CPU Scheduling

Presentation by Colin McCarthy

Runqueues

• Foundation of Linux scheduler algorithm

• Keeps track of all runnable tasks assigned to CPU

• One runqueue for each CPU

Runqueues

Active Expired

Task 1

Task 2 (lock)

Task 3

Task 4

Task 5

Priority Arrays

• O(1) performance

• Multiple tasks with same priority are scheduled round robin

Priority Arrays

Bitmap Priority Array

T1

T2 T3

1

2

3

4

5

Calculating Priority

• Static task priority– Nice value: changed with nice() system call– Scheduler never changes static priority, but

user can

• Dynamic task priority– Adds to or subtracts from static priority with

rewards and penalties– I/O-bound tasks get rewarded, CPU-bound

tasks are penalized

I/O-bound vs. CPU-bound

• Keep track of how much time a task spent sleeping (blocked on I/O) and how much time spent running on CPU

• The higher a task’s sleep average, the higher it’s dynamic priority

Calculating Timeslice

• Scale a task’s static priority onto the possible timeslice range (max and min)

• The higher the task’s static priority, the larger the timeslice it gets.

• An interactive task’s timeslice may be broken up into chunks

Other effects on calculations

• When tasks are forked, the sleep average of both the parent and child are reduced– Prevents highly interactive parents from

hogging the CPU with like children– Timeslice is not reduced, since timeslice is

based only on static priority

• Interactive tasks whose timeslice has run out will be reinserted into the active priority array as long as there are no tasks starving in the expired array.

The schedule() Function

• Picks a new task to run and switches to it1. Check that it’s not being called during an

atomic period2. Disable preemption and check how long the

current task has been running3. Look for runnable tasks. If none, try a load

balancing. If still none, idle or swap priority arrays

4. Check active priority array’s bitmap5. Switch tasks and re-enable preemption

Load Balancing

• Tasks tend to stay on the same CPU in the interest of cache hotness and memory bank proximity

• Load balancing is distributing tasks more evenly between CPUs

• Since there is a single runqueue for each CPU, it keeps track of the CPU’s load

• A migration thread is kept running on each CPU to handle load balancing

Soft RT Scheduling

• Linux 2.6 scheduler provides soft RT scheduling support in the same manner described in Nathan’s presentation on Linux 2.2

● Processes can be in three scheduling classes:– SCHED_FIFO – “Real time.”– SCHED_RR - “Real time.”– SCHED_OTHER – Normal process.● Real time processes always win out over normalprocesses.● Real time processes with higher priority alwayswin out over those with with lower priority.● FIFO/RR only applies if there are multiple RTprocesses with the same priority.

Linux 2.2 Scheduler

NUMA Scheduling

• Intended for large systems with many nodes

• Scheduler domains created to account for proximity issues with speed

• Top-level domain contains all CPUs with one group for each node, with multiple CPUs possible in each. A CPU mask is provided by the group.

NUMA Task Migration

• First balances base domains in individual CPU

• Then balances the parent domain (could be multiple CPUs) and so on