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. -- Thomas Munro http://www.enterprisedb.com
