SpotServe: Serving generative large language models on preemptible instances

Meta Info

Presented in ASPLOS 2024.

Understanding the paper

TL;DR

SpotServe — the first distributed LLM serving system on preemptible instances
Techniques
- Dynamically adapt the LLM parallelization configuration
- Minimize the cost of migrating instances for dynamic reparallelization
  - Formulated as a bipartite graph matching problem → use the KuhnMunkres algorithm to identify an optimal migration plan
- Stateful inference recovery
  - Commit inference progress at a much finer granularity
  - Resume inference upon preemption

Background

Spot instances
- Lower price than on-demand instances
- May be preempted at any time
- Grace period (e.g., 30 seconds for AWS spot instances)

Existing works

Leverage spot instances to reduce the monetary cost of DNN inference
- Example: MArk, Cocktail
- Limitation: Target small DNN models that can fit on a single spot instance with one or multiple GPU
- Handle preemptions using request rerouting or redundant computation

Challenges in serving LLMs on spot GPU instances

Dynamic reparallelization — how to quickly adapt to changes to spot instances’ availability and requests’ arrival rates?
Instance migration — how to minimize the cost of migrating GPU instances for reparallelization?
Grace period — how to leverage grace period to handle unfinished request?

Designs

Inference Server
- Deployed on a dedicated on-demand CPU instance
- Three components
  - Request Manager
    Receiving input requests
    Dynamically partition them into batches
    Assign these batches to inference instances running on spot GPU instances
    Collect generated outputs from the inference instances
    Send the results back to users
  - Meta-context Manager
    Manage the adjustment of the parallel configuration by sending instructions for context migration to all GPU instances
    Modules
    Parallelization Controller
    Adjust the parallelization configuration to improve LLM serving performance
    A parallel configuration — $C = (D, P, M, B)$
    $D$ — data parallelism degree
    $P$ — pipeline-model parallelism degree
    $M$ — tensor-model parallelism degree
    $B$ — the maximum mini-batch size
    Measure the initialization time in advance
    Adaptive optimization algorithm
    Two variables $C_t$ and at time step $t$
    $C_t$ — parallel configuration
    $N_t$ — the number of available instances
    Include newly allocated instances
    Exclude instances to be preempted
    Minimizes the end-to-end inference latency $l_{req}(C)$ while maintaining a throughput higher than $\alpha_𝑡$
    If multiple configurations can achieve similar minimum inference latency → Select the configuration with lower monetary cost (i.e., using fewer instances)
    If peak serving throughput can not exceed the request arrival rate $\alpha_𝑡$ → Maximize the overall serving throughput
    Optionally allocate on-demand instances to improve serving throughput
    Run the online algorithm, negligible overhead (i.e., less than 1s)
    Offline estimate the latency of different configurations in advance
    Device Mapper
    Use the Kuhn-Munkres (KM) algorithm to find an optimal device mapping → Maximally reuse the model parameters and KV cache on available GPU instances & minimize the total data transmission
    Device mapping — a bipartite graph $\mathcal{G} = (\mathcal{V_a}, \mathcal{V_t}, \varepsilon)$
    $u \in \mathcal{V_a}$ — a GPU device
    $v \in \mathcal{V_t}$ — a pipeline-stage-shard position of the parallel configuration
    a weighted edge $𝑒_{𝑢𝑣}$ — the amount of reusable model parameters and key/value cache when mapping GPU 𝑢 to position 𝑣 of the parallel configuration
    Build a complete bipartite graph and compute the edge weight between every $(𝑢, 𝑣)$ pair using the size of their intersection contexts
    If the new parallel configuration handles less concurrent inference requests
    Discard part of the cached results → avoid exceeding the memory capacity of the new parallel configuration
    Keep the batches of requests with more decoding progresses
    Migration Planner
    Determine the exact migration plan to finish the configuration adjustment
    Progressive migration schedule — utilize the pipeline structure and prioritize the migration of front model layers’ context
    Consider the memory usage during the progressive migration process
  - Instance Manager
    Interacts with the cloud and receives instance preemption/acquisition notifications
    Allocates on-demand and spot instances at the same time to avoid the waiting overhead when spot-instance allocation fails
    Prefer to release on-demand instances
    Keep few additional instances (e.g., 2 in experiments) to alleviate the impacts of frequent disturbance of instance availability
Inference Engine
- Deployed on each spot or on-demand GPU instance to serve LLM inference
- Components
  - Context daemon
    Manages the model parameters (i.e., model context) and intermediate activations (i.e., cache context) for different requests inside a certain GPU
  - Interruption Arranger
    Support stateful inference recovery
Stateful inference recovery
- Recover interrupted inference request without recomputation
- Context daemon maintains the cache context of an inference request
- Route the request to another inference pipeline using the cached state
- Just-in-time arrangement
  - Each spot GPU instance includes an interruption arranger that receives a notification when a grace period starts
- Fault tolerance
  - Delay the acquired instance joining and make the arrangements for prior interruptions feasible
  - One instance gets preempted before expected → Give up the cache context and only migrate the model context with the rest instances
  - All replicas of the same piece of model context are lost due to unexpected failures → Restart by loading weights locally (e.g., disk) or from remote cloud storage (e.g., S3) to fetch the required model parameters

Implementation

5.6K LoC in C++ and 2.2 LoC in Python
Built on top of FasterTransformer

Evaluation

Settings
- A real 12-hour availability trace with AWS g4dn spot instance and extract two representative 20-minute segments with different dynamic behaviors
- Two workloads
  - Stable inference request arrival workload
    Different request arrival rates for different models
    1.5 requests/s for OPT-6.7B
    0.35 requests/s for GPT-20B
    0.2 requests/s for LLaMA-30B
    Gamma request arrival process with a coefficient of variance of 6
  - Fluctuating inference request arrival workload
    Trace: Serverless-in-the-wild
- The maximum batch size $B$ is selected from $\{1,2,4,8\}$
- $𝑆_{𝑖𝑛}$ is 512 — the sequence length of the input tokens
- $𝑆_{𝑜𝑢𝑡}$ is 128 — the sequence length of output tokens
Baselines
- Rerouting — dynamically reroutes interrupted requests to other available pipelines when preemption happens
- Reparallelization — restart and reinitialize all instances without context migration
Metrics
- The average and various tail latencies
- Monetary cost — USD/token

Limitations and future work

Strongly rely on the grace period → Can explore more solutions to improve system performance (e.g., inference workload prediction, instance availability prediction)
Focus on single-type GPU instances → Can integrate heterogeneous spot instances or instances from different clouds
Take inference latency minimization as the optimization target → Can explore other targets (e.g., strict SLO, high throughput)
Can generalize to other preemptible resources (e.g., resource scheduler may preempt resources for urgent jobs with switching overheads)

Last updated 1 year ago