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TT CPU Scheduler

Task Type (TT) is an alternative CPU Scheduler for linux.

The goal of the Task Type (TT) scheduler is to detect tasks types based on their behaviours and control the schedulling based on their types. There are 5 types:

  1. REALTIME
  2. INTERACTIVE
  3. NO_TYPE
  4. CPU_BOUND
  5. BATCH

Find the descriptions and the detection rules in tasks.ods

The benefit of task types is to allow the scheduler to have more control and choose the best task to run next in the CPU.

TT gives RT tasks a -20 prio in vruntime calculations. This boosts RT tasks over other tasks. The preemption rules are purely HRRN where RT tasks have a priority since their vruntimes are relatively less than other types. The reason of using HRRN instead of hard level picking is to smooth out the preemtions and to prevent any chance of starvation.

Monitoring detected tasks

You need to compile with CONFIG_SCHED_DEBUG=y. I have added a field in the output of tasks information task_type. See and use ttdebug.sh.

Usage examples:

ttdebug.sh | grep -i realtime

watch -t "(ttdebug.sh | grep -i interactive)"

watch -t "(ttdebug.sh | egrep -i 'webco|firefox')"

Note: Tasks types are detected based on their behaviour, not by what it should be. So if systemd at some point acted like a REALTIME tasks and went for long sleep then the type would be REALTIME until it wakes up and get its type updated. You might see many sleeping tasks with incorrect types because at some point on booting time they acted like REALTIME, CPU_BOUND, or whatever type. Those tasks are sleeping for long time, so when they wake up their type will be INTERACITVE sine they have very hight HRRN value. So, don't worry about the type of sleeping system processes.

sysctls

kernel.sched_tt_max_lifetime

Default is 22s. This is the tasks' maximum life time to normalize their life time and vruntime. Similar to CacULE's cacule_max_lifetime.

kernel.sched_tt_rt_prio Default is -20. Range [-20, 19]. In case that tasks with types other than realtime are starving because of realtime tasks' priorities are too high, you can soften the priority of realtime tasks. The -20 is the highest, 19 is the least priority.

kernel.sched_tt_interactive_prio Default is -10. Range [-20, 19].

kernel.sched_tt_cpu_bound_prio Default is -15. Range [-20, 19].

kernel.sched_tt_batch_prio Default is 19. Range [-20, 19].

kernel.sched_tt_balancer_opt

It can be set to 4 values:

  • 0: Normal TT balancer
  • 1: Candidate Balancer (which is an addition to normal TT balancer - good for reponsiveness (perfomance gets affected when #CPUs > 4))
  • 2: CFS balancer (default - good for perfomance/throughput)
  • 3: Power save balancer (tries its best to avoid running tasks on idle cpus - saves power)

You can change the balancer option at run time.

kernel.sched_tt_lat_sens_enabled Default is 1. latency sensitive keeps CPUs (with no tasks) at high frequency for sometime (~1ms) in case of incoming task during this time would run faster. It reduces latency but increases power consumption. If Power save balancer is chosen, then this option has no effect (i.e. disabled, = 0).

kernel.sched_tt_dedicated_cpu_bound_enabled Default is 1. This option stick a CPU bound task to its current CPU to enhance cache locality. A CPU can only have one dedicated cpu bound task.

Support

Telegram: https://t.me/tt_sched

Thank you

Hamad