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process-scheduler.md

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Process's scheduler

Process scheduler is responsible for finding the next eligible task and switching to the context of that task. It has to balance between responsiveness and efficency. The cost of context switch is not negligible.

The option for tuning in case of CFS is /proc/sys/kernel/sched_min_granularity_ns and /proc/sys/kernel/sched_latency_ns.

# cat /proc/sys/kernel/sched_min_granularity_ns
750000
# cat /proc/sys/kernel/sched_latency_ns
6000000

Scheduler maintains per CPU run queue and picks next task for particular CPU from its run queue. When a task calls scheduler() function, the scheduler code processes run queue associated with the CPU of the calling task.

Scheduluer and policy

There are mutiple schedulers, each is responsible for a type of tasks.

stop_sched_class → rt_sched_class → fair_sched_class → idle_sched_class
  • stop_sched_class is for kernel tasks that add/remove CPUs dynamically.
  • rt_sched_class is for real time tasks.
  • fair_sched_class is for ordinary tasks. Most of tasks are of this type
  • ide_sched_class is for lowest priority tasks that only run when there are no other runable tasks.

Linux provide syscall for assign a process to a specific scheduler policy sched_setscheduler. The following are policies applicable for non realtime scheduling

  • SCHED_OTHER: the standard round-robin time-sharing policy, this is served by fair_sched_class
  • SCHED_BATCH: for "batch" style execution of processes, this is served by fair_sched_class
  • SCHED_IDLE: for running very low priority background jobs, this is served by idle_sched_class

The realtime scheduling has following policies

  • SCHED_FIFO: fifo scheduling
  • SCHED_RR: round robind scheduling

Scheduler class assigned to a process can be viewed using class field in ps -o command

# ps -e -o pid,cmd,class 
PID CMD                       CLS
1 /sbin/init                  TS
2 [kthreadd]                  TS
...
7 [watchdog/0]                FF

where

TS  SCHED_OTHER    B   SCHED_BATCH
FF  SCHED_FIFO     ISO SCHED_ISO
RR  SCHED_RR       IDL SCHED_IDLE

We can change scheduler class using command chrt

# ps -o pid,cmd,class,priority -p 1003
PID CMD                         CLS PRI
1003 cron                        TS   20
# chrt -p -b 0 1003
# ps -o pid,cmd,class,priority -p 1003
PID CMD                         CLS PRI
1003 cron                        B    20

chrt can'nt change priority for non realtime scheduler's class. We need to use renice

# renice 5 -p 1003
1003 (process ID) old priority 10, new priority 5
# ps -o pid,cmd,class,priority,nice -p 1003
PID CMD                         CLS PRI  NI
1003 cron                        B    25   5

CFS

Linux kernel from version 2.6.23 use Comppletly Faire Scheduler - CFS for process scheduling see [http://en.wikipedia.org/wiki/Completely_Fair_Scheduler].

CFS maintains per-task vruntime. As task is on CPU, this value get increased by time spending on CPU. The scheduler function check and pick task with minimum vruntime and assign CPU to it.

priority (i.e nice) is used as weight when adjusting vruntime.

References