Re: Misaligned BufferDescriptors causing major performance problems on AMD

On 2014-04-16 19:18:02 -0400, Bruce Momjian wrote:
> On Thu, Feb  6, 2014 at 09:40:32AM +0100, Andres Freund wrote:
> > On 2014-02-05 12:36:42 -0500, Robert Haas wrote:
> > > >> It may well be that your proposal is spot on.  But I'd like to see some
> > > >> data-structure-by-data-structure measurements, rather than assuming that
> > > >> alignment must be a good thing.
> > > >
> > > > I am fine with just aligning BufferDescriptors properly. That has
> > > > clearly shown massive improvements.
> > > 
> > > I thought your previous idea of increasing BUFFERALIGN to 64 bytes had
> > > a lot to recommend it.
> > 
> > Good.
> > 
> > I wonder if we shouldn't move that bit of logic:
> >     if (size >= BUFSIZ)
> >         newStart = BUFFERALIGN(newStart);
> > out of ShmemAlloc() and instead have a ShmemAllocAligned() and
> > ShmemInitStructAligned() that does it. So we can sensibly can control it
> > per struct.
> > 
> > > But that doesn't mean it doesn't need testing.
> > 
> > I feel the need here, to say that I never said it doesn't need testing
> > and never thought it didn't...
> 
> Where are we on this?

It needs somebody with time to evaluate possible performance regressions
- I personally won't have time to look into this in detail before pgcon.

Greetings,

Andres Freund

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