Memory management remains a critical bottleneck in large-scale machine learning applications, particularly when implemented in garbage-collected languages like Java. This paper presents MindsEye, a hybrid memory management system that combines explicit reference counting with Java’s garbage collection to address the memory pressure challenges inherent in deep learning workloads. Our approach includes a thread-safe reference counting framework, static analysis tooling for correctness verification, and reference-aware wrappers for Java’s foundational classes. Experimental results demonstrate significant reductions in garbage collection pressure and improved memory utilization for large neural network training tasks. Additionally, we introduce novel optimizations including copy-on-write semantics for immutable objects and pressure-sensitive cache eviction that leverage the predictable deallocation patterns enabled by reference counting.

Keywords: Memory management, Reference counting, Garbage collection, Deep learning, Static analysis, Java

1. Introduction

The increasing scale of modern deep learning models has exposed fundamental limitations in traditional memory management approaches for high-level programming languages. While languages like Python and Java offer productivity advantages through automatic memory management, their garbage collection strategies often prove inadequate for the memory-intensive, performance-critical demands of neural network training and inference.

Java’s mark-and-sweep garbage collection, while suitable for many application domains, creates several challenges in machine learning contexts:

  1. Unpredictable pause times that disrupt training convergence
  2. Memory fragmentation that limits the size of allocatable tensors
  3. Delayed reclamation of large objects leading to memory pressure
  4. Lack of explicit control over resource-intensive GPU memory allocations

This paper presents a hybrid approach that augments Java’s garbage collection with explicit reference counting for memory-intensive objects, providing the benefits of deterministic deallocation while maintaining the safety guarantees of managed memory systems.

2.1 Memory Management in Machine Learning Systems

Most high-performance machine learning frameworks have gravitated toward languages with explicit memory management. TensorFlow’s core is implemented in C++, PyTorch uses C++ with Python bindings, and frameworks like JAX rely on XLA compilation to optimize memory usage patterns.

Previous attempts to address Java’s limitations in ML contexts have focused primarily on:

However, these approaches either sacrifice Java’s memory safety guarantees or fail to address the fundamental mismatch between garbage collection patterns and ML workload characteristics.

2.2 Reference Counting in Managed Languages

Reference counting has been explored in various managed language contexts, notably in Swift’s ARC (Automatic Reference Counting) and Python’s CPython implementation. However, these implementations typically operate at the language runtime level rather than as library-level solutions.

Manual reference counting in Java has been attempted in specialized contexts (Apache Arrow’s memory management, Netty’s ByteBuf), but these efforts have been domain-specific and lack the comprehensive static analysis tooling necessary for safe adoption.

3. System Design

3.1 Reference Counting Foundation

Our system centers on a ReferenceCountingBase class that provides thread-safe reference management with the following characteristics:

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public abstract class ReferenceCountingBase {
    private final AtomicInteger referenceCount = new AtomicInteger(1);
    private volatile boolean isAlive = true;
    
    public void addRef() {
        if (!isAlive) throw new IllegalStateException("Object is dead");
        referenceCount.incrementAndGet();
    }
    
    public void freeRef() {
        if (referenceCount.decrementAndGet() == 0) {
            synchronized(this) {
                if (isAlive) {
                    isAlive = false;
                    dispose();
                }
            }
        }
    }
    
    protected abstract void dispose();
}

Key design principles:

  1. Thread Safety: All reference operations use atomic primitives
  2. Single Disposal: Objects are freed exactly once using double-checked locking
  3. Liveness Checking: Operations on dead objects throw exceptions immediately
  4. Debug Support: Optional stack trace tracking for reference operations

3.2 Gradual Adoption Strategy

Rather than requiring wholesale code conversion, our approach allows incremental adoption:

  1. Resource-intensive classes (tensors, models, GPU buffers) implement ReferenceCountingBase
  2. Container classes that hold references to counted objects are recursively included
  3. Missing operations are detected at runtime through liveness checks
  4. Memory leaks are identified when objects are finalized by the garbage collector

This gradual approach reduces migration risk and allows teams to focus reference counting efforts on the most memory-critical components.

3.3 Optimizations Enabled by Reference Counting

3.3.1 Object Pooling

Deterministic deallocation enables sophisticated object pooling through our RecycleBin implementation:

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public class RecycleBin<T extends ReferenceCountingBase> {
    private final Queue<T> pool = new ConcurrentLinkedQueue<>();
    private final Supplier<T> factory;
    
    public T get() {
        T object = pool.poll();
        return object != null ? object.reset() : factory.get();
    }
    
    public void recycle(T object) {
        if (object.getReferenceCount() == 1) {
            pool.offer(object);
        } else {
            object.freeRef();
        }
    }
}

3.3.2 Copy-on-Write Semantics

Our addAndFree pattern provides efficient in-place operations for immutable objects:

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public Tensor add(Tensor other) {
    if (this.getReferenceCount() == 1) {
        // Safe to modify in-place
        return this.addInPlace(other);
    } else {
        // Create new instance
        return new Tensor(this.data.clone().add(other.data));
    }
}

This approach combines the performance benefits of mutable operations with the safety guarantees of immutable semantics.

