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* '''Reminder:''' Week 1-3 blog posts are due for marking on Friday, January 31.
* Blog about the [[Codebase Analysis Lab]]
== Week 5 ==
=== Tuesday (Feb 4) ===
Platform-specific code is often utilized for '''Memory Barriers''' and '''Atomics Operations'''.
==== Memory Barriers ====
'''Memory Barriers''' ensure that memory accesses are sequenced so that multiple threads, processes, cores, or IO devices see a predictable view of memory.
* Leif Lindholm provides an excellent explanation of memory barriers.
** Blog series - I recommend this series - especially the introduction - as the clearest explanation of the issue.
*** Part 1 - [http://community.arm.com/groups/processors/blog/2011/03/22/memory-access-ordering--an-introduction Memory Access Ordering - An Introduction]
*** Part 2 - [http://community.arm.com/groups/processors/blog/2011/04/11/memory-access-ordering-part-2--barriers-and-the-linux-kernel Memory Access Ordering Part 2 - Barriers and the Linux Kernel]
*** Part 3 - [http://community.arm.com/groups/processors/blog/2011/10/19/memory-access-ordering-part-3--memory-access-ordering-in-the-arm-architecture Memory Access Ordering Part 3 - Memory Access Ordering in the ARM Architecture]
** Presentation at Embedded Linux Conference 2010 (Note: Acquire/Release in C++11 and ARMv8 aarch64 appeared after this presentation):
*** [http://elinux.org/images/f/fa/Software_implications_memory_systems.pdf Slides]
*** [http://free-electrons.com/pub/video/2010/elce/elce2010-lindholm-memory-450p.webm Video]
* [http://www.rdrop.com/users/paulmck/scalability/paper/whymb.2010.07.23a.pdf Memory Barriers - A Hardware View for Software Hackers] - This is a highly-rated article that explains memory barrier issues. Despite the fact that it is an introduction to the topic, it is still very technical.
* [http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka14041.html ARM Technical Support Knowlege Article - In what situations might I need to insert memory barrier instructions?] - Note that there are some additional mechanisms present in ARMv8 aarch64, including Acquire/Release.
* [https://www.kernel.org/doc/Documentation/memory-barriers.txt Kernel Documentation on Memory Barriers] - discusses the memory barrier issue generally, and the solutions used within the Linux kernel. This is part of the kernel documentation.
* Acquire-Release mechanisms
** [http://blogs.msdn.com/b/oldnewthing/archive/2008/10/03/8969397.aspx MSDN Blog Post] with a very clear explanation of Acquire-Release.
** [http://preshing.com/20130922/acquire-and-release-fences/ Preshing on Programming post] with a good explanation.
** [http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.genc010197a/index.html ARMv8 Instruction Set Architecture Manual] (ARM InfoCentre registration required) - See the section on Acquire/Release and Load/Store.
==== Atomics ====
'''Atomics''' are operations which must be completed in a single step (or appear to be completed in a single step) without interruption.
* Wikipedia has a good basic overview of the need for atomicity in the article on [http://en.wikipedia.org/wiki/Linearizability Linerarizability]
* GCC provides intrinsics (built-in functions) for atomic operations, as documented in the GCC manual:
** [http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/_005f_005fsync-Builtins.html#_005f_005fsync-Builtins Legacy __sync Built-in Functions for Atomic Memory Access]
** [http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/_005f_005fatomic-Builtins.html#_005f_005fatomic-Builtins Built-in functions for memory model aware atomic operations]
* The Fedora project has some guidelines/recommendations for the use of these GCC builtins:
** http://fedoraproject.org/wiki/Architectures/ARM/GCCBuiltInAtomicOperations
