Right-Sizing Compute for Training and Inference

Picking the right compute instance is one of the most significant levers you can pull to control AI infrastructure costs. Overprovisioning is a common and expensive habit. An engineer might request a top-tier p4de.24xlarge AWS instance with 8 A100 80GB GPUs out of an abundance of caution, only for their training job to use 15% of the available GPU memory and 30% of the compute capacity. This is akin to renting a cargo ship to deliver a single package. Conversely, underprovisioning leads to job failures, performance bottlenecks, and frustrated data scientists, which also has a cost.

Right-sizing is the continuous process of matching workload requirements to infrastructure resources to achieve the best performance at the lowest possible cost. This process is not a one-time setup but an iterative cycle of profiling, analyzing, and adjusting. The goals and techniques for right-sizing differ significantly between the long-running, resource-intensive nature of training and the latency-sensitive, steady-state demands of inference.

Right-Sizing for Model Training

Training jobs, especially for large foundation models, are characterized by their long duration and high resource consumption. The primary goal is to complete the training run successfully and as quickly as is economically feasible, directly impacting the TotalSpend in our EffectiveCost equation.

Profiling Before Provisioning

You cannot optimize what you do not measure. Before you can right-size, you must understand your workload's resource signature. This means moving past simple observation and using proper profiling tools. While nvidia-smi is useful for a quick snapshot, production-grade profiling requires time-series data.

NVIDIA's Data Center GPU Manager (DCGM) is the industry standard for this. Integrated with tools like Prometheus, it allows you to capture detailed metrics over the entire duration of a training job.

Important metrics to monitor with DCGM include:

• DCGM_FI_DEV_GPU_UTIL: The percentage of time one or more kernels were executing on the GPU. Low utilization often points to I/O or CPU bottlenecks.
• DCGM_FI_DEV_FB_USED: The amount of framebuffer (VRAM) memory used. This is essential for determining if you are close to an out-of-memory (OOM) error or if you have massively overprovisioned memory.
• DCGM_FI_DEV_MEM_COPY_UTIL: The percentage of time the memory copy engine was active. High utilization here can indicate a data loading bottleneck between CPU and GPU memory.
• DCGM_FI_DEV_TENSOR_ACTIVE: The percentage of time the Tensor Cores are active. This is a direct measure of how effectively you are using the specialized hardware for deep learning.

A common anti-pattern revealed by profiling is the "scalloped" GPU utilization graph, which indicates the GPU is frequently waiting for the next batch of data. This points not to a need for a bigger GPU, but for optimizing the data loading pipeline or using a CPU with more cores.

A data loading bottleneck where the GPU sits idle waiting for the CPU to prepare the next batch of data. Profiling reveals this as low average GPU utilization.

Strategies for Training Instances

Armed with a resource profile, you can make informed decisions.

1. Select the Right Instance Family: Not all GPUs are created equal. For a model with high memory requirements but moderate compute needs, an instance with NVIDIA A100 40GB GPUs might offer better value than one with 80GB GPUs. Conversely, for a compute-bound task, a newer generation GPU with more powerful Tensor Cores might finish the job faster, reducing total cost even if the hourly price is higher. Always analyze the memory-to-compute ratio of your workload and match it to an available instance type.

2. Use Multi-Instance GPU (MIG): For development, experimentation, or training smaller models, a full A100 or H100 GPU is often overkill. NVIDIA MIG, available on Ampere and newer architectures, allows you to partition a single physical GPU into up to seven fully isolated, hardware-backed GPU instances. Each MIG instance has its own dedicated memory, cache, and compute cores. From Kubernetes' perspective, each MIG instance appears as a distinct nvidia.com/gpu resource. This is an extremely effective way to increase utilization and share expensive accelerators across multiple teams or jobs without performance interference.

3. Balance Data Parallelism vs. Instance Size: Consider a scenario where you need to train a model that requires 64GB of VRAM. You could use a single 80GB A100 GPU. Alternatively, using a framework like PyTorch FSDP or DeepSpeed ZeRO, you could shard the model's parameters across two 40GB A100 GPUs or even four 16GB V100 GPUs. The latter approach might be significantly cheaper per hour. The trade-off is communication overhead. You must analyze whether the cost savings from using smaller, cheaper instances outweigh the performance penalty from increased network communication between nodes.

4. Don't Forget the Host CPU and RAM: A powerful GPU can be severely bottlenecked by an underpowered host. If your data loading pipeline involves complex augmentations, you need a CPU with enough cores to keep the GPU fed. Similarly, if you are loading massive datasets, you need sufficient system RAM to avoid thrashing. Monitor CPU utilization and I/O wait times alongside GPU metrics to get a complete picture.

Right-Sizing for Model Inference

Inference workloads have a different set of constraints. While training prioritizes finishing a job, inference prioritizes sustained performance under load, typically measured by latency and throughput. The cost model here shifts from cost-per-job to cost-per-inference.

$$CostPerInference = \frac{InstanceCost_{hourly}}{Throughput_{inferences\_per\_hour}}$$

The goal is to minimize this value while meeting your Service Level Objectives (SLOs) for latency, such as a p99 latency below 100ms.

Load Testing to Find the Knee

The first step in right-sizing inference is to perform load testing. Using tools like Triton's perf_analyzer, Locust, or k6, you can simulate production traffic and measure how your deployed model performs on different instance types. By plotting throughput against latency, you can identify the "knee" of the performance curve. This is the point where increasing the load further leads to a sharp, non-linear increase in latency, indicating the system is saturated.

The performance "knee" occurs around 220 inferences/sec, after which latency rapidly exceeds the 100ms SLO. This saturation point defines the maximum effective throughput for a single replica on this instance type.

Strategies for Inference Instances

1. CPU Can Be a Viable Option: It's tempting to deploy every model on a GPU, but for many use cases, this is not cost-effective. Small models, or services with low or infrequent traffic, can often be served more cheaply on CPU instances. With optimization frameworks like ONNX Runtime or OpenVINO, CPU inference can be surprisingly fast. The break-even point is a function of traffic volume. A GPU might be 10x faster but 20x more expensive per hour. If traffic is low, the GPU will sit idle, wasting money. Always benchmark a CPU-only option.

2. Use Inference-Optimized Accelerators: If a GPU is warranted, choose one designed for inference. NVIDIA's T4 and A10G GPUs offer excellent integer performance (ideal for quantized models) and a much better price/performance ratio for inference than large training GPUs like the A100. They are specifically engineered for high-throughput, low-latency serving.

3. Maximize Utilization with Model Co-location: A single powerful GPU can often serve multiple different models simultaneously, a technique known as model co-location. An advanced inference server like NVIDIA Triton allows you to load multiple models onto the same GPU, each managed independently. This dramatically increases the ResourceUtilization of the expensive GPU asset. The trick is to co-locate models with different resource profiles. For example, you could place a memory-intensive NLP model alongside a compute-intensive computer vision model to balance the load on the GPU's subsystems.

4. Combine Right-Sizing with Autoscaling: Right-sizing determines the "size" of your building block (the instance), while autoscaling determines the "number" of blocks. A Kubernetes Horizontal Pod Autoscaler (HPA) can be configured to scale the number of model server replicas based on real-time metrics like GPU utilization (via DCGM) or inferences per second. This ensures that you scale out during peak traffic and, just as importantly, scale down to a minimum number of replicas (even to zero) during idle periods, stopping you from paying for unused capacity. This dynamic approach is the pinnacle of cost-efficient inference serving.