LC-Asia: Facebook contemplates ARM servers

I don't think you need to go quite as far as saying you can't have shared writable memory at all, the issue is that hardware can't maintain cache coherency across core specific caches on a scalable or efficient basis. So what you need is for any caching of such memory to be carefully arbitrated in software, e.g. on an acquire and release basis, and for the use of shared mutable data structures to be minimized in favor of things that you can partition across cores on a relatively long term basis.