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
Introduction



Objectives
OSs analyzed
 FreeBSD 4.4 – pre SMPng
 Linux 2.4.18 – pre Preemtible kernel support
 Windows 2000
Convention
 Unit of work scheduled is Process
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
Comparison of scheduling on an uniprocessor system
 Scheduling instances
Operation
FBSD
Linux
Win2k
Process blocked by I/O
Yes
Yes
Yes
Whenever a process is runnable
Yes
Yes
Yes
Once/sec
Yes
No
No
when the process priority is changed either thru system
call (setPriority) or by kernel (priority boosting)
Yes
No
Yes
Multiple runnable processes with same priority
Yes
Yes
No
When processor affinity changes
No
No
Yes
 How scheduler picks-up a process
 Process priorities
Win2k
Kernel level priorities
User level priorities
16 – 31
1 - 15
FreeBSD
Kernel level priorities
User level priorities
50 – 127
0 - 49
Linux
Real-time priorities
Non real-time priorities
>= 1000
0 to 999
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
Process type
Formula for Goodness
Real-time
1000 + real-time priority
Non real-time
Priority + quantum left in previous epoch + 1 (if the process being
schedule shares address space with currently executed)
 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
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
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
Scheduling analysis on FreeBSD
Compute bound process
60
50
40
Priority
30
CPU
20
Comput Bound(CB) & I/O Bound(IB) Processes
10
CB Priority
40
0:14
0:13
0:12
0:11
0:10
0:09
0:08
0:07
0:06
0:05
0:04
0:03
0:02
0:00
50
0:01
0
60
Time
CB CPU
30
IB Priority
20
IB CPU
I/O bound process
10
0
40
Priority
30
CPU
20
10
Time
0:15
0:14
0:13
0:12
0:11
0:10
0:09
0:08
0:06
0:05
0:04
0:03
0:02
0
0:01
Time
50
0:00
0:
00
0:
01
0:
02
0:
03
0:
04
0:
05
0:
06
0:
07
0:
09
0:
11
0:
13
0:
00
60
Scheduling analysis on Win2k
Scheduling analysis on Win2k
Scheduling analysis on Win2k
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