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Cost-Effective Private Large Language Model Inference on Azure Kubernetes Service

Yingting Huang

• 8 min read
Cost-Effective Private Large Language Model Inference on Azure Kubernetes Service

Introduction

This article provides a method for private and cost-optimized deployment of large language models on the Azure cloud. It assumes the reader has foundational knowledge of large language models and Kubernetes.

It utilizes AKS spot instances, quantization techniques, and batching inference to reduce compute costs compared to traditional deployment approaches.

A proof-of-concept deployment demonstrates using cluster autoscaler, GPU node pools, pod affinity and tolerations on Azure Kubernetes Service to enable fast and resilient large language model inference leveraging low-cost spot instance GPU nodes.

Reasons for running large language models privately

There are several motivations for organizations to run large language models privately:

  • Customization - Private models can be customized for specific domains by providing proprietary training data. This improves accuracy for niche tasks like internal search or customer support..
  • Confidentiality - Keeping data internal allows tighter control over sensitive information like personal data, intellectual property, or competitive intelligence.
  • Control - Full control over the training data, hyperparameters, model architecture, etc. This allows you to ensure the model behaves as intended.
  • Low latency - A privately hosted model can provide very low latency responses since you don't have to make API calls over the internet to external servers.
  • Compliance - For regulated industries like healthcare and finance, running a private model may be important for compliance with data protection and privacy regulations.

Challenges of running inference cost-effectively and efficiently

Deploying large language models for inference is compute-intensive, requiring high-end GPUs and specialized hardware that can be very expensive to provision on cloud platforms. For example, an 80GB Nvidia A100 GPU on Azure can generate approximately 60 completion tokens per second when running a 13 billion parameter model like Vicuna. If we calculate the monthly cost of running just a single A100 virtual machine in Azure's East US region, it comes out to $2,680. This one GPU could generate around 157 million tokens per month.

In contrast, conversational services like ChatGPT offered through Azure OpenAI are much more cost-effective. At Azure OpenAI ChatGPT's rate of $0.0015 per token, those 157 million tokens would only cost $236.52. Additionally, ChatGPT provides a smoother conversational experience compared to a basic 13B parameter model.

Beyond raw compute costs, the development, infrastructure, and maintenance required to build and deploy production-grade private language models at scale poses further operational challenges.

Proof-of-concept architecture

This proof-of-concept leverages Azure Kubernetes Service (AKS), quantization, and batch inference to enable a cost-optimized private deployment of large language models while maintaining scalability and fault tolerance.
Key techniques:

  • AKS enables the use of spot instances and GPUs through configurable node pools. Spot instances significantly reduce compute costs compared to on-demand instance pricing. To handle potential node evictions, AKS cluster autoscaler component monitors pod resource requirements and calculates the capacity needed for scheduled GPU workloads. It then automatically adjusts node counts to match workload demands. Considering of a 5% eviction rate, a 2 replicas GPU workalod (each replica is in a seperated node) would have at least 99.75% chance alive to serve the request. This allows the cluster to leverage cost-savings from spot instances while maintaining available capacity.
  • Quantization is a technique to decrease model size and compute requirements for large language models (LLMs). It works by converting the high-precision floating point values used to represent weights and activations into lower-precision fixed-point representations that require less memory. This weight sharing through lower numeric precision allows for substantial reductions in model size and faster inference times. Some well-known quantization methods for LLMs include GGML and GPT-Q.
  • Batching groups multiple inference requests into a single batch call instead of handling them individually. This improves utilization for sporadic workloads. Batching is particularly beneficial for compute-intensive LLMs that can take several seconds per request. vllm and text-generation-inference are two common batch inference frameworks for LLMs. In the next section, we will leverage vllm to demonstrate the advantages of batching in our proof-of-concept deployment.

Architecture Design

By integrating these optimization methods, significant speedup can be achieved without sacrificing model accuracy. This results in faster inference latency and higher throughput for LLMs, enabling cost-effective and scalable private deployment.

