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SubjectRe: Memory Barrier Definitions
Alan Cox wrote:
>
> > forms of processor memory barrier instructions. It is _very_ expensive
> I think I follow
>
> You have
>
> Compiler ordering
> CPU v CPU memory ordering
> CPU v I/O memory ordering
> I/O v I/O memory ordering
>

Yep, that is a good summary. And the problem arises from the very large
penalty for the syncronization form used for CPU v I/O ordering. You
only want to pay that when necessary, certainly not when only CPU v CPU
ordering is required. The difference can be on the order of a 1000
cycles (depending on many factors, of course).

Dave.
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