1
Fork 0

Let it be so.

This commit is contained in:
Motiejus Jakštys 2022-02-08 09:52:47 +02:00 committed by Motiejus Jakštys
commit b0d23fd9bc
1 changed files with 74 additions and 0 deletions

74
README.md Normal file
View File

@ -0,0 +1,74 @@
Turbo NSS
---------
glibc nss library for passwd and group.
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, return the group name.
Assuming a member is in ~100 groups on average, that's 1M group lookups per
second. We need to convert gid to a group index quickly.
Data structures
---------------
Basic data structures that allow efficient storage:
```lang=c
// reminder:
typedef uid_t uint32;
typedef gid_t uint32;
// 6*32b = 6*4B = 24B/user
typedef struct {
uid_t uid;
gid_t gid;
name_offset uint32; // offset into *usernames
gecos_offset uint32; // offset into *gecos
shell_offset uint32; // offset into *shells
additional_groups_offset uint32; // offset into additional_groups
} user;
const char* usernames; // all concatenated usernames, fsst-compressed
const char* gecoss; // all concatenated gecos, fsst-compressed
const char* shells; // all concatenated home directories, fsst-compressed
const uint8_t additional_groups; // all additional_groups, turbo compressed
typedef struct {
gid_t gid;
name_offset uint32; // offset into *groupnames
members_offset uint32; // offset into members
}
const char* groupnames; // all concatenated group names, fsst-compressed
const uint8_8 members; // all concatenated members, turbo compressed
```
"turbo compression" encodes a list of uids/gids with this algorithm:
1. sort ascending.
2. extract deltas and subtract 1: `awk '{diff=$0-prev; prev=$0; print
diff-1}'`.
3. varint-encode these deltas into an uint32, like protobuf or utf8.
With typical group memberships (as of writing) this requires ~1.3-1.5 byte per
entry.
Indexes
-------
The following operations need to be fast, in order of importance:
1. lookup gid -> group (this is on hot path in id).
2. lookup uid -> user.
3. lookup username -> user.
4. lookup groupname -> group.
5. (optional) iterate users using a defined order (`getent passwd`).
6. (optional) iterate groups using a defined order (`getent group`).