[MISC] add cpu_kvcache_space_bytes to CacheConfig

[andyxning]

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

cpu_kvcache_space_bytes is used in cpu.py and cpu_worker.py files. Such as:

vllm/vllm/worker/cpu_worker.py

Lines 250 to 255 in 8b6e1d6

	# For CPU device, the block number will be calculated based on the
	# cpu_kvcache_space.
	cache_block_size = self.get_cache_block_size_bytes()
	num_cpu_blocks = int(self.cache_config.cpu_kvcache_space_bytes //
	cache_block_size)
	num_cpu_blocks = max(num_cpu_blocks, 0)

vllm/vllm/platforms/cpu.py

Lines 128 to 139 in 8b6e1d6

	if kv_cache_space >= 0:
	if kv_cache_space == 0:
	cache_config.cpu_kvcache_space_bytes = 4 * GiB_bytes # type: ignore
	logger.warning(
	"Environment variable VLLM_CPU_KVCACHE_SPACE (GiB) "
	"for CPU backend is not set, using 4 by default.")
	else:
	cache_config.cpu_kvcache_space_bytes = kv_cache_space * GiB_bytes # type: ignore # noqa
	else:
	raise RuntimeError(
	"Invalid environment variable VLLM_CPU_KVCACHE_SPACE"
	f" {kv_cache_space}, expect a positive integer value.")

But it is not declared in CacheConfig.

Test Plan

NA

Test Result

NA

(Optional) Documentation Update

[github-actions]

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[gemini-code-assist]

Summary of Changes

Hello @andyxning, 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 addresses a previously undeclared configuration parameter by formally adding cpu_kvcache_space_bytes to the CacheConfig class. This change ensures that the CPU K/V cache space, which is already in use by the CPU backend, is properly defined and documented within the system's central cache configuration, improving clarity and maintainability.

Highlights

• Configuration Update: Added a new field, cpu_kvcache_space_bytes, to the CacheConfig class. This parameter, which is already utilized by CPU backend components (cpu.py and cpu_worker.py), is now formally declared within the configuration structure.

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[gemini-code-assist]

Code Review

This pull request adds the cpu_kvcache_space_bytes field to the CacheConfig dataclass. This change seems appropriate as the PR description indicates this field is already in use in other parts of the codebase (cpu.py, cpu_worker.py) and this formalizes its declaration.

The main feedback is a minor stylistic improvement to the docstring of the new field for better consistency with Python conventions (PEP 257) and existing docstrings in the file.

Additionally, the PR description template has not been fully completed (checklist items are unchecked, placeholder text remains). It would be beneficial to fill this out or mark items as N/A for clarity.

[gemini-code-assist]

The docstring formatting for cpu_kvcache_space_bytes is a bit unconventional for a single effective line of documentation and inconsistent with other docstrings in this file (e.g., lines 1506-1508 for calculate_kv_scales).

According to PEP 257, for one-liner docstrings, the closing """ should be on the same line. The current multi-line formatting with the closing """ on a new, indented line is unusual for such a short docstring.

Suggesting a standard single-line format for conciseness and consistency.

    """(CPU backend only) CPU key-value cache space."""

[andyxning]

Done

[andyxning]

I do not know why pre-commit will complains about files not changed in this pr:

Will update to make pre-commit green.

[DarkLight1337]

cc @Isotr0py @bigPYJ1151 do you know about the history of this field? It doesn't seem to exist even before #16835

[Isotr0py]

Seems this field was initially added to CPU executor in the earliest CPU support PR (#3634) with declaration missing in CacheConfig. Since everything worked well after that, nobody catched this issue untill this PR. 😅

[bigPYJ1151]

In fact we thought this field is too device-specific, so didn't add it to CacheConfig😂