Device Handle Tables for Shorter PCIe and CXL Message Headers
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Solution Overview
Problem
The inclusion of relatively long domain identifiers in message headers for short messages significantly reduces the efficiency of links in computing systems, particularly in interconnect architectures like PCIe and CXL, due to increased header size.
Innovation Solution
Implementing a device handle table at both ends of the link to store mappings of domain identifiers, allowing messages to reference entries in the table using a smaller number of bits, thereby reducing the need to include full domain identifiers in the message header.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If full domain identifiers are included in message headers, then domain identification accuracy is improved, but link efficiency deteriorates due to increased header size
Solution Approach 1:
The domain identifier is segmented into two parts: a compact device handle identifier (DHI) that fits in the message header, and a full domain identifier stored in a device handle table. This segmentation allows the message header to remain small while still enabling complete domain identification through table lookup, thus resolving the contradiction between identification accuracy and link efficiency.
Solution Approach 2:
A device handle table is introduced as an intermediary data structure that maps compact device handle identifiers to full domain identifiers. This intermediary allows the system to use small headers for efficiency while maintaining the ability to identify complete domains through the table, effectively mediating between the conflicting requirements of header size and identification completeness.
2Productivity
If shorter identifiers are used in message headers, then link efficiency is improved, but domain identification capability deteriorates
Solution Approach 1:
The system performs preliminary action by pre-establishing the device handle table that contains mappings from compact identifiers to full domain identifiers. This preliminary preparation allows short identifiers to be used in messages without losing domain identification capability, as the full information is readily available in the pre-built table.
Solution Approach 2:
Instead of transmitting the full domain identifier in every message, the system uses a copy or reference (the device handle identifier) that points to the complete information stored in the device handle table. This copying approach maintains domain identification capability while significantly reducing the space required in message headers.
Data Source
AI summary
Techniques for increasing link efficiency are disclosed. In one embodiment, a device handle table is created at each end of a link. Device handle allocation messages can be used to associate a particular device handle with a particular domain identifier, such as a bus/device/function (BDF) identifier or a processor address space identifier (PASID). Once a device handle is allocated, messages can be sent between the two ends of the link that include the device handle. The device handle can be used to determine the domain identifier associated with the message. As the device handle can be fewer bits than the domain identifier, the link efficiency can be increased.