3.3.3 Pressure-Sensitive Cache Eviction

GPU memory management integrates reference counting with usage monitoring:

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public class GPUMemoryManager {
    public void checkMemoryPressure() {
        if (getUsedMemory() > PRESSURE_THRESHOLD) {
            evictCachedKernels();
            evictIntermediateResults();
            System.gc(); // Final resort
        }
    }
    
    private void evictCachedKernels() {
        kernelCache.values().stream()
            .filter(kernel -> kernel.getReferenceCount() == 1)
            .forEach(ReferenceCountingBase::freeRef);
    }
}

4. Static Analysis Framework

4.1 Motivation

Runtime detection of reference counting errors, while useful during development, cannot guarantee correctness in production systems. To address this limitation, we developed a static analysis framework using Eclipse’s Abstract Syntax Tree (AST) infrastructure.

4.2 Analysis Challenges

Static analysis of reference counting in Java presents several unique challenges:

  1. Lambda capture semantics: Determining which references are captured and their lifecycle
  2. Stream operations: Tracking references through lazy evaluation and method chaining
  3. Exception handling: Ensuring proper cleanup in exceptional control flow
  4. Inter-procedural analysis: Tracking reference transfers across method boundaries

4.3 Implementation Approach

Our analyzer performs multi-pass analysis:

Pass 1: Reference Flow Analysis

Pass 2: Lambda and Stream Handling

Pass 3: Inter-procedural Analysis

4.4 Reference-Aware Standard Library

To address the complexity of analyzing arbitrary Java code, we developed reference-aware wrappers for foundational classes:

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public class RefList<T extends ReferenceCountingBase> implements List<T> {
    private final List<T> delegate = new ArrayList<>();
    
    @Override
    public boolean add(T element) {
        element.addRef();
        return delegate.add(element);
    }
    
    @Override
    public T remove(int index) {
        T element = delegate.remove(index);
        element.freeRef();
        return element;
    }
    
    // Additional reference-safe operations...
}

These wrappers ensure correct reference counting semantics by default, reducing the burden on static analysis and improving code safety.

5. Evaluation

5.1 Memory Usage Characteristics

We evaluated our system using representative deep learning workloads including:

Results:

5.2 Static Analysis Effectiveness

Analysis of a 100,000+ line codebase revealed:

5.3 Developer Experience

Surveys of developers using the system indicated:

6. Lessons Learned

6.1 Language Integration Challenges

Implementing reference counting as a library rather than language feature creates several ongoing challenges:

6.2 Static Analysis Limitations

Despite high accuracy rates, certain patterns remain challenging:

6.3 Adoption Barriers

The primary obstacles to wider adoption appear to be:

7. Future Work

7.1 Compiler Integration

Future work could explore deeper integration with the Java compiler to provide:

7.2 Framework Integration

Broader adoption would benefit from:

7.3 Performance Optimizations

Additional performance improvements could include:

8. Conclusion

This work demonstrates that hybrid memory management approaches can successfully address the limitations of garbage collection in memory-intensive applications. By combining explicit reference counting with comprehensive static analysis tooling, we achieve the performance benefits of manual memory management while maintaining much of the safety provided by managed languages.

The key insights from this work are:

  1. Gradual adoption strategies enable incremental migration to hybrid memory management
  2. Static analysis is essential for maintaining correctness in manual memory management systems
  3. Reference-aware standard libraries significantly reduce the complexity of correct usage
  4. Domain-specific optimizations (object pooling, copy-on-write, pressure-sensitive eviction) provide substantial performance benefits

While our approach requires additional developer discipline compared to pure garbage collection, the performance improvements for memory-intensive workloads justify this complexity. The static analysis framework reduces the risk of memory safety errors to acceptable levels for production systems.

For the broader Java ecosystem, this work suggests that hybrid approaches to memory management may be necessary to remain competitive in performance-critical domains like machine learning, scientific computing, and real-time systems.

Acknowledgments

We thank the Eclipse Foundation for their robust AST infrastructure that enabled our static analysis framework, and the broader Java community for their feedback and contributions to this work.

References

[1] Bacon, D. F., & Rajan, V. T. (2001). Concurrent cycle collection in reference counted systems. European Conference on Object-Oriented Programming.

[2] Blackburn, S. M., et al. (2006). The DaCapo benchmarks: Java benchmarking development and analysis. ACM SIGPLAN Notices.

[3] Chen, T., et al. (2015). MXNet: A flexible and efficient machine learning library for heterogeneous distributed systems. Neural Information Processing Systems.

[4] Detlefs, D., et al. (2004). Garbage-first garbage collection. ACM SIGPLAN International Symposium on Memory Management.

[5] Lattner, C., & Adve, V. (2004). LLVM: A compilation framework for lifelong program analysis & transformation. International Symposium on Code Generation and Optimization.

[6] Paszke, A., et al. (2017). Automatic differentiation in PyTorch. Neural Information Processing Systems Autodiff Workshop.

This paper is based on the MindsEye open source project, available at: [github.com/author/mindseye] An interesting parallel exists between MindsEye’s reference counting approach and Rust’s ownership system. Both tackle the fundamental problem of deterministic resource cleanup, but with different trade-offs: This comparison is particularly relevant given the MindsEye framework’s sophisticated optimization algorithms, which benefit significantly from deterministic memory management during intensive computational phases. Deterministic cleanup: Both systems ensure resources are freed immediately when no longer needed, rather than waiting for garbage collection. Zero-cost abstractions: When used properly, both approaches impose minimal runtime overhead compared to their benefits. Resource safety: Both prevent use-after-free bugs through different mechanisms—Rust at compile time, MindsEye at runtime.