Re: Misaligned BufferDescriptors causing major performance problems on AMD

On 2014-02-05 11:23:29 -0500, Tom Lane wrote:
> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> > And I think somebody already thought about it (c.f. ALIGNOF_BUFFER), it
> > just wasn't updated in the last 10 years.
>
> No, ALIGNOF_BUFFER is there because we read something that said that I/O
> transfers between userspace and kernel disk cache would be faster with
> aligned buffers. There's been no particular thought given to alignment
> of other data structures, AFAIR.

But it's not aligned anymore on at last half a decade's hardware, and
it's what we already align *all* bigger ShmemAlloc() values with. And
BufferDescriptors surely counts as larger in its entirety.

> 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.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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