SLA-based Planner#

This document covers SLA-based planner in examples/common/utils/planner_core.py.

The SLA (Service Level Agreement)-based planner is an intelligent autoscaling system that monitors system performance and adjusts the number of prefill and decode workers to meet specified TTFT and ITL targets. Unlike the load-based planner that scales based on resource utilization thresholds, the SLA planner uses predictive modeling and performance interpolation to proactively scale the workers.

Note

Currently, SLA-based planner only supports disaggregated setup.

Warning

Bare metal deployment with local connector is deprecated. Please deploy the SLA planner in k8s.

Features#

  • SLA-driven scaling: Automatically scales prefill/decode workers to meet TTFT and ITL targets

  • Predictive load forecasting: Uses ARIMA, Prophet, or constant predictors to forecast future load

  • Performance interpolation: Leverages profiling results data from pre-deployment profiling for accurate scaling decisions

  • Correction factors: Adapts to real-world performance deviations from profiled data

Architecture#

The SLA planner consists of several key components:

  1. Load Predictors: Forecast future request patterns (number of requests, input/output sequence lengths)

  2. Performance Interpolators: Estimate TTFT and ITL based on profiled performance data

  3. Correction Factors: Adjust predictions based on observed vs. expected performance

  4. Scaling Logic: Calculate optimal number of prefill/decode replicas to meet SLA targets

Pre-Deployment Profiling#

SLA-based planner requires pre-deployment profiling to operate. See Pre-Deployment Profiling for more details.

Load Prediction#

The SLA planner use load predictor to predict the number of requests, ISL, and OSL in the next adjustment interval. Currently, three load prediction model is supported:

Constant Predictor#

  • Use case: Stable and long prediction interval

  • Behavior: Assumes next load equals current load

  • Configuration: load-predictor: "constant"

ARIMA Predictor#

  • Use case: Time-series data with trends and seasonality

  • Behavior: Uses auto-ARIMA to fit optimal model parameters

  • Configuration: load-predictor: "arima"

Prophet Predictor#

  • Use case: Complex seasonal patterns and trend changes

  • Behavior: Facebook’s Prophet model for time-series forecasting

  • Configuration: load-predictor: "prophet"

Scaling Algorithm#

SLA planner uses a sophisticated scaling algorithm. At each adjustment interval, SLA planner performs the following operations:

1. Metric Collection#

Every adjustment interval, collect:

  • Average Time to First Token (TTFT)

  • Average Inter-Token Latency (ITL)

  • Request count and duration

  • Input/Output sequence lengths

2. Correction Factor Calculation#

Using the collected metrics, SLA planner applies the interpolator to find out the expected TTFT/ITL and calibrate the interpolation model. This step is important because the actual TTFT/ITL can often be different than the ideal world:

  • TTFT: actual TTFT heavily depends on request queueing and prefix cache hit rate (if use kv reuse). For example, if all requests arrives at the beginning of the adjustment interval, they queue heavily and TTFT will be significantly higher. If prefix cache hit rate is very high, the actual number of tokens in the prefill will be very low and TTFT will be significantly lower.

  • ITL: actual ITL maybe affected by chunked small prefill request in decode engine.

  • Metric variances: large variances in request rate, ISL, and OSL may lead to inaccurate estimation of the TTFT/ITL since SLA only consider the average when interpolating.

SLA planner calculate the correction factor with

  • Prefill correction: actual_ttft / expected_ttft

  • Decode correction: actual_itl / expected_itl

3. Load Prediction#

SLA planner forecasts these metric in the next interval using the load predictor

  • Number of requests

  • Input sequence length

  • Output sequence length

4. Calculating Number of Replicas#

Prefill replicas: SLA planner assumes the prefill correction factor has linear affect on the prefill throughput per GPU as prefill is single-batched.

predicted_load = next_requests * next_isl / interval * min(1, prefill_correction)
prefill_replicas = ceil(predicted_load / interpolated_throughput / gpus_per_engine)

Decode replicas:

# 1. apply d_correction_factor to the ITL SLA
corrected_itl = self.args.itl / self.d_correction_factor
# 2. reversely find out what is best throughput/gpu that can achieve corrected_itl under the predicted context length
pred_decode_thpt_per_gpu = self.decode_interpolator.find_best_throughput_per_gpu(
    itl=corrected_itl,
    context_length=next_isl + next_osl / 2
)
# 3. compute number of decode replicas needed
next_num_d = math.ceil(next_num_req * next_osl / self.args.adjustment_interval / pred_decode_thpt_per_gpu / self.args.decode_engine_num_gpu)

5. Scaling#

Finally, SLA planner applies the change by scaling up/down the number of prefill and decode workers to the calculated number of replica in the next interval.

Note

SLA-planner scales up/down the P/D engines non-blockingly. If adjustment-interval is too short, the previous scaling operations may not finish before the new scaling operations are issued. Make sure to set a large enough adjustment-interval.

Deploying#

For detailed deployment instructions including setup, configuration, troubleshooting, and architecture overview, see the SLA Planner Deployment Guide.

Quick Start:

cd components/backends/vllm/deploy
kubectl apply -f disagg_planner.yaml -n {$NAMESPACE}

Note

The SLA planner requires a frontend that reports metrics at /metrics HTTP endpoint with number of requests, ISL, OSL, TTFT, ITL in the correct format. The dynamo frontend provides these metrics automatically.

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