wiki:WikiStart/Toolchain/PosixMQ

The POSIX message queues are a mean to transport samples from ports across processes, but locally (i.e. on the same machine). They are a lot more efficient than using CORBA (less copying / marshalling cost) and they are compatible with hard realtime execution.

Overview

Both orocos.rb and the supervision can automatically use the POSIX MQueues.

The main issue is with variable-sized types, i.e. types that contain std::vector "somewhere". There are mainly two ways to deal with them:

  • specify a maximum size for them in the orogen spec and/or inside the ruby script and/or in side the supervision models
  • make sure, in the C++, that the _output_port.setDataSample() method is called before the startHook() (preference goes for the constructor) with a data sample that is properly sized.

Turning it ON

Just do

Orocos::MQueue.auto = true

orocos.rb will then use MQueues if they are supported on your system (built in the RTT)

Setup using maximum size specification

  • at the type level, a maximum size can be declared globally for a type
Orocos.max_sizes '/base/samples/RigidBodyState', 'sourceFrame' => 20, 'targetFrame' => 20

This can be used in orogen files, orocos.rb scripts and supervision scripts

  • at the task model level, the maximum size can be specified with a max_sizes declaration on ports, for instance:
name 'hokuyo'
task_context 'Task' do
  output_port('scan_samples', '/base/samples/LaserScan').
    max_sizes('ranges' => 2000)
end

This declares that the 'ranges' field of the laser scans pushed on the scan_samples port is at most 2000 samples big. This specification applies on all hokuyo::Task that are running on the system.

It can be used in orogen files

  • at the task level, the max size can be set on ports themselves
task = Orocos::TaskContext.get 'hokuyo'
task.scan_samples.max_sizes('ranges' => 1000)
  • Finally, the supervision has no additional means. One has to use the previous means

For instance:

class Hokuyo::Task
  orogen_model.scan_samples.max_sizes('ranges' => 100)

  def configure
    super
    orogen_task.scan_samples.max_sizes('ranges' => 10)
  end
end

Relying on the tasks to call setDataSample

This is the "normal" way in RTT. It is not available by default in orocos.rb as we feel that the "size specification" way is more robust.

If you want to use it, do

Orocos::MQueue.auto_sizes = false
Orocos::MQueue.validate_sizes = false

Fallback to CORBA

If a MQ connection fails to be created, orocos.rb will fallback to using a CORBA connection and issue a warning.

This behaviour can be turned OFF with

Orocos::MQueue.auto_fallback_to_corba = false

System setup

  • echo 1000 > /proc/sys/fs/mqueue/msg_max
  • echo 1000000 > /proc/sys/fs/mqueue/msgsize_max
  • edit /etc/security/limits.conf and add
    * soft msgqueue unlimited
    * hard msgqueue unlimited
    

you can replace "*" by a username or a @group. See comments at the top of the limits.conf file

Last modified 8 years ago Last modified on 05/10/11 10:49:50