[Bugfix] Disable the statslogger if the api_server_count is greater than 1

[chaunceyjiang]

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.

Fixes #21954

Purpose

vllm serve /home/jovyan/qwen3-8b  --data-parallel-size 2 --data-parallel-rpc-port 25555 --data-parallel-address 127.0.0.1 --api-server-count 2
INFO 09-05 01:29:36 [__init__.py:241] Automatically detected platform cuda.
INFO 09-05 01:29:41 [api_server.py:1894] vLLM API server version 0.10.2.dev403+g14b4326b9
INFO 09-05 01:29:41 [utils.py:328] non-default args: {'model_tag': '/home/jovyan/qwen3-8b', 'api_server_count': 2, 'model': '/home/jovyan/qwen3-8b', 'data_parallel_size': 2, 'data_parallel_address': '127.0.0.1', 'data_parallel_rpc_port': 25555, 'mm_processor_cache_gb': 0}
INFO 09-05 01:29:51 [__init__.py:748] Resolved architecture: Qwen3ForCausalLM
`torch_dtype` is deprecated! Use `dtype` instead!
....
(ApiServer_1 pid=21224) WARNING 09-05 01:30:13 [async_llm.py:108] AsyncLLM created with api_server_count more than 1; disabling stats logging to avoid incomplete stats.
(ApiServer_0 pid=21221) INFO 09-05 01:30:13 [scheduler.py:222] Chunked prefill is enabled with max_num_batched_tokens=8192.
(ApiServer_0 pid=21221) WARNING 09-05 01:30:13 [async_llm.py:108] AsyncLLM created with api_server_count more than 1; disabling stats logging to avoid incomplete stats.

Test Result

(Optional) Documentation Update

[gemini-code-assist]

Code Review

This pull request introduces a bugfix to disable the statistics logger when api_server_count is greater than 1, as this configuration is not compatible. The change correctly saves the original state of the disable_log_stats argument, forces it to True when multiple API servers are used, and issues a warning to the user if the feature was previously enabled. The implementation is consistent with how other incompatible features are handled in this scenario. The code is correct and effectively addresses the issue.

[chaunceyjiang]

Hi, @njhill I have a question: when disable_log_stats is set to true, both LoggingStatLogger and PrometheusStatLogger are disabled. Is this the expected behavior?

vllm/vllm/engine/llm_engine.py

Lines 343 to 368 in 601f856

	# Metric Logging.
	if self.log_stats:
	if stat_loggers is not None:
	self.stat_loggers = stat_loggers
	else:
	# Lazy import for prometheus multiprocessing.
	# We need to set PROMETHEUS_MULTIPROC_DIR environment variable
	# before prometheus_client is imported.
	# See https://prometheus.github.io/client_python/multiprocess/
	from vllm.engine.metrics import (LoggingStatLogger,
	PrometheusStatLogger)
	self.stat_loggers = {
	"logging":
	LoggingStatLogger(
	local_interval=_LOCAL_LOGGING_INTERVAL_SEC,
	vllm_config=vllm_config),
	"prometheus":
	PrometheusStatLogger(
	local_interval=_LOCAL_LOGGING_INTERVAL_SEC,
	labels=dict(
	model_name=self.model_config.served_model_name),
	vllm_config=vllm_config),
	}
	self.stat_loggers["prometheus"].info("cache_config",
	self.cache_config)

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[njhill]

Hi, @njhill I have a question: when disable_log_stats is set to true, both LoggingStatLogger and PrometheusStatLogger are disabled. Is this the expected behavior?

@chaunceyjiang yes that's expected. Here we only want to omit LoggingStatLogger.

The API here is kind of crappy in general - if you provide custom stats loggers then they will replace the built-in ones rather than augment them. There's a separate discussion / PR proposal around that I think.

[chaunceyjiang]

/cc @njhill PTAL.

[njhill]

@chaunceyjiang I wonder if you could rebase this now that #20952 is merged?

The behaviour now is that custom stats loggers will augment the built-in ones unless disable_log_stats=True.

[chaunceyjiang]

@njhill PTAL.

[njhill]

Thanks @chaunceyjiang.

Actually we still want the prometheus metrics in this case and I think this change will disable those too.

I think the check should go here instead:

vllm/vllm/v1/metrics/loggers.py

Lines 662 to 663 in eedb2a2

	if enable_default_loggers and logger.isEnabledFor(logging.INFO):
	factories.append(LoggingStatLogger)

[njhill]

Thanks @chaunceyjiang

[njhill]

Remaining test failure is also occurring on main.