Intro to dpctl.tensor

Supported array data types

The tensor submodule provides an N-dimensional array object for a tensor whose values have the same data type from the following list:















Creating an array

Array creation functions support keyword arguments that control the device where the array is allocated as well as aspects of Unified Shared Memory allocation for the array.

These three keywords are:

Keyword arguments

Default value




type of USM allocation to make



dpctl.tensor.Device instance



Instance of dpctl.SyclQueue associated with array

Arguments sycl_queue and device are complementary to each other, and a user need only provide one of these.

A valid setting for the device keyword argument is any object that can be passed to dpctl.tensor.Device.create_device(). If both device and sycl_queue keyword arguments are specified, they must correspond to dpctl.SyclQueue instances which compare equal to one another.

A created instance of usm_ndarray has an associated dpctl.SyclQueue instance that can be retrieved using dpctl.tensor.usm_ndarray.sycl_queue property. The underlying USM allocation is allocated on dpctl.SyclDevice and is bound to dpctl.SyclContext targeted by this queue.

Execution model

When one of more instances of usm_ndarray objects are passed to a function in dpctl.tensor other than creation function, a “compute follows data” execution model is followed.

The model requires that dpctl.SyclQueue instances associated with each array compared equal to one another, signifying that each one corresponds to the same underlying sycl::queue object. In such a case, the output array is associated with the same sycl::queue and computations are scheduled for execution using this sycl::queue.


Two instances dpctl.SyclQueue may target the same sycl::device and be using the same sycl::context, but correspond to different scheduling entries, and hence be in violation of the compute-follows-data requirement. One common example of this are SyclQueue corresponding to default-selected device and using platform default context but created using different properties, e.g. one with “enable_profiling” set and another without it.

If input arrays do not conform to the compute-follows-data requirements, dpctl.utils.ExecutionPlacementError is raised. User must explicitly migrate the data to unambiguously control the execution placement.

Migrating arrays

Array content can be migrated to a different device using either dpctl.tensor.usm_ndarray.to_device() method, or by using dpctl.tensor.asarray() function.

The arr.to_device(device=target_device) method will be zero-copy if the arr.sycl_queue and the dpctl.SyclQueue instance associated with new target device have the same underlying sycl::device and sycl::context instances.

Here is an example of migration without a copy using .to_device method:

Example: Use .to_device to zero-copy migrate array content to be associated with a different sycl::queue
import dpctl
from dpctl import tensor

x = tensor.linspace(0, 1, num=10**8)
q_prof = dpctl.SyclQueue(x.sycl_context, x.sycl_device, property="enable_profiling")

timer = dpctl.SyclTimer()
# no data migration takes place here (zero-copy),
# but x and x1 arrays do not satify compute-follows-data requirements
x1 = x.to_device(q_prof)

with timer(q_prof):
    y1 = tensor.sin(2*x1)*tensor.exp(-tensor.square(x1))

# also a zero copy operation
y = y1.to_device(x.device)

host_dt, device_dt = timer.dt
print(f"Execution on device {} took {device_dt} seconds")
print(f"Execution on host took {host_dt} seconds")

Data migration when the current and the target SYCL contexts are different is performed via host. That means that data are copied from the current device to the host, and then from the host to the target device:

Example: Using .to_device to migrate data may involve copy via host
from dpctl import tensor

x_cpu = tensor.concat((tensor.ones(10, device="cpu"), tensor.zeros(1000, device="cpu")))

# data migration is performed via host
x_gpu = x_cpu.to_device("gpu")

An alternative way to migrate data is to use asarray() and specify device-placement keyword arguments:

Example: Using asarray to migrate data may involve copy via host
from dpctl import tensor

x_cpu = tensor.concat((tensor.ones(10, device="cpu"), tensor.zeros(1000, device="cpu")))

# data migration is performed via host
x_gpu = tensor.asarray(x_cpu, device="cpu")

An advantage of using the function asarray is that migration from usm_ndarray instances allocated on different devices as well migration from numpy.ndarray may be accomplished in a single call:

