mirror of
https://github.com/LadybirdBrowser/ladybird.git
synced 2024-11-25 09:00:22 +00:00
71 lines
3.5 KiB
Markdown
71 lines
3.5 KiB
Markdown
# ProcFS Indexing
|
|
|
|
## Is a ProcFS index deterministic value?
|
|
|
|
Short answer - yes. Long answer - because of the design pattern that was chosen,
|
|
each `InodeIndex` actually represent a known object, so it is guaranteed to be
|
|
the same always for global ProcFS objects. For process ID directories, once that
|
|
process has been killed, its primary segment value is no longer valid and hence
|
|
all sub-segments of it are not relevant anymore, but if the process is still alive,
|
|
it is guaranteed that accessing the same `InodeIndex` in regard to an object tied to
|
|
a process directory will provide the expected object.
|
|
|
|
## The goal - zero allocations when creating new process
|
|
|
|
The main goal is to have zero allocations happening in ProcFS when a new process is created.
|
|
The old ProcFS design followed that principle, but was quite hard to edit and to extend with new
|
|
functionality.
|
|
The current ProcFS design doesn't follow that principle, but is easier to edit and to extend.
|
|
A compromise is needed to ensure we get the advantages from both designs while minimizing the
|
|
effects of the disadvantages of each design.
|
|
|
|
## The segmented index
|
|
|
|
### The layout of the segmented index
|
|
|
|
Since it was decided that heap allocations for ProcFS are *mostly* bad, the new
|
|
design layout tries to achieve most of the principle of "Don't allocate anything
|
|
until actually needed". For that to happen, `InodeIndex` (u64 value) is split
|
|
into 3 Segments:
|
|
- The primary segment: value 0 is reserved for all non-PID inodes in the procfs.
|
|
All values from 1 to 0xFFFFFFF are valid PID indices, which represents all PIDs from 0 to 0xFFFFFFE
|
|
|
|
- The Sub-directory segment: value 0 is reserved for parent PID directory. All other values are
|
|
available for usage of sub-directories in the PID directory.
|
|
|
|
- The property segment: value 0 is reserved for parent PID directory. All other values are
|
|
available for usage of components in the PID directory or in sub-directories of the PID directory.
|
|
|
|
So, the final layout of the 64 bit index is:
|
|
|
|
```
|
|
| Primary Segment (28 bits) | Sub-directory (16 bits) | Component (20 bits) |
|
|
```
|
|
|
|
Example: To find a Thread 0 stack, for PID 1, the following encoding is applied:
|
|
|
|
```
|
|
hex(2 << 16 | 2 << (16 + 28)) == 0x200000020000
|
|
```
|
|
|
|
### Two rules for indexing
|
|
|
|
We don't want to allocate anything when a process is created, but we still want
|
|
to allocate global objects, so it's somewhat a compromise between two conflicting targets.
|
|
To do that we need to ensure that:
|
|
|
|
1. If the primary segment value equals to 0, then the sub-directory and property segmentation
|
|
is not applied, but a sequential indexing is determined instead. This is needed so ProcFS can still
|
|
use global components that were pre-allocated beforehand. This means that there might be up to
|
|
68719476735 global components (including global sub-directories objects) in the ProcFS.
|
|
Otherwise, for every primary segment value > 0, then the sub-directory and property segmentation
|
|
is applied. This means that there might be up to 65534 sub-directories in a PID directory, and
|
|
up to 1048575 (1048574 for PID directory) properties (objects) in each sub-directory.
|
|
|
|
2. If the primary segment value equals to 0, then value 0 in both artificial sub-directory
|
|
and property segments represents the root ProcFS folder.
|
|
Otherwise, for every primary segment value > 0, value 0 in both sub-directory and
|
|
property segments are reserved to represent the root PID directory.
|
|
Please note that if the sub-directory segment > 0, and property segment = 0 is a valid
|
|
index, and represents a valid property object in that sub-directory.
|
|
|