[WIP][Metrics] Re-work approach to LoRA metrics

[markmc]

Part of #10582 and discussed in #12745

The current vllm:lora_requests_info Gauge is somewhat similar to an Info metric (like cache_config_info) except the value is the current wall-clock time, and is updated every iteration.

The label names used are:

• running_lora_adapters: a list of adapters with running requests, formatted as a comma-separated string.
• waiting_lora_adapters: similar, except listing adapters with requests waiting to be scheduled.
• max_lora - the static "max number of LoRAs in a single batch." configuration.

It looks like this:

vllm:lora_requests_info{max_lora="1",running_lora_adapters="",waiting_lora_adapters=""} 1.7395575657589855e+09
vllm:lora_requests_info{max_lora="1",running_lora_adapters="test-lora",waiting_lora_adapters=""} 1.7395575723949368e+09
vllm:lora_requests_info{max_lora="1",running_lora_adapters="test-lora",waiting_lora_adapters="test-lora"} 1.7395575717647147e+09

I can't really make much sense of this. Encoding a running/waiting status for multiple adapters in a comma-separated string seems quite misguided - we should use labels to distinguish between per-adapter counts instead:

vllm:num_lora_requests_running{lora_name="test-lora",model_name="meta-llama/Llama-3.1-8B-Instruct"} 8.0
vllm:num_lora_requests_waiting{lora_name="test-lora",model_name="meta-llama/Llama-3.1-8B-Instruct"} 7.0

This was added in #9477 and there is at least one known user. If we revisit this design and deprecate the old metric, we should reduce the need for a significant deprecation period by making the change in v0 also and asking this project to move to the new metric.

TODO:

• Add a lora config info gauge - max_loras, max_lora_rank
• Add more unit test coverage of the new metrics
• Add the new metrics to V1
• Add the old metric to V0 (to ease the transition to V1)
• Deprecate the old metrics

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[markmc]

You could argue either:

1. We don't need per-adapter counts at all, just an info metric (like cache_config_info) that lists the configured adapters, or

2. Most of our metrics should be per-adapter ... just break them down per-adapter and label with lora_name=

[markmc]

See also #6275

[markmc]

ok, I took a closer look at what the Gateway API Inference Extension is doing with this metric. I've filed kubernetes-sigs/gateway-api-inference-extension#354 to invite feedback from that project.

The premise is this:

route to a model server that has the adapter already loaded, so long as there is batch capacity

and the way the metric is described:

Metric name implemented in vLLM: vllm:lora_requests_info
Metric type: Gauge
Metric value: The last updated timestamp - so the Endpoint Picker (EPP) can find the latest
Metric labels:
max_lora: The maximum number of adapters that can be loaded to GPU memory to serve a batch. Requests will be queued if the model server has reached MaxActiveAdapter and canno load the requested adapter. Example: "max_lora": "8".
running_lora_adapters: A comma separated list of adapters that are currently loaded in GPU memory and ready to serve requests. Example: "running_lora_adapters": "adapter1, adapter2"

References:

• Kubernetes LLM Instance Gateway PoC Design
• Inference Gateway for Multiplexing models
• WG Serving - LLM Instance Gateway
• Endpoint Picker Protocol

[markmc]

Given the way LRUCacheWorkerLoRAManager works, the current V0 metric implementation and what's proposed here in V1 both miss an important point - even if there was no requests for a given LoRA included in the most recent batch, that LoRA's weights could still be loaded on GPUs and it would still be efficient to route requests for it to this vLLM instance.

[varun-sundar-rabindranath]

Hi @markmc ! Thanks for doing this!
I looked through #6275. On top of what you propose (adapters + counts) the metric proposed there look very informative,

from the RFC:

 - Loading and unloading times for LoRA adapters.
 - Memory and compute resource usage by LoRA adapters.
 - Performance impact on base models when using LoRA adapters.

About,

Loading and unloading times for LoRA adapters.

I think this is good information. The max loras value combined with information about the number of running/waiting loras , will inform the users about the dynamics of LoRA loads. I believe the load times could serve as a good secondary information.
side-note: In V1, the LoRA adapter Loads are triggered here

vllm/vllm/v1/worker/gpu_model_runner.py

Line 581 in 3809458

if self.lora_config:

. In this vein, i.e. informing the users about the input preparation time, I think we should consider exposing the run-time of _prepare_inputs function in general.

About,

Memory and compute resource usage by LoRA adapters.

For memory usage, we already profile the memory usage for a forward pass in determining the available memory for KV cache. However, it is not granular enough to inform about the memory usage by particular LoRA adapters. Also, since the max_loras engine argument effectively limits the number of LoRA adapters used, I believe the memory usage by LoRA adapters would be constant and wouldn't change during runtime.
For compute usage by LoRA adapters, I am not sure how we would do this at his granularity. Perhaps, we can add a "gpu-utilitzation" metric to Stats for the users to infer.
I believe this set of metrics is less important and we can tackle the memory and compute usage metrics after the first cut maybe.

About,

even if there was no requests for a given LoRA included in the most recent batch, that LoRA's weights could still be loaded on GPUs and it would still be efficient to route requests for it to this vLLM instance.
and,
Performance impact on base models when using LoRA adapters.

I believe the existing Iteration level metrics (

vllm/vllm/engine/llm_engine.py

Line 1600 in 3809458

# Iteration stats

) and what you proposed (adapters + counts) combined should inform the user of this.

[markmc]

Implemented the V0 metric in V1 in #13504

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @markmc.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[markmc]

I've filed kubernetes-sigs/gateway-api-inference-extension#354 to invite feedback from that project.

Deferring for now, based on the feedback from above