Nachos Project Assignment 2
CPU scheduling
TA : 吳哲榮
2010/11/18
Nachos Assignment 2
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Abstract
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You have already learned how OS schedules
processes, so this assignment is to implement
one by yourself.
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Implement a system call – Sleep()
Non-preemptive shortest job first scheduler
System Call - Sleep
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Please implement Sleep(int x) system call
block the thread which called this system
call， and return to READY state after x timer
interrupts.
System Call - Sleep - Hints
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Please study
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userprog/exception.cc
userprog/syscall.h
test/start.s
to realize how system calls are implemented.
System Call - test/start.s
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Assembly language assist to make system
calls to the Nachos kernel.
Define System call stubs
system call code -- r2
arg1 -- r4
arg2 -- r5
arg3 -- r6
arg4 -- r7
The result of the system call, if any, must be put back into
r2.
System Call - test/start.s
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Take System call PrintInt as an example
.globl PrintInt
.ent PrintInt
PrintInt:
addiu $2,$0,SC_PrintInt
// put system call number in register 2
syscall
/* all parameter of this system call will be
stored in register4, 5, 6, and 7 by MIPS
machine automatically. */
j $31
.end PrintInt
Declare a new system call –
userprog/syscall.h
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Define a new system call ID.
Declare the interface for Nachos system calls,
which will be called by the user program.
ExceptionHandler userprog/exception.cc
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Fetch system call number in Register 2
Add new case to the exception handler
case SyscallException:
switch(type) {
…
}
System Call - Sleep (Hint)
1.
2.
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Modify exception.cc, syscall.h, start.s
Modify thread/alarm.cc, thread/alarm.h to
implement WaitUntil(int x) to handle
Sleep(int x) system call
Add a new class to manage these threads
blocked by calling Sleep(x).
Nachos Scheduling Policy
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The Nachos scheduling policy is simple:
threads reside on a single, unprioritized ready
list, and threads are selected in a round-robin
fashion. That is, threads are always
appended to the end of the ready list, and the
scheduler always selects the thread at the
front of the list.
SJF Scheduling
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Default Nachos scheduling algorithm is RoundRobin，we are going to implement another nonpreemptive SJF scheduling
Use n+1 = tn + (1- )n to predict next CPU burst
n : nth predicted CPU burst length
tn : nth actual CPU burst length
 : set to 0.5
(one timer interrupt means CPU burst plus 1)
SJF Hint
Begin Running
Per timer interrupt:
1.Record actual CPU burst
2.Wake up next threads
Invoke Sleep(x)
1.Set next predicted CPU burst
2.Insert this thread to Sleeping thread lists
3.Invoke thread->Sleep
SJF Hint
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You need to modify schedule.cc, alarm.cc
and the other related files，and you may add
a new class to manager those threads that
blocked by calling Sleep(x).
Begin Running
Per timer interrupt:
1.Record actual CPU burst
2.Wake up next threads
Invoke Sleep(x)
1.Set next predicted CPU burst
2.Insert this thread to Sleeping thread lists
3.Invoke thread->Sleep
Assignment Requirements
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1.Implement system call “Sleep”
2.Implement Shortest-Job-First scheduling
3.Design several test case to proof your
result
Assignment Requirements
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Assignment Report (12/23 on the class)
Please compress the following in a .zip.
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modified source code(s) with path
presentation power-point
final report
E-mail your .zip
(project1_b99901000_b99901001.zip) to TA.
Deadline: 2010/12/23 23:59
Grading Policy
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Correct Result
Report
50%
50%