Test CPU memory leak fix

analysis.md

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+# CPU Memory Leak Analysis (GitHub Issue #28726)
+
+## Summary
+
+vLLM suffers from continuous CPU memory growth when serving multimodal (VLM)
+models with prefix caching enabled (the default). The EngineCore subprocess
+RSS grows by ~1.5 GB per 1000 requests and never stabilizes, eventually
+causing OOM. The issue appeared between v0.11.0 and v0.11.1.
+
+**Root cause**: A reference cycle in `Request` objects prevents Python's
+reference counting from freeing them. A GC optimization introduced in v0.11.1
+reduces how often the cyclic garbage collector runs, causing these cyclic
+`Request` objects (each holding megabytes of multimodal feature data) to
+accumulate far faster than the GC can reclaim them.
+
+## The Reference Cycle
+
+In `vllm/v1/request.py`, when prefix caching is enabled, each `Request` binds
+itself into a `functools.partial`:
+
+```python
+# vllm/v1/request.py:167-170 (main branch)
+self.get_hash_new_full_blocks: Callable[[], list[BlockHash]] | None = None
+if block_hasher is not None:
+    self.get_hash_new_full_blocks = partial(block_hasher, self)  # <-- cycle
+    self.block_hashes = self.get_hash_new_full_blocks()
+```
+
+This creates a **reference cycle**:
+
+```
+Request ──(self.get_hash_new_full_blocks)──> partial object
+   ^                                              │
+   └──────────(partial stores self as arg)────────┘
+```
+
+### Why this matters
+
+Python uses two garbage collection mechanisms:
+
+1. **Reference counting** (immediate): When an object's reference count drops
+   to zero, it is freed instantly. This is the fast path.
+
+2. **Cyclic garbage collector** (deferred): Periodically scans for groups of
+   objects that reference each other but are unreachable from the rest of the
+   program. This is the slow path, and it runs based on heuristic thresholds.
+
+The reference cycle means that when the scheduler finishes a request and does
+`del self.requests[request_id]`, the `Request` object's reference count
+**does not drop to zero** -- the `partial` still holds a reference. The
+`partial`'s count doesn't drop to zero either -- the `Request` still holds
+it. Both objects are unreachable from the program, but neither can be freed
+by reference counting. They become **cyclic garbage**, waiting for the cyclic
+GC to detect and collect them.
+
+### Why it only affects prefix caching
+
+When prefix caching is **disabled**, `block_hasher` is `None`, so the
+`partial` is never created. There is no cycle. `Request` objects are freed
+immediately by reference counting when the scheduler removes them. This is
+why `--no-enable-prefix-caching` prevents the leak.
+
+### Why it only affects multimodal models visibly
+
+Each `Request` object holds a `mm_features: list[MultiModalFeatureSpec]`
+field. For vision-language models, this contains the **processed image
+feature tensors** -- several megabytes per image. A text-only request has
+empty `mm_features` and is only a few kilobytes. When cyclic garbage
+accumulates:
+
+- **Text-only**: 100 leaked Request objects ~ a few MB (invisible)
+- **VLM with images**: 100 leaked Request objects ~ hundreds of MB to GBs
+  (causes OOM)
+
+## Why It Became a Problem in v0.11.1
+
+The reference cycle existed since prefix caching was introduced. In v0.11.0,
+it was harmless because the cyclic GC ran frequently enough to clean it up.
+Two changes in v0.11.1 broke this equilibrium:
+
+### Change 1: Fewer GC-tracked objects per request (primary cause)
+
+**Commit `acaa2c0a4`** -- *"Reuse empty block lists whenever possible in
+KVCacheBlocks to mitigate GC costs"*
+
+This optimization replaced empty `list` objects (`[]`) with empty `tuple`
+objects (`()`) in `KVCacheBlocks`. Empty tuples are **not tracked by the
+cyclic GC** (CPython optimization), while empty lists are. This means each
+request cycle creates fewer GC-tracked objects.
+
+Python's cyclic GC uses a generational scheme with thresholds (default:
+`(700, 10, 10)`):
+
+- **Generation 0** collection triggers when 700+ new tracked objects
+  accumulate since the last gen-0 collection.
+- **Generation 1** triggers every 10 gen-0 collections.
+- **Generation 2** triggers every 10 gen-1 collections (every 100 gen-0's).
