Re: [PATCH v3] tools/memory-model: Make ppo a subrelation of po

[Jonas Oberhauser]

On 2/27/2023 11:21 PM, Paul E. McKenney wrote:
> On Mon, Feb 27, 2023 at 09:13:01PM +0100, Jonas Oberhauser wrote:
> > On 2/27/2023 8:40 PM, Andrea Parri wrote:
> > > > The LKMM doesn't believe that a control or data dependency orders a
> > > > plain write after a marked read.  Hence in this test it thinks that P1's
> > > > store to u0 can happen before the load of x1.  I don't remember why we
> > > > did it this way -- probably we just wanted to minimize the restrictions
> > > > on when plain accesses can execute.  (I do remember the reason for
> > > > making address dependencies induce order; it was so RCU would work.)
> > > >
> > > > The patch below will change what the LKMM believes.  It eliminates the
> > > > positive outcome of the litmus test and the data race.  Should it be
> > > > adopted into the memory model?
> > > (Unpopular opinion I know,) it should drop dependencies ordering, not
> > > add/promote it.
> > >
> > >     Andrea
> > Maybe not as unpopular as you think... :)
> > But either way IMHO it should be consistent; either take all the
> > dependencies that are true and add them, or drop them all.
> > In the latter case, RCU should change to an acquire barrier. (also, one
> > would have to deal with OOTA in some yet different way).
> >
> > Generally my position is that unless there's a real-world benchmark with
> > proven performance benefits of relying on dependency ordering, one should
> > use an acquire barrier. I haven't yet met such a case, but maybe one of you
> > has...
> https://www.msully.net/thesis/thesis.pdf page 128 (PDF page 141).
>
> Though this is admittedly for ARMv7 and PowerPC.

Thanks for the link.

It's true that on architectures that don't have an acquire load (and 
have to use a fence), the penalty will be bigger.

But the more obvious discussion would be what constitutes a real-world 
benchmark : )
In my experience you can get a lot of performance benefits out of 
optimizing barriers in code if all you execute is that code.
But once you embed that into a real-world application, often 90%-99% of 
time spent will be in the business logic, not in the data structure.

And then the benefits suddenly disappear.
Note that a lot of barriers are a lot cheaper as well when there's no 
contention.

Because of that, making optimization decisions based on microbenchmarks 
can sometimes lead to a very poor "time invested" vs "total product 
improvement" ratio.

Best wishes,
jonas