The CacULE CPU scheduler is a CFS patchset that is based on interactivity score mechanism. The interactivity score is inspired by the ULE scheduler (FreeBSD scheduler). The goal of this patch is to enhance system responsiveness/latency.

About CacULE Scheduler

• Each CPU has its own runqueue.
• NORMAL runqueue is a linked list of sched_entities (instead of RB-Tree).
• RT and other runqueues are just the same as the CFS's.
• Wake up tasks preempt currently running tasks if its interactivity score value is higher.

The CacULE Interactivity Score

The interactivity score is inspired by the ULE scheduler (FreeBSD scheduler). For more information see: https://web.cs.ucdavis.edu/~roper/ecs150/ULE.pdf CacULE doesn't replace CFS with ULE, it only changes the CFS' pick next task mechanism to ULE's interactivity score mechanism for picking next task to run.

sched_interactivity_factor

Sets the value m for interactivity score calculations. See Figure 1 in https://web.cs.ucdavis.edu/~roper/ecs150/ULE.pdf The default value of in CacULE is 32768 which means that the Maximum Interactive Score is 65536 (since m = Maximum Interactive Score / 2). You can tune sched_interactivity_factor with sysctl command:

sysctl kernel.sched_interactivity_factor=50

This command changes the sched_interactivity_factor from 32768 to 50.

sched_max_lifetime_ms

Instead of calculating a task IS value for infinite life time, we use sched_max_lifetime_ms which is 22s by default. Task's cacule_lifetime and vruntime shrink whenever a task life time exceeds 30s. Therefore, the rate of change of IS for old and new tasks is normalized. The value sched_max_lifetime can be changed at run time by the following sysctl command:

sysctl kernel.sched_max_lifetime_ms=60000

The value is in milliseconds, the above command changes sched_max_lifetime from 22s to 60s.

In the first round, when the task's life time became > 22s, the cacule_start_time get reset to be (current_time - 11s), then, the task will keep resetting every 15s.

Starve and Cache Scores

See here.

sched_cacule_yield

See here.

Complexity

• The complexity of Enqueue a task is O(1).
• The complexity of Dequeue a task is O(1).
• The complexity of pick the next task is in O(n).

n is the number of tasks in a runqueue (each CPU has its own runqueue).

Note: O(n) sounds scary, but usually for a machine with 4 CPUS where it is used for desktop or mobile jobs, the maximum number of runnable tasks might not exceeds 10 (at the pick next run time) - the idle tasks are excluded since they are dequeued when sleeping and enqueued when they wake up.

Tasks' Priorities

The priorities are applied as the followings: The vruntime is used in Interactivity Score as the sum of execution time. The vruntime is adjusted by CFS based on tasks priorities. The same code from CFS is used in CacULE. The vruntime is equal to sum_exec_runtime if a task has nice value of 0 (normal priority). The vruntime will be lower than sum_exec_runtime for higher tasks priorities, which make Interactivity Score thinks that those task didn't run for much time (compared to their actual run time). The vruntime will be higher than sum_exec_runtime for lower tasks priorities, which make Interactivity Score thinks that those task ran for much time (compared to their actual run time). So priorities are already taken in the acount by using vruntime in the Interactivity Score equation instead of actual sum_exec_runtime.

Response Driven Balancer (RDB)

This is an experimental load balancer for Cachy/CacULE. It is a lightweight load balancer which is a replacement of CFS load balancer. It migrates tasks based on their HRRN/Interactivity Scores (IS). Most of CFS load balancing-related updates (cfs and se updates loads) are removed. The RDB balancer follows CFS paradigm in which RDB balancing happen at the same points CFS does. RDB balancing happens in three functions: newidle_balance, idle_balance, and active_balance. The newidle_balance is called exactly at the same time as CFS did (when pick next task fails to find any task to run). The RDB newidle_balance pulls one task that is the highest HRRN/IS from any CPU. The RDB idle_balance is called in trigger_load_balance when CPU is idle, it does the same as newidle_balance but with slight changes since newidle_balance is a special case. The RDB active_balance checks if the current (NORMAL) runqueue has one task, if so, it pulls the highest of the highest HRRN/IS among all other CPUS. If the runqueue has more than one task, then it pulls any highest HRRN/IS (same as idle does). A CPU cannot pull a task from another CPU that has fewer tasks (when pull any). For the all three balancing newidle_balance, idle_balance, and active_balance, the cpu first tries to pull from a CPU that shares the same cache (cpus_share_cache). If can't pull any then it tries to pull from any CPU even though they are not in the same core. Only when pulling the highest of the highest HRRN/IS (i.e. active_balance when CPU has one task), there is no check for shared cache.

How to install

The following installation links are not only for easier installation, but they are also right configured for best CacULE experience.

Debian/Ubuntu

XanMod

Arch

AUR - ptr1337

Patched Kernel Tree

1. Go to kernel tree repository
2. Select a tag version that starts with cachy / cacule (i.e cachy-5.8-r6)
3. Download and compile

How to apply the patch

1. Download the linux kernel (https://www.kernel.org/) that is same version as the patch (i.e if patch file name is cachy-5.7.6.patch, then download https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.7.6.tar.xz)
2. Unzip linux kernel
3. Download cachy patch file and place it inside the just unzipped linux kernel folder
4. cd linux-(version)
5. patch -p1 < cachy-5.7.6.patch (or git -am)
6. To build the kernel you need to follow linux build kernel documentation and tutorials.

To confirm that CacULE is currently running:

dmesg | grep -i "cacule cpu"
[    0.122999] CacULE CPU scheduler v5.9 by Hamad Al Marri.

Suggested Configs

For a helper script to auto config use this

Benchmarks

The tests are ran 11 times and best 10 tests are picked. Between each test, a sleep for 2 minutes such the following script:

 for i in (seq 1 11); sleep 2m; <test command>; end

For the following tests, CacULE is patched on Ubuntu linux-lowlatency kernel source.

Benchmarks Data

Stress-ng test

Latency test

Please see the scripts for responsiveness/latency tests: os-scheduler-responsiveness-test

Blind Tests

I made comparison between cfs and cachy on xanmod, for blind test

• test1: https://youtu.be/DilwWlNbExg?t=14
• test2: https://youtu.be/1S3OxLrcbGY?t=14
• test3: https://youtu.be/HqaNGhThihA?t=38

to reveal the which is which go back to time 0s on the video and see uname -r output

Note: In one of the tests, the recorder seems to be freezes and lagging, I repeated this test twice, while testing system is not pausing but the recorder maybe freezing or lagging while recording.

Special Thanks to

1. Alexandre Frade (the maintainer of xanmod)
2. Raymond K. Zhao (github)
3. Peter Jung (github)
4. JohnyPeaN (github)

Contacts

Telegram: https://t.me/cacule_sched

Discussions

https://github.com/hamadmarri/cacule-cpu-scheduler/discussions