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[KV Connector][LMCache] Forward failed KV loads to the LMCache scheduler side - #59603
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…ler side update_connector_output only consumed KV events from KVConnectorOutput, so LMCache's scheduler side never learned which blocks failed to load. A resumed request was then promised the same chunk again on every resume (one request: 136 times in a long-context run, stalling the engine). Pass invalid_block_ids to the LMCache engine's record_load_failures when it exists (LMCache/LMCache#5443), so those requests recompute instead of loading again. Older LMCache versions without the method are unaffected. Signed-off-by: Muhammad Ibrahim <ibrahimnd2000@gmail.com>
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@ApostaC @NickLucche @orozery could one of you take a look, and add the It's a small change to |
…alid-blocks Signed-off-by: Muhammad Ibrahim <ibrahimnd2000@gmail.com>
…alid-blocks Signed-off-by: Muhammad Ibrahim <ibrahimnd2000@gmail.com>
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Purpose
LMCacheConnectorV1.update_connector_outputonly consumes KV events fromKVConnectorOutput, so LMCache's scheduler side never learns which blocks failed to load. With LMCache CPU offload under GPU KV-cache pressure (200k-token prompts, 13 concurrent requests, one TP8 replica), a preempted request is resumed, promised the same LMCache chunk, gets 0 tokens back, and is promised the same chunk again on the next resume. One request repeated this 136 times and the engine stalled while its health check stayed green.This change passes
KVConnectorOutput.invalid_block_idsto the LMCache engine'srecord_load_failureswhen it exists (LMCache/LMCache#5443), so those requests recompute instead of loading again. Older LMCache versions without the method are unaffected (getattrguard).For the failed blocks to be recomputed rather than failing the request, use
kv_load_failure_policy="recompute"inkv_transfer_config(the defaultfailerrors the request).Test Plan
New
TestLoadFailureForwarding: forwardsinvalid_block_ids; no call without them; no error with an LMCache engine that lacksrecord_load_failures.Test Result
(vLLM 0.28.0 with the same change.)
End to end with LMCache#5443 and
kv_load_failure_policy="recompute", same workload (200k-token prompts, 13 concurrent, 52 requests, LMCache CPU cache full): 52 / 52 requests succeeded, each short load was reported once and recovered (Recovered from KV load failure: 2 request(s) rescheduled (132 tokens affected)), no repeated promises, no engine stall; max inter-token latency 575 s -> 14.7 s.