Visibility determines when other threads will see changes made by the current thread, and whether they will see them at all. A lot of people endeavour to do something special in order to ensure visibility (like placing memory fences). However, in practice it's the most boring property, because on cache-coherent architectures (read - on all modern commodity architectures - IA-32, Intel 64, IA-64, SPARC, POWER) visibility is ensured automatically. Namely, each write is automatically propagated to all other processors/cores in a best-effort manner. There are no ways to prevent nor to speed it up. Period.
On non cache-coherent architectures changes are indeed not propagated automatically, and you need to do something special. However, such architectures are rare and unique (I saw no such archs), so I think there is no sense in discussing them in general.
However, there is an interesting deviation - on x86 architecture non-temporal stores (made with instructions like MOVNTDQ) and stores made to WC (Write-Combining) region of memory are effectively non cache-coherent. That is, they will be propagated to other processors/cores only before execution of SFENCE instruction, MFENCE instruction, LOCKed instruction or other serializing actions.
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