By: anon (anon.delete@this.anon.com), July 15, 2015 5:45 pm
Room: Moderated Discussions
Wilco (Wilco.Dijkstra.delete@this.ntlworld.com) on July 15, 2015 2:43 pm wrote:
> Linus Torvalds (torvalds.delete@this.linux-foundation.org) on July 15, 2015 2:17 pm wrote:
> > anon (anon.delete@this.anon.com) on July 14, 2015 9:37 pm wrote:
> > >
> > > At it's most general, looking at system-wide state, this is untrue. Forwarding from the store
> > > queue means that a write can be seen (from one CPU) and then not be seen (from another).
> >
> > Hmm. Yes and no.
> >
> > To some independent observer looking from the outside, what you say is obviously
> > true: the CPU that does a write can do a read and see the value of that write,
> > while another CPU that does a read slightly later still sees the old value.
> >
> > But that's not that different from relativity, where "before" and "after" sometimes don't have meaning.
> > Two independent observers may not agree on what happened before, and what happened after.
> >
> > But what does have meaning is causality. In physics, everybody still agrees about the order of two
> > causally related events, even if they might not agree on
> > the order of two non-causally related (independent)
> > events. So within the "light cone" of the events, ordering matters and is well-defined.
> >
> > And as in physics, so in x86 memory ordering. "before" and "after" are kind of ambiguous
> > things for two events that don't have anything in common. But causality is not.
> >
> > And the x86 memory ordering is explicitly causal. I'm pretty sure it's one
> > of the documented litmus tests, although I didn't look that up again.
>
> So is this reordering causal according to you? I bet most people don't find it intuitive
> either as this kind of reordering is exactly what is expected on weak memory models
Clearly it is not. This is the ordering ambiguity I was referring to.
You could say (and the Intel manual does say) that *stores* are transitively visible. But loads are not. Maybe not be quite the right wording because you can only observe the store with a load. But if you order your loads correctly with respect to your stores, then you will indeed obey causality.
> Linus Torvalds (torvalds.delete@this.linux-foundation.org) on July 15, 2015 2:17 pm wrote:
> > anon (anon.delete@this.anon.com) on July 14, 2015 9:37 pm wrote:
> > >
> > > At it's most general, looking at system-wide state, this is untrue. Forwarding from the store
> > > queue means that a write can be seen (from one CPU) and then not be seen (from another).
> >
> > Hmm. Yes and no.
> >
> > To some independent observer looking from the outside, what you say is obviously
> > true: the CPU that does a write can do a read and see the value of that write,
> > while another CPU that does a read slightly later still sees the old value.
> >
> > But that's not that different from relativity, where "before" and "after" sometimes don't have meaning.
> > Two independent observers may not agree on what happened before, and what happened after.
> >
> > But what does have meaning is causality. In physics, everybody still agrees about the order of two
> > causally related events, even if they might not agree on
> > the order of two non-causally related (independent)
> > events. So within the "light cone" of the events, ordering matters and is well-defined.
> >
> > And as in physics, so in x86 memory ordering. "before" and "after" are kind of ambiguous
> > things for two events that don't have anything in common. But causality is not.
> >
> > And the x86 memory ordering is explicitly causal. I'm pretty sure it's one
> > of the documented litmus tests, although I didn't look that up again.
>
> So is this reordering causal according to you? I bet most people don't find it intuitive
> either as this kind of reordering is exactly what is expected on weak memory models
Clearly it is not. This is the ordering ambiguity I was referring to.
You could say (and the Intel manual does say) that *stores* are transitively visible. But loads are not. Maybe not be quite the right wording because you can only observe the store with a load. But if you order your loads correctly with respect to your stores, then you will indeed obey causality.
