Re: copy.c allocation constant

On Thu, Jan 25, 2018 at 09:30:54AM +1300, Thomas Munro wrote:
> On Thu, Jan 25, 2018 at 7:19 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > On Wed, Jan 24, 2018 at 12:25 PM, Tomas Vondra
> > <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> >> At the glibc level ... I'm not so sure. AFAIK glibc uses an allocator
> >> with similar ideas (freelists, ...) so hopefully it's fine too.
> >>
> >> And then there are the systems without glibc, or with other libc
> >> implementations. No idea about those.
> >
> > My guess is that a fairly common pattern for larger chunks will be to
> > round the size up to a multiple of 4kB, the usual memory page size.
>
> See also this discussion:
>
> https://www.postgresql.org/message-id/flat/CAEepm%3D1bRyd%2B_W9eW-QmP1RGP03ti48zgd%3DK11Q6o4edQLgkcg%40mail(dot)gmail(dot)com#CAEepm=1bRyd+_W9eW-QmP1RGP03ti48zgd=K11Q6o4edQLgkcg(at)mail(dot)gmail(dot)com
>
> TL;DR glibc doesn't actually round up like that below 128kB, but many
> others including FreeBSD, macOS etc round up to various page sizes or
> size classes including 8kB (!), 512 bytes. I find this a bit
> frustrating because it means that the most popular libc implementation
> doesn't have the problem so this kind of thing probably isn't a high
> priority, but probably on most other Unices (and I have no clue for
> Windows) including my current favourite we waste a bunch of memory.

The BSD memory allocator used to allocate in powers of two, and keep the
header in a separate location. They did this so they could combine two
free, identically-sized memory blocks into a single one that was double
the size. I have no idea how it works now.

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