Re: Misaligned BufferDescriptors causing major performance problems on AMD

On Wed, Jan 28, 2015 at 10:35 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
>> On 2015-01-26 21:13:31 -0500, Robert Haas wrote:
>>> So maybe we should also do something like what LWLocks do, and make a
>>> union between the actual structure and an appropriate array of padding
>>> bytes - say either 64 or 128 of them.
>
>> Hm. That's a bit bigger patch. I'm inclined to just let it slide for the
>> moment. I still have plans to downsize some of sbufdesc's content (move
>> the io lock out) and move the content lwlock inline. Then we're not
>> going to have much choice but do this...
>
> Even if you didn't have plans like that, it would be entire folly to
> imagine that buffer headers will be exactly 64 bytes without some forcing
> function for that. Accordingly, I vote against applying any patch that
> pretends to improve their alignment unless it also does something to
> ensure that the size is a power of 2. Any notational changes that are
> forced by that would be much better done in a patch that does only that
> than in a patch that also makes functional changes to the header contents.

I entirely agree.

--
Robert Haas
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