Scaling and Resource Allocation#

This guide helps you to:

  • scale your deployments horizontally by specifying a number of replicas

  • scale up and down automatically to react to changing traffic

  • allocate hardware resources (CPUs, GPUs, etc) for each deployment

Scaling horizontally with num_replicas#

Each deployment consists of one or more replicas. The number of replicas is specified by the num_replicas field in the deployment options. By default, num_replicas is 1.

# Create with a single replica.
def func(*args):

# Scale up to 3 replicas.

# Scale back down to 1 replica.


Serve also supports a demand-based replica autoscaler. It adjusts to traffic spikes by observing queue sizes and making scaling decisions to add or remove replicas. To configure it, you can set the autoscaling_config field in deployment options.

        "min_replicas": 1,
        "initial_replicas": 2,
        "max_replicas": 5,
        "target_num_ongoing_requests_per_replica": 10,
def func(_):
    return ""
)  # The func deployment will now autoscale based on requests demand.

The min_replicas and max_replicas fields configure the range of replicas which the Serve autoscaler chooses from. Deployments will start with initial_replicas. initial_replicas is optional; it’s set to min_replicas by default.

The target_num_ongoing_requests_per_replica configuration specifies how aggressively the autoscaler should react to traffic. Serve will try to make sure that each replica has roughly that number of requests being processed and waiting in the queue. For example, if your processing time is 10ms and the latency constraint is 100ms, you can have at most 10 requests ongoing per replica so the last requests can finish within the latency constraint. We recommend you benchmark your application code and set this number based on end to end latency objective.


The Ray Serve Autoscaler is an application-level autoscaler that sits on top of the Ray Autoscaler. Concretely, this means that the Ray Serve autoscaler asks Ray to start a number of replica actors based on the request demand. If the Ray Autoscaler determines there aren’t enough available CPUs to place these actors, it responds by requesting more Ray nodes. The underlying cloud provider will then respond by adding more nodes. Similarly, when Ray Serve scales down and terminates some replica actors, it will try to do so in a way that results in the most nodes having no Ray actors or tasks running on them, at which point the Ray autoscaler will remove those nodes.

To learn about the architecture underlying Ray Serve Autoscaling, see Ray Serve Autoscaling.

autoscaling_config parameters#

There are several user-specified parameters the autoscaling algorithm takes into consideration when deciding the target number of replicas for your deployment:

min_replicas[default_value=1]: The minimum number of replicas for the deployment. min_replicas will also be the initial number of replicas when the deployment is deployed.


Ray Serve Autoscaling allows the min_replicas to be 0 when starting your deployment; the scale up will be started when you start sending traffic. There will be a cold start time as the Ray ServeHandle waits (blocks) for available replicas to assign the request.

max_replicas[default_value=1]: The maximum number of replicas for the deployment. Ray Serve Autoscaling will rely on the Ray Autoscaler to scale up more nodes when the currently available cluster resources (CPUs, GPUs, etc.) are not enough to support more replicas.

target_num_ongoing_requests_per_replica[default_value=1]: How many ongoing requests are expected to run concurrently per replica. The autoscaler scales up if the value is lower than the current number of ongoing requests per replica. Similarly, the autoscaler scales down if it’s higher than the current number of ongoing requests. Scaling happens quicker if there’s a high disparity between this value and the current number of ongoing requests.


  • It is always recommended to load test your workloads. For example, if the use case is latency sensitive, you can lower the target_num_ongoing_requests_per_replica number to maintain high performance.

  • Internally, the autoscaler will decide to scale up or down by comparing target_num_ongoing_requests_per_replica to the number of RUNNING and PENDING tasks on each replica.

