The Foundation of C++ Atomics: The Knowledge You Need to Correctly Use C++ Atomics

on YouTube ↗01h 33m

" summary

Covers concrete rules and mechanisms: std::atomic (six memory orderings) and when each actually maps to fences or machine instructions, how a core’s store queue / reorder buffer makes x86’s TSO: store can reorder with later load observable, and why loads can take hundreds of cycles and must search the store buffer for forwarding. Shows performance numbers (pipelines from 16→7 cycles, in-order vs out-of-order 16→12 cycles) and ties C++ memory orders to compiler/hardware behavior.

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" chapters (27)

1. 0:00Start
2. 3:42Why Do We Need Atomics
3. 4:14Difference between Memory Consistent and Cache Coherence
4. 5:39Can Two Processors Be in the Critical Section
5. 7:30Why Is It Important To Protect Child Data
6. 8:10Why Two Threads Can Be at the Critical Section at the Same Time
7. 11:54Sequential Consistency
8. 14:40Relaxed Memory Model
9. 19:54Why To Keep a Buffer inside of a Core
10. 21:03Pipeline Processor
11. 23:20Problem of External Fragmentation
12. 23:53Data Dependencies
13. 24:56Output Dependencies
14. 39:59Reorder Buffer
15. 50:10Dynamic Instruction Scheduling
16. 59:43Out of Order Machines
17. 1:01:10Out of Order Execution
18. 1:03:59Memory Operations
19. 1:07:07Memo Disambiguation Problem
20. 1:07:46Memory Examination Problem
21. 1:11:28How Does the out-of-Order Engine Detect the Dependence of a Load Instruction
22. 1:12:28How Does the out of Order Engine Treat the Scheduling of a Load Instruction with Regards to Previous Stores
23. 1:13:14The Aggressive Approach
24. 1:13:36Intelligent Approach
25. 1:14:21Data Forwarding between Stores and Loads
26. 1:24:38Buffer Affects the Visibility of Storage Operations
27. 1:30:30Exploit Memory Level Parallelism

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