Re: Move PinBuffer and UnpinBuffer to atomics

On 2016-03-28 11:48:46 +0530, Dilip Kumar wrote:
> On Sun, Mar 27, 2016 at 5:48 PM, Andres Freund <andres@anarazel.de> wrote:
>
> >
> > What's sizeof(BufferDesc) after applying these patches? It should better
> > be <= 64...
> >
>
> It is 72.

Ah yes, miscalculated the required alignment.  Hm. So we got to get this
smaller. I see three approaches:

1) Reduce the spinlock size on ppc. That actually might just work by  replacing "unsigned int" by "unsigned char"
2) Replace the lwlock spinlock by a bit in LWLock->state. That'd avoid  embedding the spinlock, and actually might
allowto avoid one atomic  op in a number of cases.
 
3) Shrink the size of BufferDesc by removing buf_id; that'd bring it to  64byte.

I'm a bit hesitant to go for 3), because it'd likely end up adding a bit
of arithmetic to a number of places in bufmgr.c.  Robert, what do you
think?

Andres