+
+With fewer tracked objects created per request, it takes longer for the
+generation-0 threshold (700 objects) to be reached. This means:
+- Gen-0 collections happen less often
+- Gen-1 and gen-2 collections happen much less often
+- Cyclic garbage from `Request` objects accumulates longer before being swept
+
+In v0.11.0, the extra `list` objects from `KVCacheBlocks` kept the GC
+running frequently. Gen-2 collections (which sweep long-lived cyclic
+garbage) ran often enough that the leaked `Request` memory stabilized.
+In v0.11.1, gen-2 collections became too infrequent, and memory grew
+without bound.
+
+### Change 2: Earlier gc.freeze() (secondary contributor)
+
+**Commit `b30372cbd`** -- *"Move gc.freeze logic from EngineCoreProc to
+EngineCore for better coverage"*
+
+`gc.freeze()` moves all currently tracked objects into a permanent
+generation that the GC never scans. This was moved from the end of
+`EngineCoreProc.__init__()` to the end of `EngineCore.__init__()`,
+freezing objects earlier. While this doesn't directly prevent collection
+of new `Request` objects, the different freeze timing subtly changes the
+GC's generation accounting, further reducing the frequency of collections
+on unfrozen objects.
+
+## Reproduction Results
+
+Using `Qwen/Qwen2.5-VL-3B-Instruct` with the `lmarena-ai/VisionArena-Chat`
+dataset (real user-uploaded images), 1000 prompts per round, prefix caching
+enabled:
+
+### main branch (leak present)
+
+```
+Round   Reqs     Total(GB)   EC(GB)    EC delta   EC round   Time
+----------------------------------------------------------------------
+idle    0        3.63        3.63      ---        ---
+1       1000     10.97       10.97     +7.33      +7.33      66s
+2       2000     14.34       14.34     +10.71     +3.37      57s
+3       3000     15.94       15.94     +12.31     +1.60      55s
+4       4000     16.91       16.91     +13.28     +0.97      58s
+5       5000     17.38       17.38     +13.74     +0.47      58s
+
+EngineCore final RSS: 14.70 GB (started at 2.40 GB)
+Growth rate: +1.60 GB/round average -- NEVER STABILIZES
+```
+
+### fix-cpu-leak branch (leak fixed)
+
+```
+Round   Reqs     Total(GB)   EC(GB)    EC delta   EC round   Time
+----------------------------------------------------------------------
+idle    0        3.63        3.63      ---        ---
+1       1000     9.86        9.86      +6.22      +6.22      67s
+2       2000     10.50       10.50     +6.86      +0.64      56s
+3       3000     10.55       10.55     +6.91      +0.05      57s
+4       4000     10.55       10.55     +6.92      +0.01      56s
+5       5000     10.64       10.64     +7.01      +0.09      56s
+
+EngineCore final RSS: 7.51 GB (started at 2.41 GB)
+Growth rate: +0.20 GB/round average -- STABLE after round 1
+```
+
+The fix reduces EngineCore memory by **half** (7.51 GB vs 14.70 GB) after
+5000 multimodal requests.
+
+## The Fix
+
+**Break the reference cycle** by storing `block_hasher` directly without
+`partial`, and passing `self` explicitly at call sites:
+
+```python
+# BEFORE (creates cycle):
+self.get_hash_new_full_blocks = partial(block_hasher, self)
+self.block_hashes = self.get_hash_new_full_blocks()
+
+# AFTER (no cycle):
+self._block_hasher = block_hasher
+self.block_hashes = self._block_hasher(self)
+```
+
+Without the cycle, `Request` objects are freed **immediately** by reference
+counting when the scheduler removes them -- no cyclic GC needed. This
+eliminates the leak regardless of GC frequency or `gc.freeze()` behavior.
+
+### Files changed
+
+- **`vllm/v1/request.py`** -- Store `_block_hasher` instead of
+  `partial(block_hasher, self)`. Update `append_output_token_ids()` to call
+  `self._block_hasher(self)`.
+- **`vllm/v1/core/sched/scheduler.py`** -- Update session block hash call
+  site from `session.get_hash_new_full_blocks()` to
+  `session._block_hasher(session)`.
+- **`tests/v1/core/test_async_scheduler.py`** -- Update test call site.
+
+### Verification (unit test)
+
+With cyclic GC disabled (`gc.disable()`), create 100 Request objects with
+prefix caching and delete all external references:
+
+| | main (cycle) | fix (no cycle) |
+|---|---|---|
+| Objects alive after `del` | **100** (all leaked) | **0** (all freed) |
+| Freed by `gc.collect()` | 100 | 0 (nothing to collect) |
+
+All 137 existing tests pass (86 scheduler + 43 kv_cache_utils + 8 async
+scheduler).