Am unable to understand (what I think) is a peculiar situation wrt process/thread scheduling on Linux. [Env: Ubuntu 12.10 , kernel ver 3.5.0-... ]
A 'test' application (call it sched_pthread), will have a total of three threads - 'main' + two others; main() will spawn two new threads:
Thread 1 [main()]:
Runs as SCHED_NORMAL (or SCHED_OTHER). It:
Creates two threads (Thread 2 and Thread 3 below); they will automatically inherit the
scheduling policy and priority of main.
Prints the character “m” to the terminal in a loop.
Terminates.
Thread 2 [t2]:
Sleeps for 2 seconds.
Changes it's scheduling policy to SCHED_FIFO, setting it's realtime priority to the
value passed on the command line.
Prints the character “2” to the terminal in a loop.
Terminates.
Thread 3 [t3]: Changes it's scheduling policy to SCHED_FIFO, setting it's realtime priority to the value passed on the command line plus 10. Sleeps for 4 seconds. Prints the character “3” to the terminal in a loop. Terminates.
We run it as root. As per scheduling policy, we should first see main() print 'm' for about 2s, then it should get preempted by t2 (as it awakens after 2s) and we should see '2' appearing on the terminal for about 2s, after which t3 wakes up (it was asleep for 4s); it should now preempt everyone else & emit '3' to the display; after it dies, we should see '2's until p2 dies, then 'm's until main() dies.
So okay, this works: when i test it in console mode (no X server). Of course, i take care to run it as:
sudo taskset 02 ./sched_pthrd 8
so that in effect it runs on only 1 processor core.
When I run the same thing in graphical mode (with X), after the initial 'm's by main(), there is a long-ish pause (a few seconds) during which nothing appears on the screen; then all of a sudden we get the 2's and 3's and m's slapped onto the screen! This can be explained : the X server (Xorg) was preempted by the SCHED_FIFO threads and hence could not 'paint' pixels on the screen.
However - here's the question at last - : how come the Xorg process was not scheduled / migrated onto some other core (so that it can continue updating the screen in parallel with the RT threads)?? taskset verifies that the cpu affinity mask of Xorg is 'f' (1111b) (i have 4 cores on my laptop).
Any ideas??
Here's the source code: https://dl.dropboxusercontent.com/u/9301413/code_shared/so_sched_pthrd.c -or- http://goo.gl/PLHBrC
TIA! -Kaiwan.