Re: Move PinBuffer and UnpinBuffer to atomics

On 2016-03-29 13:09:05 -0400, Robert Haas wrote:
> On Mon, Mar 28, 2016 at 9:09 AM, Andres Freund <andres@anarazel.de> wrote:
> > 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 allow to 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?
> 
> I don't have a clear idea what's going to be better here.

My gut feeling is that we should do both 1) and 2).

Dilip, could you test performance of reducing ppc's spinlock to 1 byte?
Cross-compiling suggest that doing so "just works".  I.e. replace the
#if defined(__ppc__) typedef from an int to a char.

Regards,

Andres