Demote Instruction for Cache Line Ownership Transfer
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Solution Overview
Problem
In large SMP systems, delays occur when multiple processes update the same cache lines, as processors wait for cross invalidate acknowledgments to release exclusive ownership, leading to performance degradation.
Innovation Solution
A new instruction, 'demote', allows software to indicate that a cache line is no longer needed for storage updates, enabling the processor to actively release exclusive ownership and update the cache state, allowing subsequent requests for exclusive ownership without waiting for explicit cross invalidate acknowledgments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If processors use cross invalidate acknowledgments to release exclusive ownership of cache lines, then memory consistency is maintained, but processing delays occur and performance degrades
Solution Approach 1:
The patent applies preliminary action by having the storage controller proactively change cache line ownership status from exclusive to shared before actually releasing the line. This advance preparation eliminates the need for processors to wait for cross invalidate acknowledgments, as the ownership change is already completed by the time the releasing processor finishes its operation. The storage controller performs this ownership transition in advance, allowing subsequent exclusive ownership requests to be granted immediately without blocking on acknowledgment signals.
2Reliability
If processors wait for cross invalidate acknowledgments to transfer exclusive ownership, then cache coherency is maintained, but cache line sharing overhead increases
Solution Approach 1:
The patent applies self-service by having the storage controller autonomously manage the ownership transition without requiring active participation or acknowledgment from the releasing processor. When a processor completes its store operation, the storage controller automatically detects this condition and initiates the ownership change from exclusive to shared status. This self-service mechanism eliminates the need for complex acknowledgment protocols and reduces the overhead associated with tracking and validating ownership transfers across multiple processors.
Data Source
AI summary
Systems and Program Products are created to execute a prefetch data machine instruction having an M field performs a function on a cache line of data specifying an address of an operand. The operation comprises either prefetching a cache line of data from memory to a cache or reducing the access ownership of store and fetch or fetch only of the cache line in the cache or a combination thereof. The address of the operand is either based on a register value or the program counter value pointing to the prefetch data machine instruction.