OS Kernel System Call Coalescing to Reduce Privilege Boundary Crossings
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Solution Overview
Problem
The frequent transitions between user mode and kernel mode in modern computer systems, necessitated by system calls, incur significant time costs that degrade the performance of user programs due to hardware latency and data copying processes.
Innovation Solution
An enhanced OS kernel that decodes and analyzes subsequent user program instructions to identify clustered system calls, allowing for their emulation in kernel mode, thereby reducing the number of CPU privilege boundary crossings by executing multiple system calls within a single pair of mode transitions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If system calls are handled through traditional user mode to kernel mode transitions, then system resource access is enabled, but execution time increases due to frequent CPU privilege boundary crossings
Solution Approach 1:
The patent merges multiple system call invocations into a single kernel mode transition by implementing coalescing logic that buffers successive system calls and batches them together. This allows multiple resource access requests to be handled in one unified kernel mode execution context, reducing the frequency of privilege boundary crossings while maintaining reliable system resource access.
Solution Approach 2:
The patent applies preliminary action by pre-fetching and buffering system call parameters and data in user mode before the kernel mode transition occurs. This preparation work is done in advance while in user mode, so that when the transition to kernel mode happens, the actual system call handling can proceed more efficiently with less data copying and processing overhead.
2Reliability
If multiple system calls are executed individually with separate mode transitions, then each system call is handled correctly, but the number of CPU privilege boundary crossings increases
Solution Approach 1:
The patent combines multiple individual system call handling operations into a single batched processing operation. The coalescing mechanism groups successive system calls that occur in rapid succession and processes them together in one kernel mode execution, maintaining the correctness of each individual system call while reducing the total number of mode transitions and improving overall program performance.
3Reliability
If data copying is performed during each mode transition, then system resources are accessed safely, but the time cost of each transition increases
Solution Approach 1:
The patent performs preliminary data preparation and buffering in user mode before the kernel mode transition. By pre-fetching parameters and preparing data structures in advance, the amount of data copying required during the actual mode transition is minimized, reducing the time cost of each transition while maintaining the safety of system resource access through proper validation and copying protocols.
Data Source
AI summary
In one set of embodiments, an operating system (OS) kernel of a computer system can receive an invocation of a system call by a user program running on the computer system. The OS kernel can further fetch a plurality of subsequent instructions that will be executed by the user program after the invocation of the system call and decode the plurality of subsequent instructions into a plurality of decoded instructions. The OS kernel can then analyze whether the plurality of decoded instructions include an additional system call invocation and whether other decoded instructions that appear between the invocation of the system call and the additional system call invocation are viable for emulation by the OS kernel.