns the number of non-host devices available for offloading code or data.
  • int omp_get_device_num(void)
     routine returns the device number of the device on which the calling thread is executing.
  • int omp_is_initial_device(int device_num)
     routine returns
    true
     if the current task is executing on the host device; otherwise, it returns
    false
    .
  • int omp_get_initial_device(void)
     routine returns a device number that represents the host device.
A programmer can use the environment variable
LIBOMPTARGET_DEVICETYPE = [ CPU | GPU ]
 to perform a device type selection. If a specific device type such as CPU or GPU is specified, then it is expected that the specified device type is available in the platform. If such a device is not available, then the runtime system throws an error that the requested device type is not available if the environment variable
OMP_TARGET_OFFLOAD=mandatory
, otherwise, the execution will have a fallback execution on its initial device. Additional information about device selection is available from the OpenMP 5.1 specification. Details about environment variables are available from GitHub: https://github.com/intel/llvm/blob/sycl/sycl/doc/EnvironmentVariables.md.

Product and Performance Information

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Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.