I/O Agent Cache-Line Ownership for Faster Coherent Reads
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Solution Overview
Problem
Peripheral components in computer systems often fail to utilize the relaxed-memory consistency model efficiently due to their strict transaction ordering rules and lack of caches, leading to unreasonably slow data access times.
Innovation Solution
An I/O agent is introduced to bridge peripherals to a coherent fabric, implementing coherency mechanisms that allow it to preemptively obtain exclusive ownership of cache lines, caching data before completing transactions, and reacquire ownership if lost, thus adhering to transaction ordering rules while optimizing performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If peripherals perform memory transactions serially to maintain data coherency, then data coherency is preserved, but transaction processing speed deteriorates
Solution Approach 1:
The I/O agent performs preliminary actions by obtaining exclusive ownership of cache lines before transactions are completed. This allows the agent to preemptively cache data and prepare for subsequent transactions, reducing the time needed for serial processing while maintaining coherency through ownership tracking and reacquisition mechanisms
2Device complexity
If peripherals without caches access memory directly, then device simplicity is maintained, but access efficiency deteriorates
Solution Approach 1:
The I/O agent acts as an intermediary between peripherals and the coherent fabric. It obtains exclusive ownership of cache lines, pre-caches data locally, and manages transactions on behalf of peripherals. This mediator approach allows peripherals to maintain simplicity while achieving improved access efficiency through the agent's caching and transaction management capabilities
3Reliability
If the I/O agent waits for transaction completion before starting the next transaction, then data coherency is maintained, but processing throughput deteriorates
Solution Approach 1:
The I/O agent performs preliminary actions by obtaining exclusive ownership and pre-caching data before transactions complete. This enables overlapping of transaction preparation and execution, allowing subsequent transactions to start earlier while coherency is maintained through ownership tracking and reacquisition mechanisms when needed
Data Source
AI summary
Techniques are disclosed relating to an I/O agent circuit of a computer system. The I/O agent circuit may receive, from a peripheral component, a set of transaction requests to perform a set of read transactions that are directed to one or more of a plurality of cache lines. The I/O agent circuit may issue, to a first memory controller circuit configured to manage access to a first one of the plurality of cache lines, a request for exclusive read ownership of the first cache line such that data of the first cache line is not cached outside of the memory and the I/O agent circuit in a valid state. The I/O agent circuit may receive exclusive read ownership of the first cache line, including receiving the data of the first cache line. The I/O agent circuit may then perform the set of read transactions with respect to the data.