Cache Coherency Wake Event Circuitry for Power Saving Master Devices
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Solution Overview
Problem
Existing multi-processing systems face inefficiencies in power consumption and performance due to continual polling of semaphores, especially in low power consumption scenarios, and issues with spurious wake-ups from broadcast send event notifications, which can lead to unnecessary power consumption and performance impacts.
Innovation Solution
A data processing apparatus within a coherent cache system that uses wake event circuitry to issue a wake event to a master device when a coherency action is taken in response to a coherency request, allowing the master device to exit a power saving mode without relying on send event notifications, thus reducing spurious wake-ups and improving power management.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a master device enters power saving mode by executing wait for event operation, then power consumption is reduced, but the device cannot respond to semaphore changes until woken by an event
Solution Approach 1:
The cache stores a copy of the semaphore data before the master device enters power saving mode. When another master device updates the semaphore, the cache coherency mechanism proactively detects this change and wakes up the sleeping master device, eliminating the need for continuous polling and reducing response time while maintaining low power consumption.
Solution Approach 2:
The system implements a feedback mechanism where cache coherency monitoring continuously watches for changes to cached semaphore data. When a change is detected, the system automatically triggers a wake event to the sleeping master device, creating a closed-loop system that balances power savings with timely response to external changes.
2Reliability
If broadcast send event notifications are used to wake master devices, then waiting master devices can be notified of semaphore changes, but spurious wake-ups occur causing unnecessary power consumption
Solution Approach 1:
Instead of using broad broadcast notifications, the system targets wake events specifically to the master device whose cached semaphore data has changed. The cache coherency mechanism identifies the specific owner of the modified cache line and sends a directed wake event only to that device, eliminating spurious wake-ups of unrelated master devices and reducing unnecessary power consumption.
Solution Approach 2:
The patent extracts the wake notification function from the general broadcast send event mechanism and integrates it directly into the cache coherency system. This allows the wake event to be generated as a natural byproduct of cache line ownership tracking, eliminating the need for separate notification infrastructure and reducing spurious wake-ups.
3Speed
If continuous polling of semaphores is performed, then master devices can detect semaphore changes immediately, but power consumption increases significantly
Solution Approach 1:
The system replaces continuous polling with periodic action triggered by cache coherency events. The master device polls the semaphore initially, then enters power saving mode, and is woken periodically only when the cache coherency mechanism detects an actual change in the semaphore data. This transforms continuous high-power polling into event-driven periodic checking, maintaining detection capability while dramatically reducing power consumption.
Solution Approach 2:
The cache coherency system automatically monitors for semaphore changes and manages the wake-up process without requiring the sleeping master device to actively poll. The system self-services the notification function by tracking cache line ownership and automatically triggering wake events when changes occur, eliminating the need for continuous active polling by the master device.
Data Source
AI summary
A data processing apparatus forms a portion of a coherent cache system and has a master device for performing data processing operations including a wait for event operation causing the master device to enter a power saving mode. A cache stores data values for access by the master device when performing the data processing operations. Coherency handling circuitry is responsive to a coherency request from another portion of the coherent cache system, to detect whether a data value identified by the coherency request is present in the cache, and if so, to cause a coherency action to be taken in respect of that data value stored in the cache. Wake event circuitry issues a wake event to the master device if the coherency action is taken, and the master device exits the power saving mode.