turbonss/docs/architecture.md

Design & constraints

To be fast, the user/group database (later: DB) has to be small (background). It encodes user & group information in a way that minimizes the DB size, and reduces jumping across the DB ("chasing pointers and thrashing CPU cache").

To understand how this is done efficiently, let's analyze the getpwnam_r(3) in high level. This API call accepts a username and returns the following user information:

struct passwd {
   char   *pw_name;       /* username */
   char   *pw_passwd;     /* user password */
   uid_t   pw_uid;        /* user ID */
   gid_t   pw_gid;        /* group ID */
   char   *pw_gecos;      /* user information */
   char   *pw_dir;        /* home directory */
   char   *pw_shell;      /* shell program */
};

Turbonss, among others, implements this call, and takes the following steps to resolve a username to a struct passwd*:

• Open the DB (using mmap) and interpret it's first 64 bytes as a *struct Header. The header stores offsets to the sections of the file. This needs to be done once, when the NSS library is loaded.
• Hash the username using a perfect hash function. Perfect hash function returns a number n ∈ [0,N-1], where N is the total number of users.
• Jump to the n'th location in the idx_name2user section, which contains the index i to the user's information.
• Jump to the location i of section Users, which stores the full user information.
• Decode the user information (which is all in a continuous memory block) and return it to the caller.

In total, that's one hash for the username (~150ns), two pointer jumps within the group file (to sections idx_name2user and Users), and, now that the user record is found, memcpy for each field.

The turbonss DB file is be mmap-ed, making it simple to jump across the file using pointer arithmetic. This also reduces memory usage, as the mmap'ed regions are shared. Turbonss reads do not consume any heap space.

Tight packing places some constraints on the underlying data:

• Permitted length of username and groupname: 1-32 bytes.
• Permitted length of shell and home: 1-256 bytes.
• Permitted comment ("gecos") length: 0-255 bytes.
• User name, groupname, gecos and shell must be utf8-encoded.
• User and Groups sections are up to 2^35B (~34GB) large. Assuming an "average" user record takes 50 bytes, this section would fit ~660M users. The worst-case upper bound is left as an exercise to the reader.

Sorting is stable. In v0:

• Groups are sorted by gid, ascending.
• Users are sorted by their name, ascending by the unicode codepoints (locale-independent).

remarks on id(1)

A known implementation runs id(1) at ~250 rps sequentially on ~20k users and ~10k groups. Our rps target is much higher.

To better reason about the trade-offs, it is useful to understand how id(1) is implemented, in rough terms:

• lookup user by name (getpwent_r(3)).
• get all gids for the user (getgrouplist(3)). Note: it is actually using initgroups_dyn, accepts a uid, and is very poorly documented.
• for each additional gid, get the struct group* ([getgrgid_r(3)][getgrgid_r]).

Assuming a member is in ~100 groups on average, to reach 10k id/s translates to 1M group lookups per second. We need to convert gid to a group index, and group index to a group gid/name quickly.

Caveat: struct group contains an array of pointers to names of group members (char **gr_mem). However, id does not use that information, resulting in read amplification, sometimes by 10-100x. Therefore, if argv[0] == "id", our implementation of getgrid_r(3) returns the struct group* without the members. This speeds up id by about 10x on a known NSS implementation.

Relatedly, because getgrid_r(3) does not need the group members, the group members are stored in a different DB section, reducing the Groups section and making more of it fit the CPU caches.

Turbonss header

The turbonss header looks like this:

OFFSET     TYPE     NAME                      DESCRIPTION
   0      [4]u8     magic                     f0 9f a4 b7
   4         u8     version                   0
   5         u8     endian                    0 for little, 1 for big
   6         u8     nblocks_shell_blob        max value: 63
   7         u8     num_shells                max value: 63
   8        u32     num_groups                number of group entries
  12        u32     num_users                 number of passwd entries
  16        u32     nblocks_bdz_gid           bdz_gid section block count
  20        u32     nblocks_bdz_groupname
  24        u32     nblocks_bdz_uid
  28        u32     nblocks_bdz_username
  32        u64     nblocks_groups
  40        u64     nblocks_users
  48        u64     nblocks_groupmembers
  56        u64     nblocks_additional_gids
  64        u64     getgr_bufsize
  72        u64     getpw_bufsize
  80     [48]u8     padding

magic is 0xf09fa4b7, and version must be 0. All integers are native-endian. nblocks_* is the count of blocks of a particular section; this helps calculate the offsets to all sections.

Some numbers, like nblocks_shell_blob, num_shells, would fit to smaller number of bytes. However, interpreting [2]u6 with xxd(1) is harder than interpreting [2]u8. Therefore we are using the space we have to make these integers byte-wide.

getgr_bufsize and getpw_bufsize is a hint for the caller of getgr* and getpw*-family calls. This is the recommended size of the buffer, so the caller does not receive ENOMEM.

Primitive types

User and Group entries are sorted by the order they were received in the input file. All entries are aligned to 8 bytes. All User and Group entries are referred by their byte offset in the Users and Groups section relative to the beginning of the section.

const PackedGroup = packed struct {
    gid: u32,
    padding: u3,
    groupname_len: u5,
}

PackedGroup is followed by the group name (of length groupname_len), followed by a varint-compressed offset to the groupmembers section, followed by 8b padding.

PackedUser is a bit more involved:

pub const PackedUser = packed struct {
    uid: u32,
    gid: u32,
    shell_len_or_idx: u8,
    shell_here: bool,
    name_is_a_suffix: bool,
    home_len: u6,
    name_len: u5,
    gecos_len: u11,
}

... followed by userdata: []u8:

• home.
• name (optional).
• gecos.
• shell (optional).
• additional_gids_offset: varint.

