Add message when optimal solution is found during presolve #623

adityasinghz · 2025-11-24T08:04:34Z

Description

Added an explicit log message "Optimal solution found during presolve" that is displayed when the problem is fully reduced during presolve (i.e., when the problem is reduced to 0 constraints, 0 variables, and 0 nonzeros). This provides clear feedback to users that the optimal solution was discovered during presolve rather than through the main solving process.

Changes Made:

cpp/src/mip/solve.cu: Added log message when problem.empty is true before running the MIP solver
cpp/src/mip/solver.cu: Added log message in two locations:
- When context.problem_ptr->empty is true before running presolve
- When context.problem_ptr->empty becomes true after running presolve

Example Output:
Before this change, when the optimal solution was found during presolve, users would see:

Presolve status: reduced the problem
Presolve removed: 100 constraints, 200 variables, 560 nonzeros
Presolved problem: 0 constraints, 0 variables, 0 nonzeros
Best feasible: 37.000000

After this change, users will now see:

Presolve status: reduced the problem
Presolve removed: 100 constraints, 200 variables, 560 nonzeros
Presolved problem: 0 constraints, 0 variables, 0 nonzeros
Optimal solution found during presolve
Problem fully reduced in presolve
Best feasible: 37.000000

Issue #524

Checklist

I am familiar with the Contributing Guidelines.
Testing
- New or existing tests cover these changes
- Added tests
- Created an issue to follow-up
- NA
Documentation
- The documentation is up to date with these changes
- Added new documentation
- NA

Summary by CodeRabbit

Bug Fixes
- When preprocessing fully eliminates a problem (no rows or columns remain), the solver now clearly notifies that an optimal solution was found during presolve.
- Improved presolve logging to make outcomes and solver status easier to interpret during initial processing.

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copy-pr-bot · 2025-11-24T08:04:38Z

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rg20

Move the logic of checking if presolver found solution to presolve caller or presolve function itself.

cpp/src/mip/solve.cu

cpp/src/mip/solver.cu

Move the 'Optimal solution found during presolve' message to the presolve function itself (third_party_presolve.cpp) where we can accurately detect when presolve finds the optimal solution. This ensures the message only appears when presolve actually found the solution, not when the problem is empty for other reasons. Removed the message from generic empty problem checks in: - cpp/src/mip/solve.cu (run_mip function) - cpp/src/mip/solver.cu (run_solver function - both locations)

rg20 · 2025-11-24T14:50:55Z

/ok to test 527cd60

rgsl888prabhu · 2025-11-25T12:48:06Z

@adityasinghz Is this meant for 25.12 release or 26.02.

If it is meant for 25.12, may I request you to close this PR and use branch release/25.12 branch as your base and create a PR? And also choose merge branch to be 25.12.

adityasinghz · 2025-11-26T11:36:37Z

Hi @rgsl888prabhu,

Thank you for the question. This is a user experience improvement (adding a log message when optimal solution is found during presolve), so I'm flexible on which release it targets.

Option 1: If this should go into 25.12:
I can rebase this onto release/25.12 and create a new PR targeting that branch. Should I proceed with this?

Option 2: If this can wait for 26.02:
I can keep the current PR targeting main (which will be part of 26.02).
Could you please let me know which release you'd prefer? I'm happy to adjust accordingly.

Thanks!

rgsl888prabhu · 2025-12-03T08:01:18Z

Lets move it to 26.2 since 25.12 is close to code freeze.

coderabbitai · 2025-12-03T08:04:33Z

📝 Walkthrough

Walkthrough

After presolve reductions are logged, the presolve routine now checks whether the problem has been reduced to zero rows and zero columns and logs "Optimal solution found during presolve" if so. Also updated header copyright year range.

