[jmm-dev] Sequential Consistency

[Doug Lea]

On 02/24/2014 01:52 AM, Hans Boehm wrote:
> I think it's that last comment here that needs to be emphasized: We don't really
> have a viable candidate property to replace SC, that's anywhere near as easy to
> reason about and provides significant performance advantages. Several people,
> including Doug, looked hard for such things when we were talking about  C++.
>

Yes (plus similar explorations for X10, and distributed consistency).
We are pretty sure that there is no good substitute for requiring
SC for lock-based programs. I think the main issue at hand is
how far SC applies. We cannot require SC for all uses of
mode-based/fenced/volatile accesses, because some sets of
usages clearly are not SC. The audience of people using them
seem happy to rely only on specs of ordering constraints.
So it may suffice to just leave it at that. Although people
do need to know which usages are emergently SC, so that they
can for example build locks, which may require some special
care in specification.

This is just a slightly different perspective on similar issues
and decisions in C/C++11.  Among the differences is that we
have "legacy" mode-less default volatile load/store, for
which it is not clear that requiring uniform SC guarantees
(versus only for get/set Sequential) would be doing anyone
a favor. And not clear that it wouldn't.


While I'm at it...

> On Sun, Feb 23, 2014 at 5:59 AM, Doug Lea <dl at cs.oswego.edu
> <mailto:dl at cs.oswego.edu>> wrote:
>
>     The IRIW example is a fun one in part because it is not especially
>     intuitive.  Some people do not at first think that it is a result
>     forced by SC. I occasionally present this in courses, and most
>     students' first reaction is that you should use a common lock in all
>     threads if you want to ensure agreement about order of x and y
>     here. The fact that you don't need to strikes some (but by no means
>     all) people as a magical/spooky property of SC.

A caveat: When I've done this in courses, there's usually
some student who tries to exploit this to avoid locks/sync
in some programming project. But never correctly -- the example
does not seem to generalize in any useful way. In fact, I have
never seen a program where SC-IRIW matters, so arguably,
most people are better off not even knowing about it :-)

-Doug