Proof-of-concept deployment and considerations

Before starting, ensure you have the following prerequisites:

NOTE: For production use, it is recommended to build your own custom image. This provides full control over dependencies and compatibility between the GPU accelerated application and GPU driver on the nodes. Refer to the CUDA Compatibility and Upgrades.

Define environment variables

We first need to define the following environment variables to configure the GPU spot node pool. The values for these variables can be obtained from the Azure portal or using the Azure CLI:

# AKS cluster info
export RESOURCE_GROUP_NAME=your_aks_resource_group_name
export CLUSTER_NAME=your_aks_cluster_name
export REGION=your_aks_cluster_region
# GPU VM size, use Standard_NC4as_T4_v3 for PoC purpose
export VM_SIZE=Standard_NC4as_T4_v3

Add a GPU spot node pool

Now we can add a spot node pool for GPU nodes into existing AKS cluster. The command in below will create a new node pool named gpunp with 1 node of Standard_NC4as_T4_v3 size. The node pool will be configured with GPU taints and cluster autoscaler. The GPU taints will ensure that only GPU workloads are scheduled on the node pool. The cluster autoscaler will automatically adjust the number of nodes in the node pool based on the workload demands. The node pool will also be configured with spot instances to reduce compute costs. The spot-max-price is set to -1 to ensure that the node pool will not be evicted due to price changes. The min-count and max-count are set to 1 to ensure that the node pool will always have at least 1 node available for scheduling.

az aks nodepool add \
    --resource-group $RESOURCE_GROUP_NAME \
    --cluster-name $CLUSTER_NAME \
    --name gpunp \
    --node-count 1 \
    --node-vm-size $VM_SIZE \
    --node-taints sku=gpu:NoSchedule \
    --aks-custom-headers UseGPUDedicatedVHD=true \
    --enable-cluster-autoscaler \
    --priority Spot \
    --eviction-policy Delete \
    --spot-max-price -1 \
    --min-count 1 \
    --max-count 2

Deploy LLM inference service

With the AKS GPU node pool provisioned, we can now deploy the LLM inference service. Use vllm for example, the deployment manifests are located here. There are two manifest options, each using a different storage backend for storing the model files:

The vllm-azure-disk.yaml manifest deploys a StatefulSet with 2 replicas and a Service for the LLM inference service. The StatefulSet is configured with pod anti-affinity to ensure the replicas are scheduled on different nodes. It also has tolerations to schedule the replicas on the GPU spot node pool. The Service exposes the LLM inference service on the AKS cluster using a ClusterIP. Each StatefulSet replica mounts a 16GB Azure Disk PersistentVolumeClaim for storing the model files.

The vllm-azure-files.yaml manifest deploys a Deployment with 2 replicas and a Service for the LLM inference service. The Deployment has pod anti-affinity to schedule replicas on different nodes. It also has tolerations to schedule replicas on the GPU spot node pool. The Service exposes the inference service on the AKS cluster using a ClusterIP. Each replica mounts a 16GB Azure File share PersistentVolumeClaim for storing model files, with the PV shared between replicas.

AKS supports shared disks using block volumes. However, multi-node read/write is not supported on common filesystems like ext4 or xfs - only cluster filesystems allow this. Therefore, shared disks cannot be used for storing model files that need to be accessed from multiple nodes.

AKS taints GPU spot nodes using nvidia.com/gpu:NoSchedule, sku=gpu:NoSchedule and kubernetes.azure.com/scalesetpriority=spot:NoSchedule. To schedule GPU workloads on these nodes, pods need tolerations added to their spec. Creating a namespace with following tolerations ensures all pods within it can be placed on the tainted GPU spot nodes.

apiVersion: v1
kind: Namespace
metadata:
  name: llm
  annotations:
    scheduler.alpha.kubernetes.io/defaultTolerations: '[{"Key": "kubernetes.azure.com/scalesetpriority", "Operator": "Equal", "Value": "spot", "Effect": "NoSchedule"}, {"Key": "sku", "Operator": "Equal", "Value": "gpu", "Effect": "NoSchedule"}]'

With the default tolerations set on the llm namespace, any pods created within it will automatically have the required tolerations to schedule on GPU spot instance nodes. This gives a straightforward way to deploy GPU workloads leveraging cost-efficient spot instances.

