Accelerator Support#

Accelerators (e.g. GPUs) are critical for many machine learning applications. Ray Core natively supports many accelerators as pre-defined resource types and allows tasks and actors to specify their accelerator resource requirements.

The accelerators natively supported by Ray Core are:

Accelerator

Ray Resource Name

Support Level

Nvidia GPU

GPU

Fully tested, supported by the Ray team

Intel GPU

GPU

Experimental, supported by the community

AWS Neuron Core

neuron_cores

Experimental, supported by the community

Google TPU

TPU

Experimental, supported by the community

Intel Gaudi

HPU

Experimental, supported by the community

Huawei Ascend

NPU

Experimental, supported by the community

Starting Ray Nodes with Accelerators#

By default, Ray will set the quantity of accelerator resources of a node to the physical quantities of accelerators auto detected by Ray. If you need to, you can override this.

Tip

You can set CUDA_VISIBLE_DEVICES environment variable before starting a Ray node to limit the Nvidia GPUs that are visible to Ray. For example, CUDA_VISIBLE_DEVICES=1,3 ray start --head --num-gpus=2 will let Ray only see devices 1 and 3.

Tip

You can set ONEAPI_DEVICE_SELECTOR environment variable before starting a Ray node to limit the Intel GPUs that are visible to Ray. For example, ONEAPI_DEVICE_SELECTOR=1,3 ray start --head --num-gpus=2 will let Ray only see devices 1 and 3.

Tip

You can set NEURON_RT_VISIBLE_CORES environment variable before starting a Ray node to limit the AWS Neuro Cores that are visible to Ray. For example, NEURON_RT_VISIBLE_CORES=1,3 ray start --head --resources='{"neuron_cores": 2}' will let Ray only see devices 1 and 3.

Tip

You can set TPU_VISIBLE_CHIPS environment variable before starting a Ray node to limit the Google TPUs that are visible to Ray. For example, TPU_VISIBLE_CHIPS=1,3 ray start --head --resources='{"TPU": 2}' will let Ray only see devices 1 and 3.

Tip

You can set HABANA_VISIBLE_MODULES environment variable before starting a Ray node to limit the Intel Gaudi HPUs that are visible to Ray. For example, HABANA_VISIBLE_MODULES=1,3 ray start --head --resources='{"HPU": 2}' will let Ray only see devices 1 and 3.

Tip

You can set ASCEND_VISIBLE_DEVICES environment variable before starting a Ray node to limit the Huawei Ascend NPUs that are visible to Ray. For example, ASCEND_VISIBLE_DEVICES=1,3 ray start --head --resources='{"NPU": 2}' will let Ray only see devices 1 and 3.

Note

There is nothing preventing you from specifying a larger number of accelerator resources (e.g. num_gpus) than the true number of accelerators on the machine given Ray resources are logical. In this case, Ray will act as if the machine has the number of accelerators you specified for the purposes of scheduling tasks and actors that require accelerators. Trouble will only occur if those tasks and actors attempt to actually use accelerators that don’t exist.

Using accelerators in Tasks and Actors#

If a task or actor requires accelerators, you can specify the corresponding resource requirements (e.g. @ray.remote(num_gpus=1)). Ray will then schedule the task or actor to a node that has enough free accelerator resources and assign accelerators to the task or actor by setting the corresponding environment variable (e.g. CUDA_VISIBLE_DEVICES) before running the task or actor code.

import os
import ray

ray.init(num_gpus=2)

@ray.remote(num_gpus=1)
class GPUActor:
    def ping(self):
        print("GPU ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["GPU"]))
        print("CUDA_VISIBLE_DEVICES: {}".format(os.environ["CUDA_VISIBLE_DEVICES"]))

@ray.remote(num_gpus=1)
def gpu_task():
    print("GPU ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["GPU"]))
    print("CUDA_VISIBLE_DEVICES: {}".format(os.environ["CUDA_VISIBLE_DEVICES"]))

gpu_actor = GPUActor.remote()
ray.get(gpu_actor.ping.remote())
# The actor uses the first GPU so the task will use the second one.
ray.get(gpu_task.remote())
(GPUActor pid=52420) GPU ids: [0]
(GPUActor pid=52420) CUDA_VISIBLE_DEVICES: 0
(gpu_task pid=51830) GPU ids: [1]
(gpu_task pid=51830) CUDA_VISIBLE_DEVICES: 1
import os
import ray

ray.init(num_gpus=2)

@ray.remote(num_gpus=1)
class GPUActor:
    def ping(self):
        print("GPU ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["GPU"]))
        print("ONEAPI_DEVICE_SELECTOR: {}".format(os.environ["ONEAPI_DEVICE_SELECTOR"]))

