Virtual Programmable Interrupt Timer Catch-Up Mode
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Solution Overview
Problem
Existing methods for maintaining accurate time in virtual machines are prone to time drift and 'interrupt storms' due to non-deterministic timing behavior caused by virtualization and host activity, leading to performance issues and incompatibility with multiple operating systems.
Innovation Solution
A method that adaptively corrects time loss in virtual machines by running the programmable interrupt timer at a higher frequency than nominal, using a feedback mechanism to apply time corrections only when necessary, and monitoring the interrupt rate to prevent interrupt storms, while maintaining transparency to the guest operating system.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the virtual programmable interrupt timer relies on periodic host operating system callback mechanism, then the virtualization service can emulate the programmable interrupt timer for virtual machines, but the virtual system experiences non-deterministic timing behavior and time loss due to preemption by host activity
Solution Approach 1:
The patent applies preliminary action by pre-calculating and storing expected timer interrupt times in a queue before they are needed. The virtualization service maintains a queue of pending timer interrupts with their scheduled times, allowing it to proactively deliver interrupts at the correct moments rather than relying on periodic callbacks that may be preempted. This eliminates time loss by ensuring interrupts are delivered exactly when expected, even during host preemption.
Solution Approach 2:
The patent replaces the mechanical periodic callback mechanism with a software-based event queue system. Instead of relying on hardware-like periodic interrupts from the host, the system uses a software queue that tracks expected interrupt times and delivers them when appropriate. This substitution allows precise timing control independent of host preemption cycles.
2Productivity
If the virtual programmable interrupt timer interrupt is delayed past the next expected timer period, then the interrupt arrival rate becomes less than nominal, but the interrupt merges with the next virtual programmable interrupt timer interrupt causing time loss
Solution Approach 1:
The system pre-calculates all expected timer interrupt times and stores them in a queue before execution. By having the complete schedule of expected interrupts ready in advance, the virtualization service can compare actual delivery times against the pre-calculated schedule and immediately identify any delays or merges, preventing time loss accumulation.
Solution Approach 2:
The patent implements feedback by continuously monitoring the actual interrupt delivery time against the expected interrupt time from the queue. When a delay or merge is detected, the system uses this feedback information to adjust subsequent interrupt scheduling, ensuring time accuracy is maintained despite variations in host preemption patterns.
3Measurement precision
If guest operating system time is suddenly jumped forward when time is corrected, then time accuracy is improved, but domain controllers and other time-sensitive programs are intolerant to sudden time changes
Solution Approach 1:
The system pre-calculates the exact moment when time correction should be applied based on the queue of expected interrupts. Rather than correcting time arbitrarily when drift is detected, the correction is scheduled to occur at a pre-determined future point in the interrupt queue, allowing the system to prepare for the correction and minimize disruption to time-sensitive operations.
Solution Approach 2:
The patent makes the time correction mechanism dynamic by adjusting the correction amount and timing based on actual measured drift and the current state of the interrupt queue. The system can apply corrections gradually over multiple interrupts or delay corrections until a suitable moment, rather than applying fixed corrections, thereby reducing sudden jumps while maintaining accuracy.
4Measurement precision
If the virtualization service runs the programmable interrupt timer at a higher frequency than nominal, then time accuracy is improved, but interrupt storms may occur disrupting guest operating system operations
Solution Approach 1:
The system pre-calculates the exact number and timing of interrupts needed to correct time drift based on the measured error and the queue of expected interrupts. By having this schedule prepared in advance, the system can deliver multiple interrupts in succession to catch up on lost time without creating an uncontrolled interrupt storm, as the total number of interrupts is predetermined.
Solution Approach 2:
The patent applies partial action by delivering only the specific number of additional interrupts needed to correct the measured time drift, rather than continuously running the timer at higher frequency. The system calculates the precise correction amount needed and delivers exactly that many interrupts, avoiding excessive action that would cause interrupt storms while still achieving time accuracy.
Data Source
AI summary
A catch-up mode that runs a virtual programmable interrupt timer faster than a nominal rate to prevent time loss in a virtual machine can be implemented. If time loss is determined, a catch-up mode can be initiated to cause increased firings, beyond a nominal rate, of the programmable interrupt timer to adjust the clock of the virtual machine to the clock of the host system. The virtual programmable interrupt timer can also be readjusted to a predetermined nominal rate when the time loss in the guest operating system is determined approximately within a predetermined tolerance range. The catch-up mode can be monitored to avoid “interrupt storms” on the virtual machine. The virtual programmable interrupt timer can be altered by the guest operating system to accommodate different operating systems.