Concurrency and memory models

Question

I\'m watching this video by Herb Sutter on GPGPU and the new C++ AMP library. He is talking about memory models and mentions Weak Memory Models and then

Answer 1

Yes, you are right - the difference between Weak and Strong memory models is a difference in what optimizations are available (order of reads/write and related fences).

You can specify a memory model by starting with a sequentially consistent model (the most restrictive, or strongest model), and then specify how reads and writes from a single thread can be introduced, removed, or moved with respect to one another

In this model (sequentially consistent) the memory is independent of any of the processors (threads) that use it. The memory is connected to each of the threads by a controller that feeds read and write requests from each thread. The reads and writes from a single thread reach memory in exactly the order specified by the thread, but they might be interleaved with reads and writes from other threads in an unspecified way

Understand the Impact of Low-Lock Techniques in Multithreaded Apps

However there's no exact bound between strong and weak memory models, unless you consider sequentilly consistent model vs others. Some of them are just stronger/weaker and therefore more open to optimizations by reordering than others. For example, memory model in .NET 2.0 for x86 allows a bit more optimizations that the verison in .NET 1.1 so it can be considered as a weaker model.