Should disclose: x86 specific [LWN.net]

Should disclose: x86 specific

Posted Oct 3, 2007 0:21 UTC (Wed) by ncm (subscriber, #165)
In reply to: Should disclose: x86 specific by mikov
Parent article: Memory part 2: CPU caches

"The text doesn't touch on memory consistency at all"

Sorry, not true. It describes how caches of different x86 CPUs interact, but doesn't say it only describes x86, falsely suggesting that is how every other machine does it too. It leaves the reasonable reader under the impression that programmers don't need to know anything about memory consistency. That's not entirely true even on x86, but is just false on most non-x86 platforms. If Ulrich is writing for people programming only x86, the article should say so without quibbling. If not, it should call out places where it is describing x86-specific behavior.

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Should disclose: x86 specific

Posted Oct 3, 2007 1:14 UTC (Wed) by mikov (subscriber, #33179) [Link]

I think that you may be confusing cache coherency with memory consistency. Although they are obviously related, in the context of the article the latter is not important.

To the best of my knowledge, the description in the article applies to all cache coherent systems, including the ones listed in your previous post. It has nothing to do with memory consistency, which is an issue mostly internal to the CPU.

I am very possibly wrong, of course - I am not a hardware system designer - so I am glad to discuss it. Can you describe how the cache/memory behavior in an Alpha (for example; or any other weak consistency system) differs from the article ?

Should disclose: x86 specific

Posted Oct 3, 2007 1:48 UTC (Wed) by ncm (subscriber, #165) [Link]

I'm sorry, I should have pointed out this quote from the article: "All processors are supposed to see the same memory content at all times.". (My emphasis.)

I agree that coding with memory barriers (etc.!) is a big subject, and beyond the scope of this installment. It would have sufficed, though, to mention that (and where) it is a matter for concern, and why.