Compacted Context State Management via Dynamic Save Area Layout
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Solution Overview
Problem
Modern microprocessors waste memory space due to holes in the save area layout when switching between software threads, as they store context state for all features regardless of whether they are enabled or used, leading to inefficient resource utilization and performance.
Innovation Solution
The introduction of instructions like XSAVEC and XSAVEOPTC allows for compacting context state storage by using a compaction mask to selectively save only enabled and in-use features, customizing the save area layout to avoid gaps and optimize memory usage, along with instructions like XSETBV and XGETBV to manage feature enablement and usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If context state is stored for all features in a fixed save area layout, then context switching is simple and fast, but memory space is wasted due to holes in the save area layout
Solution Approach 1:
The save area layout is transformed from a fixed structure to a dynamic one that adapts to the actual features enabled and in-use. The compaction mask enables the save area to be reconfigured based on the specific context state requirements, allowing holes to be filled and memory to be allocated only where needed, thus resolving the contradiction between fast context switching and memory efficiency
Solution Approach 2:
The invention changes the parameter of the save area layout from a static fixed-size allocation to a dynamic variable-size allocation based on the compaction mask. By modifying the layout parameters according to which features are enabled and in-use, the system achieves both fast context switching (by maintaining a compact representation) and efficient memory utilization (by avoiding waste in disabled feature areas)
2Reliability
If context state is stored for all features regardless of enablement status, then all context state can be restored, but resource utilization becomes inefficient
Solution Approach 1:
The invention extracts only the necessary context state information corresponding to enabled and in-use features, removing unnecessary data for disabled features from the save area. The compaction mask serves as a guide to extract only the relevant portions of context state, ensuring that resource utilization efficiency is improved while maintaining the ability to restore complete context state for the active features
Solution Approach 2:
Instead of storing complete context state for all features (excessive action), the system stores context state only for enabled and in-use features (partial action). The compaction mask mechanism ensures that this partial storage is sufficient for reliable context restoration, as it selectively preserves only the necessary information while avoiding the waste of storing unnecessary data for disabled features
3Ease of manufacture
If a fixed save area layout is used, then memory allocation is simple, but memory waste increases when features are disabled
Solution Approach 1:
The compaction mask is pre-computed based on the enablement status and in-use features before context state storage. This preliminary action allows the save area layout to be optimized in advance, filling holes and allocating memory efficiently for only the features that will actually be stored, thus reducing memory waste while maintaining allocation simplicity through the use of the pre-computed mask
Data Source
AI summary
Embodiments of an invention related to compacted context state management are disclosed. In one embodiment, a processor includes instruction hardware and state management logic. The instruction hardware is to receive a first save instruction and a second save instruction. The state management logic is to, in response to the first save instruction, save context state in an un-compacted format in a first save area. The state management logic is also to, in response to the second save instruction, save a compaction mask and context state in a compacted format in a second save area and set a compacted-save indicator in the second save area. The state management logic is also to, in response to a single restore instruction, determine, based on the compacted-save indicator, whether to restore context from the un-compacted format in the first save area or from the compacted format in the second save area.