Re: Move PinBuffer and UnpinBuffer to atomics

On Fri, Apr 8, 2016 at 7:39 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:

> On 2016-04-07 16:50:44 +0300, Alexander Korotkov wrote:
> > On Thu, Apr 7, 2016 at 4:41 PM, Andres Freund <andres(at)anarazel(dot)de>
> wrote:
> >
> > > On 2016-03-31 20:21:02 +0300, Alexander Korotkov wrote:
> > > > ! BEGIN_BUFSTATE_CAS_LOOP(bufHdr);
> > > >
> > > > ! Assert(BUF_STATE_GET_REFCOUNT(state) > 0);
> > > > ! wasDirty = (state & BM_DIRTY) ? true : false;
> > > > ! state |= BM_DIRTY | BM_JUST_DIRTIED;
> > > > ! if (state == oldstate)
> > > > ! break;
> > >
> > > I'm doubtful that this early exit is entirely safe. None of the
> > > preceding operations imply a memory barrier. The buffer could
> previously
> > > have been marked dirty, but cleaned since. It's pretty critical that we
> > > re-set the dirty bit (there's no danger of loosing it with a barrier,
> > > because we hold an exclusive content lock).
> > >
> >
> > Oh, I get it.
> >
> >
> > > Practically the risk seems fairly low, because acquiring the exclusive
> > > content lock will have implied a barrier. But it seems unlikely to have
> > > a measurable performance effect to me, so I'd rather not add the early
> > > exit.
> > >
> >
> > Ok, let's just remove it.
>
> Here's my updated version of the patch. I've updated this on an
> intercontinental flight, after a otherwise hectic week (moving from SF
> to Berlin); so I'm planning to look over this once more before pushing (.
>

Ok.

I've decided that the cas-loop macros are too obfuscating for my
> taste. To avoid duplicating the wait part I've introduced
> WaitBufHdrUnlocked().
>

That's OK for me. Cas-loop macros looks cute, but too magic.

> As you can see in
>
> http://archives.postgresql.org/message-id/CA%2BTgmoaeRbN%3DZ4oWENLvgGLeHEvGZ_S_Z3KGrdScyKiSvNt3oA%40mail.gmail.com
> I'm planning to apply this sometime this weekend, after running some
> tests and going over the patch again.
>
> Any chance you could have a look over this?
>

I took a look at this. Changes you made look good for me.
I also run test on 4x18 Intel server.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company