[PD] Emit inactive KV blocks for decode affinity - #50732
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Publish GPU block inactivity when the last request reference is released so external schedulers can maintain accurate decode load signals. Signed-off-by: wenkea <tianmengx12@gmail.com> Co-authored-by: Cursor <cursoragent@cursor.com>
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When prefix caching is off, cache_blocks is a no-op so Conductor never sees Decode HBM occupancy. Emit GPU BlockStored on kv_consumer promote.
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Publish GPU block inactivity when the last request reference is released so external schedulers can maintain accurate decode load signals.
Purpose
Add a new KV cache event,
BlockInactive, so external PD schedulers can track when a GPU prefix-cached block is no longer actively referenced.Today, KV event consumers can observe
BlockStoredandBlockRemoved, but cannot reliably detect that the last request released a block while it may still remain in the prefix cache. Without a refcount-zero signal, decode-affinity load accounting (active_blocks) tends to only increase or needs fragile heuristics.This PR:
BlockInactivetoKVEventBatch, emitted fromBlockPool.free_blocks()whenref_cntdrops to 0 for a hashed GPU block (medium=MEDIUM_GPU).take_events()/ ZMQ). No new scheduler-loop wiring.kv_role == "kv_producer"still emitsBlockStored/BlockRemoved, but does not emitBlockInactive, so Prefill frees do not pollute decode load signals.kv_consumer), union (kv_both), and non-PD setups continue to emit Inactive viaKVTransferConfig.should_emit_block_inactive(kv_role != "kv_producer").Related issue: #50730
Test Plan
pytest tests/v1/core/test_prefix_caching.py
-k "free_blocks_emits_block_inactive or free_blocks_skips_block_inactive or kv_transfer_should_emit_block_inactive"
-q
Coverage:
free_blocks emits BlockInactive when enable_block_inactive_events=True.
Prefill-style gate (enable_block_inactive_events=False) still emits BlockStored, but not BlockInactive.
KVTransferConfig.should_emit_block_inactive is False for kv_producer, True for kv_consumer / kv_both / default.
Test Result
Unit tests above pass locally.
Existing BlockStored / BlockRemoved behavior is unchanged.
Old KV event consumers can ignore the unknown BlockInactive type.
Validated end-to-end in a 1P1D (one Prefill + one Decode) setup. Additional tests are still in progress.
supported_models.mdandexamplesfor a new model.