[Perf] Disable chunked local attention by default with llama4

[LucasWilkinson]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Currently using the hybrid kv-cache with llama4s chunked local attention causes a latency ~2ms since when the hybrid kv-cache manager is used we end up with 3 ChunkedLocalAttention kv-cache spec groups. We end up with the following groups:

(FullAttention x 12) (ChunkedLocalAttention x 12) (ChunkedLocalAttention x 12) (ChunkedLocalAttention x 12)

This results in attn metadata and local virtual batches for the local layers being constructed 3 times adding latency:

Enabled: 

vllm serve meta-llama/Llama-4-Scout-17B-16E-Instruct -tp 4 --trust-remote-code --max-model-len 16384 --port 8081  --disable-log-requests

============ Serving Benchmark Result ============
Successful requests:                     100       
Benchmark duration (s):                  9.11      
Total input tokens:                      6299      
Total generated tokens:                  12509     
Request throughput (req/s):              10.97     
Output token throughput (tok/s):         1372.85   
Total Token throughput (tok/s):          2064.16   
---------------Time to First Token----------------
Mean TTFT (ms):                          61.84     
Median TTFT (ms):                        61.53     
P99 TTFT (ms):                           106.66    
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          28.46     
Median TPOT (ms):                        29.17     
P99 TPOT (ms):                           30.99     
---------------Inter-token Latency----------------
Mean ITL (ms):                           28.44     
Median ITL (ms):                         28.65     
P99 ITL (ms):                            38.05     
==================================================


Disabled:

vllm serve meta-llama/Llama-4-Scout-17B-16E-Instruct -tp 4 --trust-remote-code --max-model-len 16384 --port 8081  --disable-log-requests --disable-hybrid-kv-cache-manager

============ Serving Benchmark Result ============
Successful requests:                     100       
Benchmark duration (s):                  8.84      
Total input tokens:                      6299      
Total generated tokens:                  12297     
Request throughput (req/s):              11.32     
Output token throughput (tok/s):         1391.49   
Total Token throughput (tok/s):          2104.26   
---------------Time to First Token----------------
Mean TTFT (ms):                          58.69     
Median TTFT (ms):                        59.23     
P99 TTFT (ms):                           90.65     
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          26.48     
Median TPOT (ms):                        27.32     
P99 TPOT (ms):                           28.90     
---------------Inter-token Latency----------------
Mean ITL (ms):                           26.55     
Median ITL (ms):                         26.54     
P99 ITL (ms):                            39.40     
==================================================

Test Plan

see: #21707

Test Result

see: #21707

(Optional) Documentation Update

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[gemini-code-assist]

Code Review

This pull request correctly addresses a performance regression by disabling chunked local attention with the hybrid KV cache manager by default, while providing an environment variable to re-enable it. The implementation is sound. My only suggestion is to update a comment to more accurately reflect that the change is a performance optimization, which will improve code clarity and maintainability.

[gemini-code-assist]

This comment is slightly misleading as it suggests the feature is unsupported, whereas the PR description and warning log indicate it's a performance regression. To improve clarity for future maintenance, it would be better to state the performance-related reason for disabling it.

Suggested change

	# Hybrid KV cache manager is not yet supported with chunked
	# local attention.
	# Disable hybrid KV cache manager with chunked local attention
	# due to a performance regression.

[mgoin]

I kind of agree with Gemini here, although you say this in your log

[mgoin]

LGTM for the moment

[mgoin]

I kind of agree with Gemini here, although you say this in your log

[mgoin]

nit: warning_once

[mgoin]

Merging to solve the regression since we have better solutions on the way

[luccafong]

@LucasWilkinson will we reduce metadata creation with refactoring?

[LucasWilkinson]

thats the plan; we are working towards: https://vllm-dev.slack.com/archives/C07R5Q1Q2BB/p1753727605258469?thread_ts=1753202489.248869&cid=C07R5Q1Q2BB but that will be a followup PR