Part 6 answers: How many requests can be in flight?
A link that waits for each answer before asking again wastes most of its bandwidth. Keeping several requests in flight fixes that, and brings tags, reordering and new ways to run out of resources.
This part covers latency versus throughput, Little's law, outstanding transactions, tags, out-of-order completion, reorder buffers, interleaving, efficiency, queueing, tail latency and performance modelling before RTL.
Use Little's law to size outstanding requests and tag fields.
Turn a link's line rate into its real payload bandwidth.
Explain why latency explodes near full load.
Answer each in your head before opening it. If two or more feel shaky, revisit Part 5: Transactions first.
A read's data comes back only after a full round trip, so the requester waits. A write can be handed off and, if posted, forgotten.
Knowledge of when, or whether, it completed: no response comes back, so errors and completion order are not visible to the writer.
Targets and address translation work in 4 KB pages, so a burst that crossed one could reach two different targets or translations.
# | Lesson | Level | Status |
|---|---|---|---|
6.1 | F · Foundation | Ready | |
6.2 | W · Working | Ready | |
6.3 | W · Working | Ready | |
6.4 | W · Working | Ready | |
6.5 | S · Senior | Ready | |
6.6 | S · Senior | Ready | |
6.7 | S · Senior | Ready | |
6.8 | S · Senior | Ready | |
6.9 | W · Working | Ready | |
6.10 | W · Working | Ready | |
6.11 | S · Senior | Ready | |
6.12 | P · Principal | Ready | |
6.13 | S · Senior | Ready | |
6.14 | P · Principal | Ready | |
6.15 | P · Principal | Ready | |
— | all levels | Ready | |
— | all levels | Ready |
Every lesson in this part is ready. Work through them in order: each one sets up the problem the next one solves.
This course teaches why protocols are designed the way they are. These lessons show how to build or sign off the same ideas, so this part links to them instead of repeating them.