trim_overshoot: cap swa_evicted_seqlen + unit test#22900
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This reverts commit 6e3351f.
…ncates output_ids)
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…swa-cap # Conflicts: # python/sglang/srt/mem_cache/session_aware_cache.py
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/rerun-test test_streaming_session.py test_streaming_session_unit.py test_session_control.py test_session_latency.py |
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Problem
_trim_overshoot(added in #22897) capskv_committed_lenandkv_allocated_lenattarget = origin + finished_len, but leavesreq.swa_evicted_seqlenuntouched._free_tail(the match_prefix counterpart on the sameSessionAwareCache) caps all three. The asymmetry causes a SWA pool leak when a finishing round overshoots while SWA was actively evicting.swa_evicted_seqlenis a cursor —_evict_swareads it as the start position for the next eviction sweep:If overshoot pushed the cursor past the trim boundary (e.g.
swa_evicted = 42buttarget = 38),save_from_reqpropagates the stale 42 into the slot and next turn'srestore_to_reqhands it back. Next turn's prefill writes new SWA slots at positions[38, 42), but_evict_swastarts scanning from 42 — those slots are skipped forever and accumulate as a leak.Fix
One-line cap to mirror
_free_tail:This is the only other downward-cap site for
swa_evicted_seqlenoutside_free_tail(andreset_for_retract, which clears to 0). Both downward-moving paths now enforce the same invariant:swa_evicted_seqlen <= origin + committed.Test
Adds
test_trim_overshoot_postconditioncovering the full postcondition in one shot:kv_committed_len,kv_allocated_len,swa_evicted_seqlenall capped at target,output_idstruncated tofinished_len, and the page-aligned tail freed. Without the swa cap, the test fails onswa_evicted_seqlen == target(assert 42 == 38).