What is the priority of background processes in linux environment

All processes running at the same nice value will get an equal cpu-timeslice.

Here is a simple test that launches 2 processes, both performing the exact same operations. One is launched in the background and the other in the foreground.

dd if=/dev/zero of=/dev/null bs=1 &dd if=/dev/zero of=/dev/null bs=1

The relevant extract from subsequently running the top command

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND 1366 root      20   0  1576  532  436 R  100  0.0   0:30.79 dd 1365 root      20   0  1576  532  436 R  100  0.0   0:30.79 dd

Next, if both the processes are restricted to the same CPU,

taskset -c 0 dd if=/dev/zero of=/dev/null bs=1 &taskset -c 0 dd if=/dev/zero of=/dev/null bs=1

Again the relevant extract from subsequently running the top command shows

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND 1357 root      20   0  1576  532  436 R   50  0.0   0:38.74 dd 1358 root      20   0  1576  532  436 R   50  0.0   0:38.74 dd

both the processes compete for CPU-timeslice and are equally prioritised.

Finally,

kill -SIGINT 1357 &kill -SIGINT 1358 &kill -SIGINT 1365 &kill -SIGINT 1366 &

results in similar amounts of data copied and throughput.

25129255+0 records in25129255+0 records out25129255 bytes (25 MB) copied, 34.883 s, 720 kB/s

Slight discrepancies in output may occur in the throughput due to differences in the exact moment the individual processes respond to the break-signal and stop running.

However also note that sched_autogroup_enabled exists.

if enabled, sched_autogroup_enabled ensures that the fairness in distributing cpu-timeslice is now performed between individual shells. By distributing cpu equally amongst the various active shells.

Thus if a shell launches 1 process A,
and another shell launches 2 processes B and C,
then the CPU execution timeslice will typically be distributed as

A <-- 50%  <---- shell1 50% B <-- 25%  <-. C <-- 25%  <--`- shell2 50%

(though all 3 processes A, B & C are running at the same nice level.)

The process priorities in Linux kernel is given by NICE values.Refer to the link http://en.wikipedia.org/wiki/Nice_(Unix)

The nice values (ranging between -20 to +19) define the process priorities, -20 being the highest priority task. Usually the user-space processes are given default nice values of '0'. You can check the nice values for the running processes on your shell using the below command.

ps -alF S   UID   PID  PPID  C PRI  NI ADDR SZ WCHAN  TTY          TIME CMD0 S  1039  1268 16889  0  80   0 - 11656 poll_s pts/8    00:00:08 vim0 S  1047  1566 17683  0  80   0 -  2027 wait   pts/18   00:00:00 arm-linux-andro0 R  1047  1567  1566 21  80   0 -  9143 ?      pts/18   00:00:00 cc10 R  1031  1570 15865  0  80   0 -  2176 -      pts/24   00:00:00 ps0 R  1031 17357 15865 99  80   0 -  2597 -      pts/24   00:03:29 top

So from above output if you see the 'NI' column shows your nice values. When i tried running a background process, that too got a nice value of '0' (top is that process with PID 17357). That would mean, it will also be queued up for like a foreground process and will be scheduled likewise.