Use cached default device in tensor

Profiling of an example of GPU Max revealed that repeated calls to
dpctl.select_default_device() took 200x longer than actual computatations.

Introduced dpctl._sycl_device_factory._cached_default_device() and deployed
it in tensor (in _usmarray and in _device).

Author: oleksandr-pavlyk

dpctl/_sycl_device_factory.pxd

@@ -36,3 +36,4 @@ cpdef int get_num_devices(backend=*, device_type=*)
 cpdef cpp_bool has_gpu_devices()
 cpdef cpp_bool has_cpu_devices()
 cpdef cpp_bool has_accelerator_devices()
+cpdef SyclDevice _cached_default_device()

dpctl/_sycl_device_factory.pyx

@@ -45,6 +45,8 @@ from ._backend cimport (  # noqa: E211
     _device_type,
 )
 
+from contextvars import ContextVar
+
 from ._sycl_device import SyclDeviceCreationError
 from .enum_types import backend_type
 from .enum_types import device_type as device_type_t
@@ -59,6 +61,7 @@ __all__ = [
     "has_cpu_devices",
     "has_gpu_devices",
     "has_accelerator_devices",
+    "_cached_default_device",
 ]
 
 
@@ -355,3 +358,48 @@ cpdef SyclDevice select_gpu_device():
         raise SyclDeviceCreationError("Device unavailable.")
     Device = SyclDevice._create(DRef)
     return Device
+
+
+cdef class _DefaultDeviceCache:
+    cdef dict __device_map__
+
+    def __cinit__(self):
+        self.__device_map__ = dict()
+
+    cdef get_or_create(self):
+        """Return instance of SyclDevice and indicator if cache
+        has been modified"""
+        key = 0
+        if key in self.__device_map__:
+            return self.__device_map__[key], False
+        dev = select_default_device()
+        self.__device_map__[key] = dev
+        return dev, True
+
+    cdef _update_map(self, dev_map):
+        self.__device_map__.update(dev_map)
+
+    def __copy__(self):
+        cdef _DefaultDeviceCache _copy = _DefaultDeviceCache.__new__(
+            _DefaultDeviceCache)
+        _copy._update_map(self.__device_map__)
+        return _copy
+
+
+_global_default_device_cache = ContextVar(
+    'global_default_device_cache',
+    default=_DefaultDeviceCache()
+)
+
+
+cpdef SyclDevice _cached_default_device():
+    """Returns a cached devide selected by default selector.
+
+    Returns:
+        :class:`dpctl.SyclDevice`: A cached default-selected SYCL device.
+
+    """
+    cdef _DefaultDeviceCache _cache = _global_default_device_cache.get()
+    d_, changed_ = _cache.get_or_create()
+    if changed_: _global_default_device_cache.set(_cache)
+    return d_

dpctl/tensor/_device.py

@@ -14,6 +14,7 @@
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
 import dpctl
+from dpctl._sycl_device_factory import _cached_default_device
 from dpctl._sycl_queue_manager import get_device_cached_queue
 
 __doc__ = "Implementation of array API mandated Device class"
@@ -73,7 +74,7 @@ def create_device(cls, device=None):
                 )
         else:
             if dev is None:
-                _dev = dpctl.SyclDevice()
+                _dev = _cached_default_device()
             else:
                 _dev = dpctl.SyclDevice(dev)
             obj.sycl_queue_ = get_device_cached_queue(_dev)

dpctl/tensor/_usmarray.pyx

@@ -36,6 +36,8 @@ cimport dpctl as c_dpctl
 cimport dpctl.memory as c_dpmem
 cimport dpctl.tensor._dlpack as c_dlpack
 
+from .._sycl_device_factory cimport _cached_default_device
+
 import dpctl.tensor._flags as _flags
 from dpctl.tensor._tensor_impl import default_device_fp_type
 
@@ -208,7 +210,7 @@ cdef class usm_ndarray:
             if q is not None:
                 dtype = default_device_fp_type(q)
             else:
-                dev = dpctl.select_default_device()
+                dev = _cached_default_device()
                 dtype = "f8" if dev.has_aspect_fp64 else "f4"
         typenum = dtype_to_typenum(dtype)
         if (typenum < 0):