Maybe fix for improper reuse #8830
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Signed-off-by: thorjohnsen <41591019+thorjohnsen@users.noreply.github.com>
📝 WalkthroughWalkthroughChanges refine block lifecycle management in the eviction policy by conditionally clearing free block iterators, adjusting duration parameter handling, and ensuring blocks are properly re-queued and re-parameterized when matched or copied during cache operations. Changes
Estimated code review effort🎯 3 (Moderate) | ⏱️ ~25 minutes
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⚠️ Outside diff range comments (1)
cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp (1)
1248-1266: Don’t release the freshly cloned blockIn the partial-copy path we overwrite
matchingBlockwith the new clone before the release check, so the subsequentreleaseBlockpushes that newly allocated block straight back to the free queue while we continue using it. That opens the door to double allocation, data corruption, and the exact cache state reuse bug this PR is trying to eliminate. Please ensure the release (if any) happens before reassigning the pointer so the original shared block is considered, not the clone.auto newBlock = getFreeBlock( sequence, matchingBlock->getPriority(), matchingBlock->getDurationMs(), mode, directory); mTransferManager->onboard(matchingBlock, newBlock, mPools, numMatched, mode, directory); // TODO: (optional) Send out event - matchingBlock = newBlock; - if (blockItr != blockKeys.end()) - { - matchingBlock->setBlockKey( - *blockItr, blockItr->uniqueTokens.size() == static_cast<size_t>(mTokensPerBlock)); - } - matchingBlock->setHash(); - if (!matchingBlock->hasRefs()) - { - // Return matchingBlock to free queue since we have copied it and won't be using matchingBlock itself - mEvictionPolicy->releaseBlock(matchingBlock); - } + if (!matchingBlock->hasRefs()) + { + // Return the original block to the free queue; we are switching to the clone. + mEvictionPolicy->releaseBlock(matchingBlock); + } + matchingBlock = newBlock; + if (blockItr != blockKeys.end()) + { + matchingBlock->setBlockKey( + *blockItr, blockItr->uniqueTokens.size() == static_cast<size_t>(mTokensPerBlock)); + } + matchingBlock->setHash();
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cpp/tensorrt_llm/batch_manager/evictionPolicy.cpp(1 hunks)cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp(3 hunks)
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cpp/tensorrt_llm/batch_manager/evictionPolicy.cppcpp/tensorrt_llm/batch_manager/kvCacheManager.cpp
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🧠 Learnings (8)
📓 Common learnings
Learnt from: thorjohnsen
Repo: NVIDIA/TensorRT-LLM PR: 6910
File: cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp:0-0
Timestamp: 2025-08-14T21:04:50.248Z
Learning: In KV cache onboarding logic during prefill in cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp, when calculating which blocks fall within the attention window, use getTokensPerBlock() to advance token indices rather than block->getUniqueTokens().size(), because the calculation needs to consider the post-prefill state where blocks will be filled to capacity, not their current token count.
Learnt from: eopXD
Repo: NVIDIA/TensorRT-LLM PR: 6768
File: cpp/include/tensorrt_llm/batch_manager/kvCacheManager.h:0-0
Timestamp: 2025-08-20T06:48:45.368Z
Learning: There is a planned refactoring to move cache block bookkeeping utilities from BlockManager/WindowBlockManager into the GenerationRequest class itself to improve code organization and make responsibilities clearer.
Learnt from: eopXD
Repo: NVIDIA/TensorRT-LLM PR: 6768
File: cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp:2010-2045
Timestamp: 2025-08-21T09:41:49.347Z
Learning: In cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp, updateSequenceCacheBlockOffsets is specifically for updating bookkeeping when blocks are added during the context phase, not for refreshing offsets after detach operations. During detach operations, GenerationRequest::removeFrontBlock handles the necessary cache block bookkeeping internally.
📚 Learning: 2025-08-21T09:41:49.347Z
Learnt from: eopXD
Repo: NVIDIA/TensorRT-LLM PR: 6768
File: cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp:2010-2045
Timestamp: 2025-08-21T09:41:49.347Z
Learning: In cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp, updateSequenceCacheBlockOffsets is specifically for updating bookkeeping when blocks are added during the context phase, not for refreshing offsets after detach operations. During detach operations, GenerationRequest::removeFrontBlock handles the necessary cache block bookkeeping internally.
Applied to files:
cpp/tensorrt_llm/batch_manager/evictionPolicy.cppcpp/tensorrt_llm/batch_manager/kvCacheManager.cpp
📚 Learning: 2025-08-20T06:48:45.368Z
Learnt from: eopXD
Repo: NVIDIA/TensorRT-LLM PR: 6768
File: cpp/include/tensorrt_llm/batch_manager/kvCacheManager.h:0-0
Timestamp: 2025-08-20T06:48:45.368Z
Learning: In cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp, updateSequenceCacheBlockOffsets is only called when adding a sequence, not during detach operations. During detach, the cache block bookkeeping is handled by GenerationRequest::removeFrontBlock.
