System Management Guide: Operating System and Devices

RT_MPC and RT_GRQ

The use of multiple queues increases the processor affinity of threads, but there is a special situation where you might want to counteract this effect. When there is only one run queue, a thread that has been awakened (the waking thread) by another running thread (the waker thread) would normally be able to use the CPU immediately on which the waker thread was running. With multiple run queues, the waking thread may be on the run queue of another CPU which cannot notice the waking thread until the next scheduling decision. This may result in up to a 10 ms delay.

This is similar to scenarios in earlier releases of this operating system which migjht have occurred using the bindprocessor option. If all CPUs are constantly busy, and there are a number of interdependent threads waking up, there are two options available.

• The first option, which uses one run queue, is to set the environment variable RT_GRQ=ON which forces unbound selected threads to be dispatched off the global run queue.
• Alternatively, POWER-based platform users can choose the real time kernel option (type the command bosdebug -R on and then bosboot) and the RT_MPC=ON environment variable for selected processes. It is essential to maintain a performance log of your systems to closely monitor the impact of any tuning you attempt. This option is not useful to Itanium-based platforms because the architecture does not perceive the result as a real-time kernel.