Turbo NSS

glibc nss library for passwd and group.

Checking out and building

$ git clone --recursive https://git.sr.ht/~motiejus/turbonss

Alternatively, if you forgot --recursive:

$ git submodule update --init

And run tests:

$ zig build test

... the other commands will be documented as they are implemented.

This project uses [git subtrac][git-subtrac] for managing dependencies.

Steps

A known implementation runs id(1) at ~250 rps sequentially. Our goal is 10k ID/s.

id(1) works as follows:

• lookup user by name.
• get all additional gids (an array attached to a member).
• for each additional gid, get the group name.

Assuming a member is in ~100 groups on average, that's 1M group lookups per second (cmph can do 1M in <200ms). We need to convert gid to a group index quickly.

API

The following operations need to be fast, in order of importance:

1. lookup gid -> group (this is on hot path in id) with or without members (2 separate calls).
2. lookup uid -> user.
3. lookup groupname -> group.
4. lookup username -> user.
5. lookup uid -> list of gids.
6. (optional) iterate users using a defined order (getent passwd).
7. (optional) iterate groups using a defined order (getent group).

Indexes

Preliminary results of playing with [cmph][cmph]:

BDZ: tried b=3, b=7 (default), and b=10.

• BDZ algorithm stores 1M values in (900KB, 338KB, 306KB) respectively.
• Latency for 1M keys: (170ms, 180ms, 230ms).
• Packed vs non-packed latency differences are not meaningful.

CHM retains order, however, 0M keys weigh 8MB. 10k keys are ~20x larger with CHM than with BDZ, eliminating the benefit of preserved ordering.

Full file structure

The file structure stars with magic and version number, followed by a list of User, Group records and their indices. All indices are number of bytes, relative to the beginning of the file.

// /home/motiejusMotiejus.Jakstys is 16+30=46b for "my" record.
const User = struct {
    uid: u32,
    gid: u32,
    additional_gids_offset: u29,
    shell_here: u1, // whether it's stored "here" or in another place. Docs TBD
    shell_len: u6,
    home_len: u6,
    username_len: u6,
    gecos_len: u8,
    // a variable-sized array that will be stored immediately after this
    // struct.
    stringdata []u8;
}

`magic` must be 0xf09fa4b7, and `version` must be `0x00`. The remaining fields
are indexes to further sections of the file with their sizes in bytes. All
numbers are little-endian. Each field may be aligned to 64B (L1D cache size) or
4KB (standard page size), to be decided.

[git-subtrac]: https://github.com/apenwarr/git-subtrac/
[cmph]: http://cmph.sourceforge.net/