Applied to files:
cpp/tensorrt_llm/batch_manager/evictionPolicy.cppcpp/tensorrt_llm/batch_manager/kvCacheManager.cpp
📚 Learning: 2025-08-20T06:56:02.889Z
Learnt from: eopXD
Repo: NVIDIA/TensorRT-LLM PR: 6768
File: cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp:577-579
Timestamp: 2025-08-20T06:56:02.889Z
Learning: In cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp, maxSequenceLength is now enforced as a non-optional argument in the BlockManager constructor, so concerns about std::nullopt defaulting to 0 are not applicable. When windowSize > maxSequenceLength, a warning should be added instead of handling optional parameter cases.
Applied to files:
cpp/tensorrt_llm/batch_manager/evictionPolicy.cppcpp/tensorrt_llm/batch_manager/kvCacheManager.cpp
📚 Learning: 2025-08-15T06:46:54.897Z
Learnt from: eopXD
Repo: NVIDIA/TensorRT-LLM PR: 6767
File: cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp:0-0
Timestamp: 2025-08-15T06:46:54.897Z
Learning: In cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp addToken function, newly allocated blocks are unshared by design. The beam search path in addToken (when sequence.getNumTokens() > windowSize) is currently broken/non-functional with SWA, so the block allocation doesn't follow a shared-then-unshared pattern.
Applied to files:
cpp/tensorrt_llm/batch_manager/evictionPolicy.cppcpp/tensorrt_llm/batch_manager/kvCacheManager.cpp
📚 Learning: 2025-08-14T21:04:50.248Z
Learnt from: thorjohnsen
Repo: NVIDIA/TensorRT-LLM PR: 6910
File: cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp:0-0
Timestamp: 2025-08-14T21:04:50.248Z
Learning: In KV cache onboarding logic during prefill in cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp, when calculating which blocks fall within the attention window, use getTokensPerBlock() to advance token indices rather than block->getUniqueTokens().size(), because the calculation needs to consider the post-prefill state where blocks will be filled to capacity, not their current token count.
Applied to files:
cpp/tensorrt_llm/batch_manager/evictionPolicy.cppcpp/tensorrt_llm/batch_manager/kvCacheManager.cpp
📚 Learning: 2025-08-06T08:18:28.669Z
Learnt from: zhengd-nv
Repo: NVIDIA/TensorRT-LLM PR: 6633
File: cpp/tensorrt_llm/batch_manager/dataTransceiverImpl.cpp:145-155
Timestamp: 2025-08-06T08:18:28.669Z
Learning: In cpp/tensorrt_llm/batch_manager/dataTransceiverImpl.cpp, the existing `mMtxForMap` mutex in DataSenderImpl is sufficient to synchronize measurement file operations in the `release` method, as all file operations occur within the same critical section that protects the `mRequestToSession` map access.
Applied to files:
cpp/tensorrt_llm/batch_manager/evictionPolicy.cppcpp/tensorrt_llm/batch_manager/kvCacheManager.cpp
📚 Learning: 2025-08-20T06:48:45.368Z
Learnt from: eopXD
Repo: NVIDIA/TensorRT-LLM PR: 6768
File: cpp/include/tensorrt_llm/batch_manager/kvCacheManager.h:0-0
Timestamp: 2025-08-20T06:48:45.368Z
Learning: There is a planned refactoring to move cache block bookkeeping utilities from BlockManager/WindowBlockManager into the GenerationRequest class itself to improve code organization and make responsibilities clearer.
Applied to files:
cpp/tensorrt_llm/batch_manager/kvCacheManager.cpp
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- GitHub Check: Pre-commit Check
| if (durationMs.has_value()) | ||
| { | ||
| block->setDurationMs(durationMs); | ||
| } |
There was a problem hiding this comment.
Duration must be reset when no value is provided
With this change a caller can no longer clear a block’s duration by passing std::nullopt; the old value survives the claim. The second claimBlock call in the reuse paths now leaves whatever retention duration the block had previously, so blocks can continue to expire unexpectedly under the wrong policy. Please restore the ability to reset the optional.
- if (durationMs.has_value())
- {
- block->setDurationMs(durationMs);
- }
+ block->setDurationMs(durationMs);📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| if (durationMs.has_value()) | |
| { | |
| block->setDurationMs(durationMs); | |
| } | |
| block->setDurationMs(durationMs); |
🤖 Prompt for AI Agents
In cpp/tensorrt_llm/batch_manager/evictionPolicy.cpp around lines 188 to 191,
the current logic only sets block->setDurationMs(durationMs) when
durationMs.has_value(), which prevents callers from clearing a previously set
duration; change the branch so that when durationMs has a value you set it on
the block, and when it does not you explicitly clear/reset the block’s duration
(e.g., call the block’s reset/clear-duration API or set its optional duration to
std::nullopt) so an absence of duration truly removes the prior retention.
1 participant
Summary by CodeRabbit
Bug Fixes
Description
This is a possible fix for a corner case were we end up reusing the wrong KV cache state when the reusable block has been offloaded. This happens when we are trying to reuse an offloaded block that is also at the front of the secondary free queue and the block at the front of the primary free queue has stored KV state. In this case we end up reusing the state of the block at the front of the primary free queue instead of the state of the matched block.
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