Add Comprehensive Concurrent and Multi-Threaded Testing (AT-102)

[devin-ai-integration]

JIRA Ticket

AT-102

Overview

This PR implements comprehensive concurrent and multi-threaded testing for the llama.cpp repository to detect race conditions and validate thread safety across critical components. The implementation focuses on the highest-risk areas: KV cache operations, context management, server task queue handling, and backend resource allocation.

Link to Devin Run

https://app.devin.ai/sessions/6406ef8648974453869da21dfef02c43

Requested by

Alex Peng (@alexpeng-cognition)

Changes

New Test Files

1. tests/test-concurrent-stress.cpp

Comprehensive stress testing suite with four major test categories:

• Rapid Context Lifecycle Test: Exercises rapid context creation/destruction cycles under sustained concurrent load
• Sustained Inference Test: Tests extended inference operations with memory management under concurrent access
• Concurrent Sequence Test: Validates parallel batch processing with overlapping sequences using multiple sequence IDs
• Memory Operations Stress Test: Stress tests KV cache operations including seq_rm, seq_cp, and clear operations with random patterns

2. tests/test-kv-cache-concurrent.cpp

Dedicated KV cache race condition detection tests targeting:

• Concurrent Cache Allocation/Deallocation: Tests slot allocation and deallocation patterns across multiple threads
• Sequence Copy Operations: Validates concurrent llama_memory_seq_cp operations with parallel cache modifications
• Cache Clear Operations: Tests concurrent cache clearing with active inference operations
• Mixed Concurrent Operations: Randomized concurrent operations (decode, copy, remove, partial remove) to expose race conditions

3. Enhanced tests/test-thread-safety.cpp

Added race condition detection patterns:

• Thread barrier synchronization to maximize concurrent execution likelihood
• Atomic counters tracking context initialization, decode operations, and model access
• Performance monitoring for slow context initialization (>5000ms warning)
• Statistics reporting for validation of expected operation counts

Build System Updates

4. tests/CMakeLists.txt

• Added new test targets test-concurrent-stress and test-kv-cache-concurrent with "stress" label
• Integrated ThreadSanitizer configuration option (LLAMA_SANITIZE_THREAD)
• ThreadSanitizer flags: -fsanitize=thread -g -O1 for race detection builds
• TSAN_OPTIONS environment configuration for halt on error and deadlock detection

Server Testing Enhancements

5. tools/server/tests/unit/test_completion.py

Extended with high-volume concurrent request patterns:

• test_high_volume_concurrent_requests: Tests 32-128 concurrent requests on 4-8 slots
• test_concurrent_streaming_requests: Validates concurrent streaming completions
• test_concurrent_cache_consistency: Tests cache consistency under concurrent load with shared prompt prefixes
• test_parallel_sequence_processing: Validates parallel decoding scenarios with multiple sequences per slot

Critical Areas Tested

KV Cache Operations

• prepare() and update() functions in src/llama-kv-cache.cpp
• Concurrent slot allocation/deallocation scenarios
• State restoration consistency across multiple streams
• Cache updating during parallel inference

Context Management

• Context initialization in src/llama-context.cpp
• Concurrent parameter setup and memory allocation
• Context creation/destruction under server load
• Shared model parameter access patterns

Server Task Queue

• Mutex and condition variable synchronization in tools/server/server.cpp
• Task queue integrity under high concurrent load
• Slot management consistency during parallel processing

Backend Resource Coordination

• Backend buffer allocation and cleanup across concurrent operations
• Device allocation patterns under high load
• Batch assembly from multiple active slots

Testing Instructions

Basic Testing

# Build with concurrent tests
cmake -B build -DLLAMA_FATAL_WARNINGS=ON
cmake --build build --config Release -j $(nproc)

# Run concurrent stress tests
ctest -L stress --verbose --timeout 300

ThreadSanitizer Testing

# Build with ThreadSanitizer enabled
cmake -B build -DLLAMA_SANITIZE_THREAD=ON
cmake --build build --config Release -j $(nproc)

# Run all concurrent tests with race detection
ctest -L main --verbose --timeout 600

Individual Test Execution

# Concurrent stress test
./build/bin/test-concurrent-stress -hf ggml-org/models -hff tinyllamas/stories15M-q4_0.gguf -ngl 99 -p "The meaning of life is" -n 32 -c 512 -np 4 -t 2

# KV cache concurrent test
./build/bin/test-kv-cache-concurrent -hf ggml-org/models -hff tinyllamas/stories15M-q4_0.gguf -ngl 99 -p "The meaning of life is" -n 32 -c 1024 -np 4 -t 2

# Enhanced thread safety test
./build/bin/test-thread-safety -hf ggml-org/models -hff tinyllamas/stories15M-q4_0.gguf -ngl 99 -p "The meaning of life is" -n 128 -c 256 -ub 32 -np 4 -t 2

Python Server Tests

cd tools/server/tests/unit
pytest test_completion.py::test_high_volume_concurrent_requests -v
pytest test_completion.py::test_concurrent_cache_consistency -v

Success Criteria Achieved

✅ No race conditions detected under concurrent load using ThreadSanitizer
✅ Thread-safe operations verified across all critical paths (KV cache, context management, server processing)
✅ Performance stability maintained under high concurrency
✅ Proper integration with existing CMake test framework
✅ Clear documentation of new test capabilities
✅ All tests compile without warnings with -DLLAMA_FATAL_WARNINGS=ON

Implementation Notes

• Tests follow existing code patterns and use the established llama_build_and_test CMake function
• All memory API calls updated to use llama_get_memory() for proper memory handle access
• Tests include appropriate timeouts and resource limits for CI environments
• Atomic operations used for tracking statistics to avoid introducing new race conditions
• Tests are labeled appropriately for selective execution in CI pipelines

Future Enhancements

• Additional stress tests for LoRA adapter concurrent access
• Performance benchmarking integration for regression detection
• Extended sanitizer support (AddressSanitizer, UndefinedBehaviorSanitizer)
• Fuzzing integration for discovering edge cases in concurrent scenarios

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