From: | Andres Freund <andres(at)anarazel(dot)de> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, Merlin Moncure <mmoncure(at)gmail(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, YUriy Zhuravlev <u(dot)zhuravlev(at)postgrespro(dot)ru>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Move PinBuffer and UnpinBuffer to atomics |
Date: | 2016-03-29 17:13:08 |
Message-ID: | 20160329171308.GD25907@awork2.anarazel.de |
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Lists: | pgsql-hackers |
On 2016-03-29 13:09:05 -0400, Robert Haas wrote:
> On Mon, Mar 28, 2016 at 9:09 AM, Andres Freund <andres(at)anarazel(dot)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(at)anarazel(dot)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
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