Re: Misaligned BufferDescriptors causing major performance problems on AMD

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.

regards, tom lane