@ray.remote(num_gpus=1)
def gpu_task():
    print("GPU ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["GPU"]))
    print("ONEAPI_DEVICE_SELECTOR: {}".format(os.environ["ONEAPI_DEVICE_SELECTOR"]))

gpu_actor = GPUActor.remote()
ray.get(gpu_actor.ping.remote())
# The actor uses the first GPU so the task will use the second one.
ray.get(gpu_task.remote())
(GPUActor pid=52420) GPU ids: [0]
(GPUActor pid=52420) ONEAPI_DEVICE_SELECTOR: 0
(gpu_task pid=51830) GPU ids: [1]
(gpu_task pid=51830) ONEAPI_DEVICE_SELECTOR: 1
import os
import ray

ray.init(resources={"neuron_cores": 2})

@ray.remote(resources={"neuron_cores": 1})
class NeuronCoreActor:
    def ping(self):
        print("Neuron Core ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["neuron_cores"]))
        print("NEURON_RT_VISIBLE_CORES: {}".format(os.environ["NEURON_RT_VISIBLE_CORES"]))

@ray.remote(resources={"neuron_cores": 1})
def neuron_core_task():
    print("Neuron Core ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["neuron_cores"]))
    print("NEURON_RT_VISIBLE_CORES: {}".format(os.environ["NEURON_RT_VISIBLE_CORES"]))

neuron_core_actor = NeuronCoreActor.remote()
ray.get(neuron_core_actor.ping.remote())
# The actor uses the first Neuron Core so the task will use the second one.
ray.get(neuron_core_task.remote())
(NeuronCoreActor pid=52420) Neuron Core ids: [0]
(NeuronCoreActor pid=52420) NEURON_RT_VISIBLE_CORES: 0
(neuron_core_task pid=51830) Neuron Core ids: [1]
(neuron_core_task pid=51830) NEURON_RT_VISIBLE_CORES: 1
import os
import ray

ray.init(resources={"TPU": 2})

@ray.remote(resources={"TPU": 1})
class TPUActor:
    def ping(self):
        print("TPU ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["TPU"]))
        print("TPU_VISIBLE_CHIPS: {}".format(os.environ["TPU_VISIBLE_CHIPS"]))

@ray.remote(resources={"TPU": 1})
def tpu_task():
    print("TPU ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["TPU"]))
    print("TPU_VISIBLE_CHIPS: {}".format(os.environ["TPU_VISIBLE_CHIPS"]))

tpu_actor = TPUActor.remote()
ray.get(tpu_actor.ping.remote())
# The actor uses the first TPU so the task will use the second one.
ray.get(tpu_task.remote())
(TPUActor pid=52420) TPU ids: [0]
(TPUActor pid=52420) TPU_VISIBLE_CHIPS: 0
(tpu_task pid=51830) TPU ids: [1]
(tpu_task pid=51830) TPU_VISIBLE_CHIPS: 1
import os
import ray

ray.init(resources={"HPU": 2})

@ray.remote(resources={"HPU": 1})
class HPUActor:
    def ping(self):
        print("HPU ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["HPU"]))
        print("HABANA_VISIBLE_MODULES: {}".format(os.environ["HABANA_VISIBLE_MODULES"]))

@ray.remote(resources={"HPU": 1})
def hpu_task():
    print("HPU ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["HPU"]))
    print("HABANA_VISIBLE_MODULES: {}".format(os.environ["HABANA_VISIBLE_MODULES"]))

hpu_actor = HPUActor.remote()
ray.get(hpu_actor.ping.remote())
# The actor uses the first HPU so the task will use the second one.
ray.get(hpu_task.remote())
(HPUActor pid=52420) HPU ids: [0]
(HPUActor pid=52420) HABANA_VISIBLE_MODULES: 0
(hpu_task pid=51830) HPU ids: [1]
(hpu_task pid=51830) HABANA_VISIBLE_MODULES: 1
import os
import ray

ray.init(resources={"NPU": 2})

@ray.remote(resources={"NPU": 1})
class NPUActor:
    def ping(self):
        print("NPU ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["NPU"]))
        print("ASCEND_VISIBLE_DEVICES: {}".format(os.environ["ASCEND_VISIBLE_DEVICES"]))

@ray.remote(resources={"NPU": 1})
def npu_task():
    print("NPU ids: {}".format(ray.get_runtime_context().get_accelerator_ids()["NPU"]))
    print("ASCEND_VISIBLE_DEVICES: {}".format(os.environ["ASCEND_VISIBLE_DEVICES"]))

npu_actor = NPUActor.remote()
ray.get(npu_actor.ping.remote())
# The actor uses the first NPU so the task will use the second one.
ray.get(npu_task.remote())
(NPUActor pid=52420) NPU ids: [0]
(NPUActor pid=52420) ASCEND_VISIBLE_DEVICES: 0
(npu_task pid=51830) NPU ids: [1]
(npu_task pid=51830) ASCEND_VISIBLE_DEVICES: 1

Inside a task or actor, ray.get_runtime_context().get_accelerator_ids() will return a list of accelerator IDs that are available to the task or actor. Typically, it is not necessary to call get_accelerator_ids() because Ray will automatically set the corresponding environment variable (e.g. CUDA_VISIBLE_DEVICES), which most ML frameworks will respect for purposes of accelerator assignment.