  • target_num_ongoing_requests_per_replica is only a target value used for autoscaling (not a hard limit), the real ongoing requests number can be higher than the config.

downscale_delay_s[default_value=600.0]: How long the cluster needs to wait before scaling down replicas.

upscale_delay_s[default_value=30.0]: How long the cluster needs to wait before scaling up replicas.


downscale_delay_s and upscale_delay_s control the frequency of doing autoscaling work. For example, if your application takes a long time to do initialization work, you can increase downscale_delay_s to make the downscaling happen slowly.

smoothing_factor[default_value=1.0]: The multiplicative factor to speed up or slow down each autoscaling step. For example, when the application has high traffic volume in short period of time, you can increase smoothing_factor to scale up the resource quickly. You can think of this as a “gain” factor to amplify the response of the autoscaling algorithm.

metrics_interval_s[default_value=10]: This controls how often each replica sends metrics to the autoscaler. (Normally you don’t need to change this config.)

Resource Management (CPUs, GPUs)#

You may want to specify a deployment’s resource requirements to reserve cluster resources like GPUs. To assign hardware resources per replica, you can pass resource requirements to ray_actor_options. By default, each replica reserves one CPU. To learn about options to pass in, take a look at the Resources with Actors guide.

For example, to create a deployment where each replica uses a single GPU, you can do the following:

@serve.deployment(ray_actor_options={"num_gpus": 1})
def func(*args):
    return do_something_with_my_gpu()

Fractional CPUs and Fractional GPUs#

Suppose you have two models and each doesn’t fully saturate a GPU. You might want to have them share a GPU by allocating 0.5 GPUs each.

To do this, the resources specified in ray_actor_options can be fractional. For example, if you have two models and each doesn’t fully saturate a GPU, you might want to have them share a GPU by allocating 0.5 GPUs each.

@serve.deployment(ray_actor_options={"num_gpus": 0.5})
def func_1(*args):
    return do_something_with_my_gpu()

@serve.deployment(ray_actor_options={"num_gpus": 0.5})
def func_2(*args):
    return do_something_with_my_gpu()

In this example, each replica of each deployment will be allocated 0.5 GPUs. The same can be done to multiplex over CPUs, using "num_cpus".

Custom Resources, Accelerator types, and more#

You can also specify custom resources in ray_actor_options, for example to ensure that a deployment is scheduled on a specific node. For example, if you have a deployment that requires 2 units of the "custom_resource" resource, you can specify it like this:

@serve.deployment(ray_actor_options={"resources": {"custom_resource": 2}})
def func(*args):
    return do_something_with_my_custom_resource()

You can also specify accelerator types via the accelerator_type parameter in ray_actor_options.

Below is the full list of supported options in ray_actor_options; please see the relevant Ray Core documentation for more details about each option:

  • accelerator_type

  • memory

  • num_cpus

  • num_gpus

  • object_store_memory

  • resources

  • runtime_env

Configuring Parallelism with OMP_NUM_THREADS#

Deep learning models like PyTorch and Tensorflow often use multithreading when performing inference. The number of CPUs they use is controlled by the OMP_NUM_THREADS environment variable. Ray sets OMP_NUM_THREADS=<num_cpus> by default. To avoid contention, Ray sets OMP_NUM_THREADS=1 if num_cpus is not specified on the tasks/actors, to reduce contention between actors/tasks which run in a single thread. If you do want to enable this parallelism in your Serve deployment, just set num_cpus (recommended) to the desired value, or manually set the OMP_NUM_THREADS environment variable when starting Ray or in your function/class definition.

OMP_NUM_THREADS=12 ray start --head
OMP_NUM_THREADS=12 ray start --address=$HEAD_NODE_ADDRESS
class MyDeployment:
    def __init__(self, parallelism: str):
        os.environ["OMP_NUM_THREADS"] = parallelism
        # Download model weights, initialize model, etc.

    def __call__(self):


Some other libraries may not respect OMP_NUM_THREADS and have their own way to configure parallelism. For example, if you’re using OpenCV, you’ll need to manually set the number of threads using cv2.setNumThreads(num_threads) (set to 0 to disable multi-threading). You can check the configuration using cv2.getNumThreads() and cv2.getNumberOfCPUs().