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DOC: internals: document the shm-stats-file format/mapping

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Add some documentation about shm stats file structure to help writing
tools that can parse the file to use the shared stats counters.

This file was written for shm stats file version 1.0 specifically,
it may need to be updated when the shm stats file structure changes
in the future.
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Aurelien DARRAGON 2025-09-16 18:44:02 +02:00
parent 644b6b9925
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doc/internals/shm-stats-file-description.txt Normal file
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2025/09/16 - SHM stats file storage description and hints
Shm stats file (used to share thread-groupable statistics over multiple
process through the "shm-stats-file" directive) is made of:
- a main header which describes the file version, the processes making
use of it, the common clock source and hints about the number of
objects that are currently stored or provisionned in the file.
- an indefinite number of "objects" blocks coming right after the
main header, all blocks have the same size which is the size of the
maximum underlying object that may be stored. The main header tells
how many objects are stored in the file.
File header looks like this (32/64 bits systems):
0 8 16 32 48 64
+-------+---------+----------------+-------------------+-------------------+
| VERSION | 2 bytes | global_now_ms (global mono date in ms)|
|MAJOR | MINOR | hole | |
+----------------------------------+---------------------------------------+
| global_now_ns (global mono date in ns) |
+--------------------------------------------------------------------------+
| now_offset (offset applied to global monotonic date |
| on startup) |
+--------------------------------------------------------------------------+
| Process slot : | 1byte x 64
| pid | heartbeat (ticks) |
+----------------------------------+---------------------------------------+
| objects | objects slots |
| (used objects) | (available for use) |
+----------------------------------+---------------------------------------+
| padding (for future use) | 128 bytes
+--------------------------------------------------------------------------+
Object block looks like this:
0 8 16 32 48 64
+-------+---------+----------------+-------------------+-------------------+
| GUID | 128 bytes
+ (zero terminated) +
| |
+-------+---------+--------------------------------------------------------+
| tgid | type | padding |
+-------+---------+--------------------------------------------------------+
| users (bitmask of process slots making use of the obj) |
+--------------------------------------------------------------------------+
| object data |
| (version dependent) |
| struct be_counters_shared_tg or |
| struct fe_counters_shared_tg |
+--------------------------------------------------------------------------+
| padding (to anticipate evolutions) | 64 bytes
+--------------------------------------------------------------------------+