Part 13 answers: What if there are several copies of one value?
Caches make copies. Copies go stale. A coherence protocol is the set of messages that keeps every copy of a location consistent with the single-writer, many-readers rule. This part teaches the concepts; the Cache Coherency course holds the protocol detail.
This part covers why coherence is needed, invalidate versus update, MSI to MOESI, snooping versus directories, ownership and forwarding, coherence versus consistency, and I/O coherence.
State the single-writer, multiple-reader invariant.
Say what each MESI state adds.
Explain when snooping stops scaling.
Answer each in your head before opening it. If two or more feel shaky, revisit Part 12: Power first.
The shortest idle period for which entering a low-power state saves more energy than the entry and exit cost.
A transaction in flight when power goes away is lost or corrupted, and the side waiting for it may hang.
It still has work in flight or is about to receive some; denying keeps the request from cutting it off mid-transaction.
# | Lesson | Level | Status |
|---|---|---|---|
13.1 | W · Working | Ready | |
13.2 | W · Working | Ready | |
13.3 | W · Working | Ready | |
13.4 | S · Senior | Ready | |
13.5 | S · Senior | Ready | |
13.6 | S · Senior | Ready | |
13.7 | S · Senior | 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.