Heap Reference GC State Tracking for Concurrent Collection
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Solution Overview
Problem
Existing garbage collection systems cause applications to pause during non-concurrent operations, which can disrupt the execution of programs, and there is a need for methods to minimize these pauses while allowing concurrent garbage collection.
Innovation Solution
Implementing garbage collection states (colors) to track the progress of garbage collection operations with heap references, using reference load and write barriers to ensure that references are in a good state before being used, and storing GC states in a non-addressable portion of the reference to avoid disrupting application execution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If concurrent garbage collection operations are implemented, then productivity is improved by reducing pause times, but device complexity increases due to the need for tracking GC states and implementing barriers
Solution Approach 1:
The reference structure is segmented into different portions: an addressable portion for memory addresses and a non-addressable portion for GC state tracking. This segmentation allows the GC system to maintain state information without interfering with normal reference dereferencing operations, enabling concurrent GC while managing complexity through structured organization.
Solution Approach 2:
GC state tracking acts as an intermediary mechanism between the garbage collector and application threads. By introducing a dedicated state tracking system with barriers, the patent enables concurrent operations while mediating the interaction between GC progress and application execution, allowing both to proceed simultaneously without direct interference.
2Measurement precision
If GC states are stored with heap references, then measurement precision is improved for tracking GC progress, but object-affected harmful factors increase due to potential disruption of application execution
Solution Approach 1:
Different portions of the reference structure have different properties: the addressable portion maintains canonical form for safe dereferencing, while the non-addressable portion stores GC state information. This local differentiation allows precise GC state tracking in one location while maintaining safe access properties in another, preventing harmful disruptions to application execution.
Solution Approach 2:
The patent adds a new dimension to the reference structure by incorporating GC state information in the non-addressable portion. This dimensional expansion allows the system to track GC progress without affecting the addressable portion's ability to safely dereference, separating measurement functions from execution functions.
3Reliability
If reference load and write barriers are implemented, then reliability is improved by ensuring references are in good state, but device complexity increases due to additional barrier operations
Solution Approach 1:
Write barriers perform preliminary actions by ensuring GC state information is properly set before a reference is written to the heap. Load barriers perform preliminary checks to verify the reference is in a good state before dereferencing. These preliminary actions prevent reliability issues by catching state inconsistencies early, while the systematic approach manages complexity through standardized barrier implementations.
Data Source
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AI summary
Garbage collection (GC) states are stored within references stored on a heap memory to track a progress of GC operations with respect to the references. GC state may be stored in a non-addressable portion of references. Based on the GC state of a particular reference, a set of GC operations are selected and performed for the reference. However, references stored on a call stack do not include any indication of GC state. Hence, loading a reference from heap to call stack involves removing the indication of GC state. Writing a reference to heap involves adding the indication of GC state. References embedded within a compiled method also do not indicate any GC state. Metadata of the compiled method indicate a GC state, which is implicated to the embedded references. GC operations are selected and performed for each embedded reference based on the GC state of the compiled method.