First byte of home is stored right after the gecos_len field, and its length is home_len. The same logic applies to all the stringdata fields: there is a way to calculate their relative position from the length of the fields before them.

PackedUser employs two data-oriented compression techniques:

• shells are often shared across different users, see the "Shells" section.
• name is frequently a suffix of home. For example, /home/vidmantas and vidmantas. In this case storing both name and home is wasteful. Therefore name has two options:
  1. name_is_a_suffix=true: name is a suffix of the home dir. Then name starts at the home_len - name_len'th byte of home, and ends at the same place as home.
  2. name_is_a_suffix=false: name begins one byte after home, and it's length is name_len.

The last field additional_gids_offset: varint points to the additional_gids section for this user.

Shells

Normally there is a limited number of separate shells even in huge user databases. A few examples: /bin/bash, /usr/bin/nologin, /bin/zsh among others. Therefore, "shells" have an optimization: they can be pointed by in the external list, or, if they are unique to the user, reside among the user's data.

255 most popular shells (i.e. referred to by at least two User entries) are stored externally in "Shells" area. The less popular ones are stored with userdata.

Shells section consists of two sub-sections: the index and the blob. The index is an array of offsets: the i'th shell starts at offsets[i] byte, and ends at offsets[i+1] byte. If there is at least one shell in the shell section, the index contains a sentinel index as the last element, which signifies the position of the last byte of the shell blob.

shell_here=true in the User struct means the shell is stored with userdata, and it's length is shell_len_or_idx. shell_here=false means it is stored in the Shells section, and it's index is shell_len_or_idx (and the actual string start and end offsets are resolved as described in the paragraph above).

Variable-length integers (varints)

Varint is an efficiently encoded integer (packed for small values). Same as protocol buffer varints, except the largest possible value is u64. They compress integers well. Varints are stored for group memberships.

Group memberships

There are two group memberships at play:

1. Given a group (gid/name), resolve the members' names (e.g. getgrgid).
2. Given a username, resolve user's group gids (for initgroups(3)).

When group's memberships are resolved in (1), the same call also requires other group information: gid and group name. Therefore it makes sense to store a pointer to the group members in the group information itself. However, the memberships are not always necessary (see remarks about id(1)), therefore the memberships will be stored separately, outside of the groups section.

Similarly, when user's groups are resolved in (2), they are not always necessary (i.e. not part of struct user*), therefore the memberships themselves are stored out of bound.

groupmembers and additional_gids store group and user memberships respectively. Membership IDs are packed — not necessitating random access, thus suitable for compression.

• groupmembers consists of a number X followed by a list of offsets to User records, because getgr* returns pointers to membernames, thus a name has to be immediately resolvable.
• additional_gids is a list of gids, because initgroups_dyn (and friends) returns an array of gids.

Each entry of groupmembers and additional_gids starts with a varint N, which is the number of upcoming elements. Then N delta-compressed varints, which are:

• additional_gids a list of gids.
• groupmembers byte-offsets to the User records in the users section.

Indices

Now that we've sketched the implementation of id(3), it's clearer to understand which operations need to be fast; in order of importance:

1. lookup gid -> group info (this is on hot path in id) without members.
2. lookup username -> user's groups.
3. lookup uid -> user.
4. lookup groupname -> group.
5. lookup username -> user.

These indices can use perfect hashing like bdz from cmph: a perfect hash hashes a list of bytes to a sequential list of integers. Perfect hashing algorithms require some space, and take some time to calculate ("hashing duration"). I've tested BDZ, which hashes [][]u8 to a sequential list of integers (not preserving order) and CHM, preserves order. BDZ accepts an optional argument 3 <= b <= 10.

• BDZ algorithm requires (b=3, 900KB, b=7, 338KB, b=10, 306KB) for 1M values.
• Latency to resolve 1M keys: (170ms, 180ms, 230ms, respectively).
• Packed vs non-packed latency differences are not meaningful.

CHM retains order, however, 1M keys weigh 8MB. 10k keys are ~20x larger with CHM than with BDZ, eliminating the benefit of preserved ordering: we can just have a separate index.

None of the tested perfect hashing algorithms makes the distinction between existing (in the initial dictionary) and new keys. In other words, HASH(value) will be pointing to a number n ∈ [0,N-1], regardless whether the value was in the initial dictionary. Therefore one must always confirm, after calculating the hash, that the key matches what's been hashed.

idx_* sections are of type []u32 and are pointing from hash(key) to the respective Groups and Users entries (from the beginning of the respective section). Since User and Group records are 8-byte aligned, the actual offset to the record is acquired by right-shifting this value by 3 bits.

Database file structure

Each section is padded to 64 bytes.

SECTION               SIZE             DESCRIPTION
header                128              see "Turbonss header" section
bdz_gid               ?                bdz(gid)
bdz_groupname         ?                bdz(groupname)
bdz_uid               ?                bdz(uid)
bdz_username          ?                bdz(username)
idx_gid2group         len(group)*4     bdz->offset Groups
idx_groupname2group   len(group)*4     bdz->offset Groups
idx_uid2user          len(user)*4      bdz->offset Users
idx_name2user         len(user)*4      bdz->offset Users
shell_index           len(shells)*2    shell index array
shell_blob            <= 65280         shell data blob (max 255*256 bytes)
groups                ?                packed Group entries (8b padding)
users                 ?                packed User entries (8b padding)
groupmembers          ?                per-group delta varint memberlist (no padding)
additional_gids       ?                per-user delta varint gidlist (no padding)