To begin the deployment by using StatefulSet, execute this command:

kubeclt apply -f vllm/manifests/vllm-azure-disk.yaml

To begin the deployment by using Deployment, execute this command:

kubeclt apply -f vllm/manifests/vllm-azure-files.yaml

NOTE: The manifests are configured to use the image ghcr.io/huangyingting/llm-inference-vllm:main, you can change it to your own image.

AKS cluster autoscaler

We deployed two replicas but initially there was only 1 GPU node. The cluster autoscaler detected the insufficient resources and scaled the node pool up to 2 nodes. The status of the autoscaler can be checked by running this command:

kubectl describe pod vllm-1 -n llm

Name:             vllm-1
Namespace:        llm
Priority:         0
...
Node-Selectors:              <none>
Tolerations:                 kubernetes.azure.com/scalesetpriority=spot:NoSchedule
                             node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
                             node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
                             nvidia.com/gpu:NoSchedule op=Exists
                             sku=gpu:NoSchedule
Events:
  Type     Reason                  Age                    From                     Message
  ----     ------                  ----                   ----                     -------
  Normal   NotTriggerScaleUp       6m42s                  cluster-autoscaler       pod didn't trigger scale-up: 2 max node group size reached
  Normal   TriggeredScaleUp        3m5s                   cluster-autoscaler       pod triggered scale-up: [{aks-gpunp-98036146-vmss 1->2 (max: 2)}]
  Warning  FailedScheduling        2m12s (x2 over 7m14s)  default-scheduler        0/2 nodes are available: 2 Insufficient nvidia.com/gpu. preemption: 0/2 nodes are available: 2 No preemption victims found for incoming pod..
  Warning  FailedScheduling        34s                    default-scheduler        0/3 nodes are available: 1 node(s) had untolerated taint {node.kubernetes.io/network-unavailable: }, 2 Insufficient nvidia.com/gpu. preemption: 0/3 nodes are available: 1 Preemption is not helpful for scheduling, 2 No preemption victims found for incoming pod..
  Normal   Scheduled               19s                    default-scheduler        Successfully assigned llm/vllm-1 to aks-gpunp-98036146-vmss000004
  Normal   SuccessfulAttachVolume  9s                     attachdetach-controller  AttachVolume.Attach succeeded for volume "pvc-d3ccf681-99ae-411a-8d72-33063bf9e422"
  Normal   Pulling                 4s                     kubelet                  Pulling image "ghcr.io/huangyingting/llm-inference-vllm:main"

We can simulate a node eviction to trigger scaling by the cluster autoscaler. This is done by running the following command, which will evict a node and cause the autoscaler to scale up the node pool:

az vmss simulate-eviction --resource-group MC_$RESOURCE_GROUP_NAME_$CLUSTER_NAME_$REGION --name $NODE_POOL --instance-id 0

Test LLM inference

The deployment comes with a default model lmsys/vicuna-7b-v1.5 and an OpenAI API compatiable endpoint. We can test the inference service by running the following command:

# port forwarding to the service
kubectl port-forward svc/vllm 9090:8080 -n llm

curl http://localhost:8192/v1/chat/completions -H "Content-Type: application/json" -d '{
     "model": "lmsys/vicuna-7b-v1.5",
     "max_tokens": 1024,
     "messages": [{"role": "user", "content": "What is large language model?"}],
     "temperature": 0.9 
   }'

Wrap-up

In summary, this article has outlined an approach to deploy large language models on Azure Kubernetes Service in a cost-optimized and scalable manner for private inference. By combining cloud-native techniques like spot instances and autoscaling with model optimization methods like quantization and batching, organizations can unlock the advanced AI capabilities of large models while maintaining control, low latency, and reduced costs compared to public services.

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