Why is CONFIG_MAIN_THREAD_PRIORITY=7 in mqtt_simpe?

[Yusuke]

CONFIG_MAIN_THREAD_PRIORITY=7

I read prj.conf of mqtt_simple. Why is the value of 7 given?

https://github.com/NordicPlayground/fw-nrfconnect-nrf/blob/master/samples/nrf9160/mqtt_simple/prj.conf

This is definition of  CONFIG_MAIN_THREAD_PRIORITY.

https://docs.zephyrproject.org/1.13.0/reference/kconfig/CONFIG_MAIN_THREAD_PRIORITY.html

Does larger value mean higher priority?

Why is it set 7 while the default value is 0?

Thanks.

[Martin Lesund]

Hi Yusuke, 

Please check out the documentation of Thread priority.

Lower values mean higher priority.

Typical values in NCS are:

-2 for MCUboot

0 for SPM

7 for regular applications

[Yusuke]

Thanks!

I see. Can I configure CONFIG_MAIN_THREAD_PRIORITY=1,2,3,4,5 or 6?

[Martin Lesund]

Hi Yusuke,

Yes, since the main thread is a preemptible thread it can have non-negative priority values. (e.g. 0,1,2,3..)

("The kernel supports a virtually unlimited number of thread priority levels")

Just set the value inside the prj.conf:

CONFIG_MAIN_THREAD_PRIORITY=7

This will affect how the scheduler allows the threads to share the CPU. 

More details on this are described in the Scheduling documentation.

Best regards,
Martin L.

[Yusuke]

I see. So I shouldn't basically change the value, right?

[Martin Lesund]

That depends on what you are planing with your application. 

You do not need to change it, but you could if you want some threads to be prioritized over the other.

To understand how this works I would recommend checking out the documentation links posted in my last post.