Example: asarray may migrate multiple arrays
from dpctl import tensor
import numpy

x_cpu = tensor.ones((10, 10), device="cpu")
x_gpu = tensor.zeros((10, 10), device="opencl:gpu")
x_np = numpy.random.randn(10, 10)

# Array w has shape (3, 10, 10)
w = tensor.asarray([x_cpu, x_gpu, x_np], device="level_zero:gpu")

Migration may also occur during calls to other array creation functions, e.g., full() when the fill_value parameter is an instance of usm_ndarray. In such a case default values of device placement keywords are interpreted to avoid data migration, i.e., the new array is created on the same device where fill_value array was allocated.

Example: Using usm_ndarray as arguments to array construction _dpctl_tensor_utility_functions
from dpctl import tensor

# Zero-dimensional array allocated on CPU device
pi_on_device = tensor.asarray(tensor.pi, dtype=tensor.float32, device="cpu")

# x will also be allocated on CPU device
x = tensor.full(shape=(100, 100), fill_value=pi_on_device)

# Create array on GPU. Migration of `pi_on_device` to GPU via host
# takes place under the hood
y_gpu = tensor.full(shape=(100, 100), fill_value=pi_on_device, device="gpu")

Combining arrays with different USM types

For functions with single argument the returned array has the same usm_type as the input array.

Functions that combine several usm_ndarray instances the usm_type of the output array is determined using the following coercion rule:
















If assigning USM-type “device” a score of 0, USM-type “shared” a score of 1, and USM-type “host” a score of 2, the USM-type of the output array has the smallest score of all its inputs.

The convenience function get_coerced_usm_type() is a convenience function to determine the USM-type following this convention:

from dpctl.utils import get_coerced_usm_type

# r1 has value "device"
r1 = get_coerced_usm_type(["device", "shared", "host"])

# r2 has value "shared"
r2 = get_coerced_usm_type(["shared", "shared", "host"])

# r3 has value "host"
r3 = get_coerced_usm_type(["host", "host", "host"])

Sharing data between devices and Python

Python objects, such as sequences of int, float, or complex objects, or NumPy arrays can be converted to dpctl.tensor.usm_ndarray using dpctl.tensor.asarray() function.

>>> from dpctl import tensor as dpt
>>> import numpy as np
>>> import mkl_random

>>> # Sample from true random number generator
>>> rs = mkl_random.RandomState(brng="nondeterm")
>>> x_np = rs.uniform(-1, 1, size=(6, 512)).astype(np.float32)

>>> # copy data to USM-device (default) allocated array
>>> x_usm = dpt.asarray(x_np)
>>> dpt.max(x_usm, axis=1)
usm_ndarray([0.9998379 , 0.9963589 , 0.99818915, 0.9975991 , 0.9999802 ,
            0.99851537], dtype=float32)
>>> np.max(x_np, axis=1)
array([0.9998379 , 0.9963589 , 0.99818915, 0.9975991 , 0.9999802 ,
      0.99851537], dtype=float32)

The content of dpctl.tensor.usm_ndarray may be copied into a NumPy array using dpctl.tensor.asnumpy() function:

from dpctl import tensor as dpt
import numpy as np

def sieve_pass(r : dpt.usm_ndarray, v : dpt.usm_ndarray) -> dpt.usm_ndarray:
    "Single pass of sieve of Eratosthenes"
    m = dpt.min(r[r > v])
    r[ (r > m) & (r % m == 0) ] = 0
    return m

def sieve(n : int) -> dpt.usm_ndarray:
    "Find primes <=n using sieve of Erathosthenes"
    idt = dpt.int32
    s = dpt.concat((
      dpt.arange(2, 3, dtype=idt),
      dpt.arange(3, n + 1, 2, dtype=idt)
    lb = dpt.zeros(tuple(), dtype=idt)
    while lb * lb < n + 1:
        lb = sieve_pass(s, lb)
    return s[s > 0]

# get prime numbers <= a million into NumPy array
# to save to disk
ps_np = dpt.asnumpy(sieve(10**6))

np.savetxt("primes.txt", ps_np, fmt="%d")