Document of memory_lock

[Zard-C]

use lock memory as non-root users (sshd_config: UsePAM yes)

Required Info:

• Operating System:
  • Ubuntu: focal
• Installation type:
  • from source
• Version or commit hash:
  • humble
• DDS implementation:
  • fastrtps
• Client library (if applicable):
  • rclcpp

Steps to reproduce issue

Before setting memlock limits:

ulimit -l
65536

Do as

ros2-realtime-examples/minimal_memory_lock/README.md

Line 14 in f61f93f

- Adjust permissions for memory locking. Add to `/etc/security/limits.conf`

said to adjust memlock limits.

run minimal_memory_lock with "--allocate-in-node 50 --lock-memory"

Expected behavior

Process memory before node creation:  5 MB
Sleeping here so all the middleware threads are created
Process memory before locking:  20 MB
Process memory locked
Process memory after locking:  106 MB
Process memory before spin:  106 MB
Total page faults before spin [Minor: 22671, Major: 0]
[WARN] [1697725046.436525305] [minimal_publisher]: New page faults during spin: [minor: 12822, major: 0]
[INFO] [1697725046.907732793] [minimal_publisher]: New page faults during spin: [minor: 0, major: 0]
[INFO] [1697725047.407619649] [minimal_publisher]: New page faults during spin: [minor: 0, major: 0]
[INFO] [1697725047.907790732] [minimal_publisher]: New page faults during spin: [minor: 0, major: 0]
[INFO] [1697725048.407655468] [minimal_publisher]: New page faults during spin: [minor: 0, major: 0]
...

Actual behavior

Process memory before node creation:  5 MB
Sleeping here so all the middleware threads are created
Process memory before locking:  20 MB
terminate called after throwing an instance of 'std::runtime_error'
  what():  mlockall failed. Error code Cannot allocate memory
[1]    3244501 abort     

and check the memlock limits with

ulimit -l
65536

It remains unchanged, and it's fewer than --allocate-in-node 50 --lock-memory need. I believe that's the reason why mlockall() failed.

Additional information

Since I was logging with ssh, so there's an additional step to take:

Add to /etc/ssh/sshd_config (as sudo)

UsePAM yes

Then check the limits

ulimit -l       
266144

Then it works!

Feature request

Feature description

• Maybe we should add the additional step above to README.md for other developers use ssh to login 😄
• In addition to using stress-ng to evaluate the entire system, we can also use perf to monitor the CPU usage of individual process minimal_memory_lock:
  perf stat -e cpu-clock,context-switches,branches,branch-misses,cache-references,cache-misses,instructions,cycles -D 30 -p $pid

[Zard-C]

CC @christophebedard

[christophebedard]

This should probably be directed towards @carlossvg instead.