Page-Protected Memory Barrier Traps for Garbage Collection Latency
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Solution Overview
Problem
Conventional garbage collection mechanisms face inefficiencies, particularly in concurrent page-based memory access, where memory access barrier traps are expensive in terms of cycle time and latency, and do not scale well in multiprocessor systems, leading to performance degradation.
Innovation Solution
Modifying the operating system kernel to implement page-protection based memory access barrier traps by computing a user-mode bit for each virtual page, allowing for fast translation of access protection faults into user-traps, thereby reducing the need for costly kernel-mode triage and improving scalability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional memory access barrier traps are used during garbage collection, then memory access safety is ensured, but cycle time and latency increase significantly
Solution Approach 1:
The patent segments the trap handling process by introducing a flag mechanism that distinguishes between GC-related access barriers and other access protection faults. This segmentation allows the system to quickly identify and handle GC barriers without full kernel-mode triage, reducing latency while maintaining safety.
Solution Approach 2:
The patent performs preliminary action by setting a flag in the page table entry before the actual garbage collection operation. This pre-marking allows the trap handler to immediately recognize GC-related accesses and route them appropriately without requiring complex analysis during trap handling, thus reducing cycle time.
2Reliability
If conventional memory access barrier traps are used during garbage collection, then memory access control is maintained, but scalability in multiprocessor systems deteriorates
Solution Approach 1:
The patent segments the trap handling workload by introducing a classification mechanism (the flag) that separates GC-related traps from other traps. This segmentation enables parallel processing of different trap types across multiple processors, improving scalability without compromising access control.
Solution Approach 2:
The patent introduces a flag as an intermediary mechanism between the garbage collection process and the trap handler. This intermediary allows the system to communicate GC-related information through the page table without requiring complex inter-processor communication protocols, thereby improving scalability in multiprocessor environments.
3Productivity
If page-protection based memory access barrier traps are implemented with flag computation, then performance and scalability are improved, but kernel complexity increases
Solution Approach 1:
The patent uses a simplified copying approach by replicating the flag bit in the page table entry structure. This copying mechanism allows the kernel to track GC-related pages without requiring complex data structures or additional kernel components, thus limiting the increase in kernel complexity.
Solution Approach 2:
The patent changes a simple parameter (adding a flag bit to page table entries) to enable the enhanced trap handling functionality. This minimal parameter change approach achieves performance and scalability improvements while avoiding significant increases in kernel complexity.
Data Source
AI summary
A method, apparatus and computer program product for providing page-protection based memory access barrier traps is presented. A value for a user-mode bit (u-bit) is computed for each extant virtual page in an address space, the u-bit indicative that an object on the virtual page is being moved by a Garbage Collector process. An instruction is executed which causes an access protection fault. The state of the u-bit for the virtual page associated with the access protection fault is consulted when the access protection fault is encountered. Additionally, the access protection fault is translated into a user-trap (utrap) and the utrap is serviced when the u-bit is set.