1. A Comparison Study of Process Scheduling in FreeBSD, Linux and Win2k
Introduction
Comparison of scheduling based on some parameters
On Uniprocessor system – most of it
On Multiprocessor system
Problems addressed by the schedulers

2. Introduction Objectives OSs analyzed FreeBSD 4.4 – pre SMPng
Linux – pre Preemtible kernel support
Windows 2000
Convention
Unit of work scheduled is Process

3. Comparison of scheduling on an uniprocessor system
Responsibility
Win2k – Kernel Dispatcher, running in the context of user process
Linux, FreeBSD – Scheduler process
Process types considered
I/O and interactive processes
CPU bound
Real-time processes – Not in FreeBSD

4. Comparison of scheduling on an uniprocessor system
Scheduling instances
How scheduler picks-up a process
Process priorities
Win2k FreeBSD Linux
Operation
FBSD
Linux
Win2k
Process blocked by I/O
Yes
Whenever a process is runnable
Once/sec No
when the process priority is changed either thru system call (setPriority) or by kernel (priority boosting)
Multiple runnable processes with same priority
When processor affinity changes
16 – 31
1 - 15
50 – 127
0 - 49
>= 1000
0 to 999
Kernel level priorities
Kernel level priorities
Real-time priorities
User level priorities
User level priorities
Non real-time priorities

5. Comparison of scheduling on an uniprocessor system
How scheduler picks-up a process
Win2k picks a process with higher priority to run or if there are multiple processes with same priority then it runs them one by one for a quantum each
Linux picks a process with higher Goodness value
FreeBSD picks a process with lowest priority value
Scheduler data structure
Win2k, FreeBSD uses multilevel priority queue
Linux uses linked list of runnable processes
Wait queue
Process type
Formula for Goodness
Real-time
real-time priority
Non real-time
Priority + quantum left in previous epoch + 1 (if the process being schedule shares address space with currently executed)

6. Comparison of scheduling on an uniprocessor system
Quantum of process execution
Trade-off in quantum
Win2k - configurable but it’s 20ms for Professional and 120ms on uniprocessor server
FreeBSD - it’s 100ms
Linux
No quantum for real-time FIFO processes
Epoch
Priority and quantum are related
Quantum = Quantum/2 + priority, for next epoch
Low priority process will never get to run in Win2k and FreeBSD but not the case in Linux
Same priority processes share the quantum in FreeBSD and Linux but will for quantum each in Win2k

7. Comparison of scheduling on an uniprocessor system
Priority boosting
Win2k, priority boosting for I/O and interactive processes (1-15)
E.g. 1 for disk I/O, 6 – keyboard
FreeBSD, priority is boosted based on CPU usage
When a process is awakened in kernel level then it’s assigned a kernel level priority ( 0 – 49)
User level dynamic priority (50 – 127) is calculated by,
Priority=50 + accumulated clock ticks on previous execution/4 + 2*nice
Accumulated clocks ticks = (2*load)/(2*load+1) * Accumulated clock ticks + nice
If a process is waiting on I/O for more than a sec.
Accumulated clock ticks = ((2*load)/(2*load+1))# of seconds waiting * Accumulated clock ticks
Linux, Priority + quantum left in previous epoch + 1

8. Scheduling analysis on FreeBSD

9. Scheduling analysis on Win2k

10. Scheduling analysis on Win2k

11. Scheduling analysis on Win2k

12. Multiprocessor scheduling & problems addressed
Comparison of multiprocessor scheduling
Win2k, Scheduler chooses a processor based on whether it ran the scheduled process before (cache reuse) or it’s the ideal processor of the process
Linux
Boosts priority by 15 if the process was last running on the processor
Scheduling based on Cache Flush Time
Problems addressed by the schedulers
Win2k – priority inversion
FreeBSD – thrashing, solved by high-level scheduler
Linux – Quantum sharing between parent and child processes during forking