SAS Controller Link List Manager for Queue Reordering
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Solution Overview
Problem
Traditional SAS controllers face inefficiencies in managing command and data requests due to their FIFO nature, which makes it difficult to revert requests to the queue in the same order, leading to inefficient use of SAS links and high overhead in link-list operations, especially in systems with thousands of nodes and frequent back-offs.
Innovation Solution
A link list with a dispatcher cache architecture that allows for efficient insertion, deletion, and traversal of link list entries, including backing off requests, to manage command and data requests, and handle conditions like Open_Reject, fairness control, SATA atomic sequences, and maximum connection handling, reducing memory accesses and improving performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If traditional FIFO queue management is used in SAS controllers, then simple queue structure is maintained, but link usage efficiency deteriorates due to inability to revert requests to queue in same order
Solution Approach 1:
The patent divides the queue management into multiple independent link lists, each associated with a specific target device. This segmentation allows requests for different targets to be managed separately, enabling efficient reordering and reverting of requests without affecting other targets. Each link list can be independently manipulated to optimize link usage while maintaining overall system organization.
Solution Approach 2:
The patent introduces a dispatcher cache as an intermediary structure between the link lists and the execution engine. This cache stores metadata about pending requests and facilitates efficient management of the link lists, including operations like reverting requests to the queue. The dispatcher cache acts as a mediator that simplifies the complexity of link list operations while improving link usage efficiency.
2Productivity
If link list operations are implemented without optimization, then request reordering capability is achieved, but overhead in link-list operations increases
Solution Approach 1:
The patent pre-allocates and pre-organizes link list structures and dispatcher cache entries before requests need to be processed. By preparing the data structures in advance and establishing the link list topology beforehand, the system reduces the time required for actual link list operations during request processing. This preliminary setup minimizes overhead when reordering or reverting requests.
Solution Approach 2:
The patent maintains copies of request metadata in the dispatcher cache alongside the actual link lists. This copying approach allows the system to perform link list operations on the metadata copies rather than directly manipulating the full request structures, significantly reducing the overhead of link-list operations while maintaining the ability to reorder and revert requests efficiently.
3Reliability
If requests are backed off frequently to handle device unavailability, then device availability handling is improved, but SAS link efficiency deteriorates due to repeated reordering operations
Solution Approach 1:
The patent implements dynamic link list management where the structure and ordering of requests can adapt in real-time based on device availability status. When a device becomes unavailable, the system dynamically reorganizes the corresponding link list to reflect the current state, allowing efficient back-off and retry operations. This dynamic adaptation maintains reliability while minimizing the performance impact of frequent back-off operations.
Data Source
AI summary
A controller comprising a transport layer, an internal memory, and a link list manager block. The internal memory stores pending instruction entries. The link list manager block is configured to read instructions stored in an external memory, update an active vector, the active vector for storing indications of instructions from the external memory; update the pending instruction entries in the internal memory; and update the instructions stored in the external memory. The link list manager block configured to dispatch a instruction from the pending instruction entries in the internal memory to the transport layer.