Virtual Legacy Wire Interrupt Routing for SoC Power Management
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Solution Overview
Problem
Current computer systems with shared resources, such as shared memory, face issues like unfairness and starvation due to fixed priority arbitration systems, and lack of consideration for power states, leading to inefficient resource allocation and power management.
Innovation Solution
A system-on-a-chip (SoC) with a virtual legacy wire (VLW) functionality that virtualizes physical interrupts, broadcasts them only to awake agents, and uses a shared memory fabric with quality of service (QoS) and anti-starvation algorithms to ensure fair and efficient access to shared resources, considering power states and latency requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a fixed priority arbitration system is used to allocate access to shared resources, then resource allocation is simplified and deterministic, but unfairness and starvation of agents occur
Solution Approach 1:
The arbitration system dynamically adjusts priority levels based on agent states (awake/sleep) and resource availability. Instead of fixed priorities, the system modifies arbitration weights in real-time to prevent starvation while maintaining efficient resource allocation. This dynamic adaptation resolves the contradiction between deterministic allocation and fairness.
2Productivity
If arbitration grants access to shared resources without considering power state, then resource availability is maximized, but power consumption increases due to unnecessary wake-ups
Solution Approach 1:
The system applies different arbitration rules based on the local state of each agent. Awake agents are subject to normal arbitration, while sleeping agents are excluded from arbitration decisions. This localized treatment based on power state prevents unnecessary wake-ups while maintaining resource availability for active components.
Solution Approach 2:
The arbitration mechanism changes its behavior based on the power state parameter of agents. When agents are in low-power states, the arbitration logic modifies its parameters to exclude sleeping agents from resource allocation decisions, thereby reducing power consumption while maintaining system productivity.
3Ease of operation
If virtual legacy wire broadcasts interrupts to all agents, then interrupt delivery is simplified, but power consumption increases due to waking sleeping agents
Solution Approach 1:
The virtual legacy wire mechanism introduces an intermediary layer between the interrupt source and agent destinations. This intermediary filters interrupts based on agent power states, routing only relevant interrupts to awake agents while preventing wake-up of sleeping agents. This maintains simple interrupt delivery logic while reducing power consumption.
4Reliability
If round robin arbitration is used to allocate shared resources, then fairness is improved, but system complexity and latency increase
Solution Approach 1:
The system uses dynamic priority adjustment based on agent activity states rather than static round-robin scheduling. This dynamic approach achieves fairness by adapting to current system conditions (which agents are awake and need resources) without requiring complex rotation logic, thereby reducing arbitration mechanism complexity while maintaining fairness.
Data Source
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AI summary
In an example, a system on chip (SoC) includes virtual legacy wire (VLW) functionality. The VLW signal virtualizes a physical interrupt existing in legacy systems to enable a peripheral to cause a processor to enter an interrupt handling routine relevant to the peripheral. The VLW interrupt is broadcast to all cores or agents within the SoC. However, to save power, if one or more agents are asleep when the interrupt occurs, the agents are not awakened to receive the interrupt. Rather, the VLW is broadcast with a mask to exclude those agents, and the state of those agents is stored in a register or buffer. Once a power management agent notifies the VLW broadcaster that an agent has newly awakened. The VLW interrupt is then re-broadcast, with a mask that excludes all but the newly-awakened agent.