Re: better atomics - v0.2

On Tue, Nov 19, 2013 at 10:16 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>> On the other hand, if the complaint is
>> "my hardware doesn't support 64-bit CAS", it's not reasonable to tell them
>> to buy a new server.
>
> Agreed. I've am even wondering about 32bit CAS since it's not actually
> that hard to emulate it using spinlocks. Certainly less work than
> arguing about removing stone-age platforms ;)
>
> There's no fundamental issue with continuing to support semaphore based
> spinlocks I am just wondering about the point of supporting that
> configuration since it will always yield horrible performance.
>
> The only fundamental thing that I don't immediately see how we can
> support is the spinlock based memory barrier since that introduces a
> circularity (atomics need barrier, barrier needs spinlocks, spinlock
> needs atomics).

We've been pretty much assuming for a long time that calling a
function in another translation unit acts as a compiler barrier.
There's a lot of code that isn't actually safe against global
optimization; we assume, for example, that memory accesses can't move
over an LWLockAcquire(), but that's just using spinlocks internally,
and those aren't guaranteed to be compiler barriers, per previous
discussion. So one idea for a compiler barrier is just to define a
function call pg_compiler_barrier() in a file by itself, and make that
the fallback implementation. That will of course fail if someone uses
a globally optimizing compiler, but I think it'd be OK to say that if
you want to do that, you'd better have a real barrier implementation.
Right now, it's probably unsafe regardless.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company