[Misc] Improve logging for dynamic shape cache compilation

[kyolebu]

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Purpose

When a graph is compiled for dynamic shapes, the previous log message ("Cache the graph of shape None for later use") is ambiguous. This update makes it clear that the cached graph supports dynamic inputs, improving clarity of the compilation process.

Slack Issue: https://vllm-dev.slack.com/archives/C08K1FAHFPH/p1751506800619009

Test Plan

Test Result

(Optional) Documentation Update

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[gemini-code-assist]

Summary of Changes

Hello @kyolebu, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on a minor but helpful improvement to the logging within the compilation backend. The change ensures that when a graph is cached, the log message accurately reflects whether a specific static shape or a dynamic shape is being cached, enhancing observability during compilation.

Highlights

• Improved Logging: I've updated the informational log message emitted when a graph is cached. Previously, it always attempted to log a specific shape. Now, if the runtime_shape is None (indicating a dynamic shape), the log message will explicitly state "Cache the graph for DYNAMIC shape for later use", providing clearer context for dynamic shape compilation. Otherwise, it will continue to log the specific shape.

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[gemini-code-assist]

Code Review

This pull request improves logging for dynamic shape cache compilation by providing a more descriptive message when runtime_shape is None.

[gemini-code-assist]

This if/else block can be simplified using a conditional expression for constructing the log message, improving readability.

                shape_log_msg = "for DYNAMIC shape" if runtime_shape is None else f"of shape {runtime_shape}"
                logger.info(f"Cache the graph {shape_log_msg} for later use")

[tlrmchlsmth]

Looks good, thanks!

For future improvement, it looks like there are some other places we should hit as well:

vllm/vllm/compilation/backends.py

Lines 155 to 157 in ddf39ff

	logger.info(
	"Directly load the compiled graph(s) for shape %s "
	"from the cache, took %.3f s", str(runtime_shape), elapsed)

It'd also be nice to be consistent about general vs dynamic shape in the logs:

vllm/vllm/compilation/backends.py

Lines 196 to 201 in ddf39ff

	if runtime_shape is None:
	logger.info("Compiling a graph for general shape takes %.2f s",
	elapsed)
	else:
	logger.info("Compiling a graph for shape %s takes %.2f s",
	runtime_shape, elapsed)

[kyolebu]

Looks good, thanks!

For future improvement, it looks like there are some other places we should hit as well:

vllm/vllm/compilation/backends.py

Lines 155 to 157 in ddf39ff

	logger.info(
	"Directly load the compiled graph(s) for shape %s "
	"from the cache, took %.3f s", str(runtime_shape), elapsed)

It'd also be nice to be consistent about general vs dynamic shape in the logs:

vllm/vllm/compilation/backends.py

Lines 196 to 201 in ddf39ff

	if runtime_shape is None:
	logger.info("Compiling a graph for general shape takes %.2f s",
	elapsed)
	else:
	logger.info("Compiling a graph for shape %s takes %.2f s",
	runtime_shape, elapsed)

Updated other log areas to be consistent with the new changes for dynamic shape. Also changed wording from "general" to "dynamic." Thanks for the feedback!

[mgoin]

LGTM, thanks