Packet Transmission Scheduling Fairness Through DT Window Protection
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Solution Overview
Problem
Existing packet transmission systems in data centers face challenges in maintaining fair bandwidth allocation after flow control is de-asserted, leading to potential packet scheduling delays, underruns, and starvation issues due to the drift of departure times (DT) and last departure times (LDT) values, which can cause unfairness and malfunctions in weighted fair queuing (WFQ) algorithms.
Innovation Solution
The system adjusts and maintains departure time (DT) values within a valid window range by using window protection mechanisms, such as adjusting DT values to stay within a moving window and maintaining relative ordering, ensuring fair bandwidth allocation across nodes and ports by updating last departure time (LDT) values and using piggyback sorting operations to re-sort DT values post-flow control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If flow control is applied to pause packet transmission for congested nodes, then packet drops and latency are reduced, but bandwidth allocation fairness deteriorates when flow control is de-asserted due to drift between departure time (DT) and last departure time (LDT) values
Solution Approach 1:
The system performs preliminary actions by adjusting DT values to stay within a valid window range before flow control is de-asserted. This prevents DT-LDT drift from causing unfairness when transmission resumes, ensuring that nodes released from flow control do not experience bandwidth allocation issues or scheduling delays.
Solution Approach 2:
The system implements feedback mechanisms by monitoring the relationship between DT and LDT values during flow control assertion. When flow control is de-asserted, the system detects any drift between these values and applies corrective adjustments to maintain fair bandwidth allocation, creating a closed-loop control system that responds to actual system state.
2Productivity
If weighted fair queuing (WFQ) arbitration is performed among nodes, then bandwidth is allocated based on priority weights, but scheduling delays and starvation issues occur when DT and LDT values drift apart during flow control
Solution Approach 1:
The system adjusts DT values in advance to remain within a valid window range before flow control de-assertion occurs. This preliminary adjustment ensures that when WFQ arbitration resumes, the scheduling calculations based on DT and LDT values remain accurate, preventing scheduling delays and starvation issues that would otherwise occur due to value drift.
3Adaptability or versatility
If departure time (DT) values are allowed to drift during flow control assertion, then nodes can accumulate packets for later transmission, but unfairness and malfunctions occur in WFQ algorithms due to excessive drift between DT and last departure time (LDT) values
Solution Approach 1:
The system performs preliminary adjustments to DT values to ensure they remain within a valid window range before flow control de-assertion. This prevents excessive drift between DT and LDT values that would cause WFQ algorithm malfunctions, while still allowing nodes to accumulate packets during flow control assertion for later transmission.
Solution Approach 2:
The system implements feedback by monitoring DT-LDT drift during flow control assertion and applying corrective adjustments when flow control is de-asserted. This feedback mechanism ensures that DT values remain within acceptable ranges, maintaining WFQ algorithm correctness while preserving the ability of nodes to accumulate packets during congestion.
Data Source
AI summary
Examples described herein relate to a network interface device comprising a packet transmission scheduler. In some examples, the packet transmission scheduler is to: perform packet transmit arbitration among nodes, wherein based on a first node of the nodes having transmission paused by flow control, the perform packet transmit arbitration among nodes comprises retain relative priority of a packet departure time for the first node with respect to a second packet departure time associated with a second node of the nodes during a duration of flow control. In some examples, retaining relative priority of a packet departure time for the first node with respect to a second packet departure time associated with a second node of the nodes during a duration of flow control comprises adjust the packet departure time and the second packet departure time to stay within a time window but not rollover.