Changes

Cohort / File(s)	Summary
Presolve logging enhancement `cpp/src/mip/presolve/third_party_presolve.cpp`	In `third_party_presolve_t<i_t,f_t>::apply`, after logging presolve reductions, added a check that when both row and column counts are zero it logs "Optimal solution found during presolve". Also extended header copyright year range to 2025-2026. No control-flow or behavior changes beyond logging.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run `@coderabbitai generate docstrings` to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding a log message when presolve fully reduces the problem to zero rows and columns, indicating an optimal solution was found.

✨ Finishing touches

📝 Generate docstrings

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📥 Commits

Reviewing files that changed from the base of the PR and between 17a7c33 and 1d85420.

📒 Files selected for processing (1)

cpp/src/mip/presolve/third_party_presolve.cpp

🧰 Additional context used

📓 Path-based instructions (3)

**/*.{cu,cuh,cpp,hpp,h}

📄 CodeRabbit inference engine (.github/.coderabbit_review_guide.md)

**/*.{cu,cuh,cpp,hpp,h}: Track GPU device memory allocations and deallocations to prevent memory leaks; ensure cudaMalloc/cudaFree balance and cleanup of streams/events
Validate algorithm correctness in optimization logic: simplex pivots, branch-and-bound decisions, routing heuristics, and constraint/objective handling must produce correct results
Check numerical stability: prevent overflow/underflow, precision loss, division by zero/near-zero, and use epsilon comparisons for floating-point equality checks
Validate correct initialization of variable bounds, constraint coefficients, and algorithm state before solving; ensure reset when transitioning between algorithm phases (presolve, simplex, diving, crossover)
Ensure variables and constraints are accessed from the correct problem context (original vs presolve vs folded vs postsolve); verify index mapping consistency across problem transformations
For concurrent CUDA operations (barriers, async operations), explicitly create and manage dedicated streams instead of reusing the default stream; document stream lifecycle
Eliminate unnecessary host-device synchronization (cudaDeviceSynchronize) in hot paths that blocks GPU pipeline; use streams and events for async execution
Assess algorithmic complexity for large-scale problems (millions of variables/constraints); ensure O(n log n) or better complexity, not O(n²) or worse
Verify correct problem size checks before expensive GPU/CPU operations; prevent resource exhaustion on oversized problems
Identify assertions with overly strict numerical tolerances that fail on legitimate degenerate/edge cases (near-zero pivots, singular matrices, empty problems)
Ensure race conditions are absent in multi-GPU code and multi-threaded server implementations; verify proper synchronization of shared state
Refactor code duplication in solver components (3+ occurrences) into shared utilities; for GPU kernels, use templated device functions to avoid duplication
Check that hard-coded GPU de...

Files:

cpp/src/mip/presolve/third_party_presolve.cpp

**/*.{cpp,hpp,h}

📄 CodeRabbit inference engine (.github/.coderabbit_review_guide.md)

**/*.{cpp,hpp,h}: Check for unclosed file handles when reading MPS/QPS problem files; ensure RAII patterns or proper cleanup in exception paths
Validate input sanitization to prevent buffer overflows and resource exhaustion attacks; avoid unsafe deserialization of problem files
Prevent thread-unsafe use of global and static variables; use proper mutex/synchronization in server code accessing shared solver state

Files:

cpp/src/mip/presolve/third_party_presolve.cpp

**/*.{cu,cpp,hpp,h}

📄 CodeRabbit inference engine (.github/.coderabbit_review_guide.md)

Avoid inappropriate use of exceptions in performance-critical GPU operation paths; prefer error codes or CUDA error checking for latency-sensitive code

Files:

cpp/src/mip/presolve/third_party_presolve.cpp

🧠 Learnings (17)

📓 Common learnings

Learnt from: CR
Repo: NVIDIA/cuopt PR: 0
File: .github/.coderabbit_review_guide.md:0-0
Timestamp: 2025-11-25T10:20:49.822Z
Learning: Applies to **/*.{cu,cuh,cpp,hpp,h} : Validate algorithm correctness in optimization logic: simplex pivots, branch-and-bound decisions, routing heuristics, and constraint/objective handling must produce correct results