Introduction

without further notes, all contents are copy-verbatim or derived from this book:;

A Primer on Memory Consistency and Cache Coherence (2nd Edition), Synthesis Lectures on Computer Architecture, Vijay Nagarajan, Daniel J.Sorin, Mark D.Hill and David A. Wold
https://pages.cs.wisc.edu/~markhill/papers/primer2020_2nd_edition.pdf

this section is under CC-BY-SA-4.0. The above notice must be preserved.

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§ consistency and coherence

• Memory -> Consistency : (memory model), a precise, architecturally visible definition of shared memory correctness; rules about loads and stores and how they act upon memory. Also relaxed (weak) memory model w.r.t. re-ordering.
• Cache -> Coherence : seems to make caches of a shared-memory system as functionally invisible; it does so by propagating a processor’s write to other processor’s caches

Consistency vs. Coherence
“It is worth stressing that unlike consistency which is an architectural specification that defines shared memory correctness, coherence is a means to supporting a consistency model”

• The coherence protocol provides the processor core pipeline an abstraction of a memory system. It alone cannot determine memory behaviour
• A memory consistency implementation can use cache coherence as a “black box”

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§ self-test quizs

• what’s the difference / relationship between memory consistency and cache coherence (or just explain them)
• what is the fundamental issue that causes the incoherence? (multiple actors with access to caches and memory)
• (cont.) which actors are there? (processor cores, DMA engines, external devices that can read/write caches and memory)

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§ Glossary

• LLC : last-level cache
• HLL : high-level language
• RAS : Raliability (prevent or correct errors), Availability (uptime) and Serviceability (diagnose and repair)

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Notes for future readings

• M. L. Scott. Shared-Memory Synchronization. Synthesis Lectures on Computer Architecture.

• Lamport, “How to Make a Multiprocessor Computer That Correctly Executes Multiprocess Programs,” in IEEE Transactions on Computers, vol. C-28, no. 9, pp. 690-691, Sept. 1979, doi: 10.1109/TC.1979.1675439. keywords: {Computer design;concurrent computing;hardware correctness;multiprocessing;parallel processing}
  https://lamport.azurewebsites.net/pubs/lamport-how-to-make.pdf

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