Re: Atomics hardware support table & supported architectures

On 2014-06-23 09:28:19 -0700, Tom Lane wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
> > On Mon, Jun 23, 2014 at 11:16 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> >> Since fetch-and-add is trivially implemented using CAS, there's not much
> >> need to distinguish between CAS and CAS + fetch_and_add. From my POV the
> >> restriction to just CAS/fetch_and_add isn't actually buying much. Pretty
> >> much all platforms but older gcc ones have atomic intrinsics since
> >> forever. Once you've dug up the documentation for one operation not
> >> adding two more or so doesn't save much.
> 
> > Again, the concern that was expressed at the developer's meeting was
> > that the performance characteristics of the CAS loop might be
> > significantly different from a native atomic op as to cause us
> > heartburn.  I think that's a valid concern... but if everything in
> > common use has both CAS and fetch-and-add, then I think there's
> > probably no issue here.
> 
> What I want to know is whether we are going to agree that the set of
> atomics is going to be CAS plus fetch_and_add plus *nothing else*.

It's going to be TAS, CAS, fetch_and_add, right? Since TAS is the only
thing supported on some platforms?

The only op I'm currently wondering about adding is a atomic exchange,
without compare to that set. All platforms that support CAS also have a
non-comparing version of it.

Right now the patch also uses __sync_fetch_and_sub() in the generic gcc
implementation instead of doing the negation itself, but that's easily
"fixable".

> Andres seems to envision that those will be a minimal set and then
> we'll freely use any other atomic op that seems handy as long as we can
> provide a fallback implementation of it.

Well, I *do* also want pg_atomic_fetch_and/or_u32() - but I'm totally
fine with those two only being implemented with CAS. On all
platforms. Otherwise the next scalability patch I'm going to submit will
just have to open code a CAS loop for it which doesn't seem to help.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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