External Memory Interface Handbook Volume 3: Reference Material

ID 683841
Date 7/24/2019
Public
Document Table of Contents

5.2.1. Port Scheduling

The evaluation of absolute priority ensures that ports carrying higher-priority traffic are served ahead of ports carrying lower-priority traffic. The scheduler recognizes eight priority levels, with higher values representing higher priorities. Priority is absolute; for example, any transaction with priority seven will always be scheduled before transactions of priority six or lower.

When ports carry traffic of the same absolute priority, relative priority is determined based on port weighting. Port weighting is a five-bit value, and is determined by a weighted round robin (WRR) algorithm.

The scheduler can alter priority if the latency target for a transaction is exceeded. The scheduler tracks latency on a per-port basis, and counts the cycles that a transaction is pending. Each port has a priority escalation register and a pending counter engagement register. If the number of cycles in the pending counter engagement register elapse without a pending transaction being served, that transaction’s priority is esclated.

To ensure that high-priority traffic is served quickly and that long and short bursts are effectively interleaved on ports, bus transactions longer than a single DRAM burst are scheduled as a series of DRAM bursts, with each burst arbitrated separately.

The scheduler uses a form of deficit round robin (DRR) scheduling algorithm which corrects for past over-servicing or under-servicing of a port. Each port has an associated weight which is updated every cycle, with a user-configured weight added to it and the amount of traffic served subtracted from it. The port with the highest weighting is considered the most eligible.

To ensure that lower priority ports do not build up large running weights while higher priority ports monopolize bandwidth, the hard memory controller’s DRR weights are updated only when a port matches the scheduled priority. Hence, if three ports have traffic, two being priority 7 and one being priority 4, the weights for both ports at priority 7 are updated but the port with priority 4 remains unchanged.

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