On Thu, Jan 25, 2018 at 09:30:54AM +1300, Thomas Munro wrote: > On Thu, Jan 25, 2018 at 7:19 AM, Robert Haas <robertmh...@gmail.com> wrote: > > On Wed, Jan 24, 2018 at 12:25 PM, Tomas Vondra > > <tomas.von...@2ndquadrant.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.gmail.com#CAEepm=1bRyd+_W9eW-QmP1RGP03ti48zgd=k11q6o4edqlg...@mail.gmail.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. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + As you are, so once was I. As I am, so you will be. + + Ancient Roman grave inscription +