Note: The remote function or actor defined above doesn’t actually use any accelerators. Ray will schedule it on a node which has at least one accelerator, and will reserve one accelerator for it while it is being executed, however it is up to the function to actually make use of the accelerator. This is typically done through an external library like TensorFlow. Here is an example that actually uses accelerators. In order for this example to work, you will need to install the GPU version of TensorFlow.

@ray.remote(num_gpus=1)
def gpu_task():
    import tensorflow as tf

    # Create a TensorFlow session. TensorFlow will restrict itself to use the
    # GPUs specified by the CUDA_VISIBLE_DEVICES environment variable.
    tf.Session()

Note: It is certainly possible for the person to ignore assigned accelerators and to use all of the accelerators on the machine. Ray does not prevent this from happening, and this can lead to too many tasks or actors using the same accelerator at the same time. However, Ray does automatically set the environment variable (e.g. CUDA_VISIBLE_DEVICES), which will restrict the accelerators used by most deep learning frameworks assuming it’s not overridden by the user.

Fractional Accelerators#

Ray supports fractional resource requirements so multiple tasks and actors can share the same accelerator.

ray.init(num_cpus=4, num_gpus=1)

@ray.remote(num_gpus=0.25)
def f():
    import time

    time.sleep(1)

# The four tasks created here can execute concurrently
# and share the same GPU.
ray.get([f.remote() for _ in range(4)])
ray.init(num_cpus=4, num_gpus=1)

@ray.remote(num_gpus=0.25)
def f():
    import time

    time.sleep(1)

# The four tasks created here can execute concurrently
# and share the same GPU.
ray.get([f.remote() for _ in range(4)])

AWS Neuron Core doesn’t support fractional resource.

Google TPU doesn’t support fractional resource.

Intel Gaudi doesn’t support fractional resource.

ray.init(num_cpus=4, resources={"NPU": 1})

@ray.remote(resources={"NPU": 0.25})
def f():
    import time

    time.sleep(1)

# The four tasks created here can execute concurrently
# and share the same NPU.
ray.get([f.remote() for _ in range(4)])

Note: It is the user’s responsibility to make sure that the individual tasks don’t use more than their share of the accelerator memory. Pytorch and TensorFlow can be configured to limit its memory usage.

When Ray assigns accelerators of a node to tasks or actors with fractional resource requirements, it will pack one accelerator before moving on to the next one to avoid fragmentation.

ray.init(num_gpus=3)

@ray.remote(num_gpus=0.5)
class FractionalGPUActor:
    def ping(self):
        print("GPU id: {}".format(ray.get_runtime_context().get_accelerator_ids()["GPU"]))

fractional_gpu_actors = [FractionalGPUActor.remote() for _ in range(3)]
# Ray will try to pack GPUs if possible.
[ray.get(fractional_gpu_actors[i].ping.remote()) for i in range(3)]
(FractionalGPUActor pid=57417) GPU id: [0]
(FractionalGPUActor pid=57416) GPU id: [0]
(FractionalGPUActor pid=57418) GPU id: [1]

Workers not Releasing GPU Resources#

Currently, when a worker executes a task that uses a GPU (e.g., through TensorFlow), the task may allocate memory on the GPU and may not release it when the task finishes executing. This can lead to problems the next time a task tries to use the same GPU. To address the problem, Ray disables the worker process reuse between GPU tasks by default, where the GPU resources is released after the task process exits. Since this adds overhead to GPU task scheduling, you can re-enable worker reuse by setting max_calls=0 in the ray.remote decorator.

# By default, ray will not reuse workers for GPU tasks to prevent
# GPU resource leakage.
@ray.remote(num_gpus=1)
def leak_gpus():
    import tensorflow as tf

    # This task will allocate memory on the GPU and then never release it.
    tf.Session()

Accelerator Types#

Ray supports resource specific accelerator types. The accelerator_type option can be used to force to a task or actor to run on a node with a specific type of accelerator. Under the hood, the accelerator type option is implemented as a custom resource requirement of "accelerator_type:<type>": 0.001. This forces the task or actor to be placed on a node with that particular accelerator type available. This also lets the multi-node-type autoscaler know that there is demand for that type of resource, potentially triggering the launch of new nodes providing that accelerator.

from ray.util.accelerators import NVIDIA_TESLA_V100

@ray.remote(num_gpus=1, accelerator_type=NVIDIA_TESLA_V100)
def train(data):
    return "This function was run on a node with a Tesla V100 GPU"

ray.get(train.remote(1))

See ray.util.accelerators for available accelerator types.