Transcript Kickstart Intro to Java

Some Theory Review
COMP346/5461 - Operating Systems
Revision 1.2
September 30, 2003
May 21, 2002
Serguei A. Mokhov,
[email protected]
1
Topics
• OS
– Need
– Multiprogramming vs.
Uniprogramming
• I/O
– Interrupts vs. Polling
– DMA
• Programs, Processes and
Threads Revisited
May 21, 2002
• Synchronous vs. Asynchronous
– Message Passing
– Trap and Interrupts
• Common Problems in
Multiprogramming
– Synchronization
– Deadlock
– Mutual exclusion, critical
sections…
– Security
Serguei A. Mokhov,
[email protected]
2
OS: The Need
• A typical example:
– You have a responsibility of designing a dedicated
system to control something (water pumps, conveyor
production, etc).
– Dedicated implies a single user and a single process
dedicated process.
• Discussion:
– Would you still include an OS into the design of such a
system? (key things: security and abstraction)
May 21, 2002
Serguei A. Mokhov,
[email protected]
3
OS: Multiprogramming vs.
Uniprogramming
• A typical problem:
– There a need to have a system where throughput
(number of jobs completed within a given time) is the
most important factor.
– Experiments show, however, that the context switch
time is a lot more than a typical execution time of an
average process in the mm-OS.
• Discussion:
– Again, as a system designer, would you go with
multiprogramming or uniprogramming?
May 21, 2002
Serguei A. Mokhov,
[email protected]
4
I/O: Interrupts vs. Polling (1)
• Polling (sync. I/O): CPU routinely checks device’s
status via controller:
–
–
–
–
–
CPU: “Dear device driver (DD), is the device done?”
DD: “Nope”
CPU: “Dear device driver (DD), is the device done?”
DD: “Nope, not yet”
CPU: “Dear device driver (DD), is the device finally
done???? I’m kinda wasting my time waiting here!”
– DD: “Patience, it almost done. A couple more
cycles…”
May 21, 2002
Serguei A. Mokhov,
[email protected]
5
I/O: Interrupts vs. Polling (2)
• Interrupts (asynchronous I/O):
• CPU: “Dear device driver, say the device to do this and that,
OK?”
• DD: “Sure. [To device]: CPU wants you to do this and that. Let
us know when you’re done, k?”
• Device: No prob. [starts doing I/O]
• CPU: [to itself]: oh, I have so much to do! [goes and does smth
else]
• Device: [busy doing I/O an finally finishes]: Hey CPU!
• CPU: [suddenly interrupted by the Device] What is it?
• Device: I’m done with I/O. I though you might want to
know…
May 21, 2002
Serguei A. Mokhov,
[email protected]
6
I/O: No DMA
• Usually it’s the CPU who does the transfer of the data from a device to
the memory and back:
– CPU: “Hey DD! I need some data for that process. Could you arrange
some?”
– DD: “Yep.” [To HDD]: “Hey HDD,CPU needs some data for that process,
do you have it?”
– HDD: “Here’s some”
– DD: “Oh cool, CPU, here’s some data!”
– CPU: [gets the data, and places it to the memory and continues with the
process]
– HDD: “Here’s some more, I forgot since last time”
– DD: “OK”
– CPU: “What, again data? OK…” [takes it from DD and places to the
memory of the process]
May 21, 2002
Serguei A. Mokhov,
[email protected]
7
I/O: DMA
• CPU is more free from useless work when it has a
friendly DMA (Direct Memory Access) controller:
– CPU: “DD, I need that data for that process from HDD.
It should be between 0x46F46F and 0x46FFFF about
this big.” [goes and does smth else]
– DD: “Hey DMA! Wake up, you heard what CPU
said…”
– DMA: “Yep.” [Contacts HDD, grabs specified block of
data, and puts it to the specified place]: “CPU: Me
done!”
May 21, 2002
Serguei A. Mokhov,
[email protected]
8
Programs, Processes, and
Threads
• Once again, what are a program, a process
and a thread?
• Which one static, dynamic?
• What does a process have (consist of)?
• How threads are different from processes?
– What do they consist of?
May 21, 2002
Serguei A. Mokhov,
[email protected]
9
Synchronous vs. Asynchronous
• Message Passing
– A typical async example is email. The sender doesn’t
wait for the recipient to get it.
– A sync example is a bounded buffer. Sync is needed,
so that the sender waits for the receiver to get the
message or a part of it before proceeding to the next
part.
– Side question: why would be advisable to have
processes’ mailboxes for message passing in the
kernel space? Problems (hint: size and data loss)?
Solutions?
May 21, 2002
Serguei A. Mokhov,
[email protected]
10
Synchronous vs. Asynchronous
(2)
• Interrupts
– System Call Interface – switch
between user and kernel mode – a
syscall executes trap – a sync
interrupt (which is processed in the
kernel as ordinary interrupt)
– From devices – async. Why?
May 21, 2002
Serguei A. Mokhov,
[email protected]
11
Interrupts and Trap Revisited
• Neither interrupt nor trap are function calls!
• Interrupt is an event, or a signal (do not confuse with
UNIX signals), notifying the system that something
happened.
• Interrupts are not necessarily used to switch between
processes (in scheduling), but cause a context switch.
• Trap is a software interrupt, it's synchronous, whereas an
ordinary interrupt is asynchronous.
• Trap is a CPU instruction, which is invoked when a user
program wants either some service from an OS (system
call) or causes an error (division by zero, segmentation
fault, page fault, etc.).
May 21, 2002
Serguei A. Mokhov,
[email protected]
12
Common Interface:
open, close, read, write
The “Big” Picture
System Call
Interface
App
App
App
App
App
User mode
SW Kernel mode
User-running
syscall() - trap
exit()
MM
Kernel-running
kernel
wait()
PM
HDDD PTRDD CDDD
Blocked
int
Zombie
Ready
Ready queue
Bus
HW
CPU: add
mult
mov
trap
May 21, 2002
DMA Ctl
int
Mon Ctl
Ptr Ctl
CD Ctl
int
(IRQ)
Serguei A. Mokhov,
[email protected]
13
Common Problems in
Multiprogramming
• Synchronization (act of communication).
• Mutual exclusion and critical sections (only one process at a
time executing a given section of a code, usually operating on a
shared resource).
• Deadlock (two or more processes waiting for each other on some
resources).
• Context switch overhead.
• Security (processes must not interfere with other processes, and
unauthorized users shouldn’t be able to interfere with other users).
• All the above make the system much harder to implement.
• Why is multiprogramming is usually still better? (hint: CPU
utilization)
May 21, 2002
Serguei A. Mokhov,
[email protected]
14
Basic Security
• Security != Protection
• OS Protection is needed between:
– Processes of one user from others in multiuser
environment.
– Processes of the same user (isolation).
– OS and the users (various resource managers
and device drivers vs. application software)
– Various components of the OS itself.
May 21, 2002
Serguei A. Mokhov,
[email protected]
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Basic Security (2)
• OS on the network:
• The system has to provide CIAN:
–
–
–
–
–
–
Confidentiality
Integrity
Availability
Authentication
Authorization
Access Control
• Hence, the compilation of the protocols.
May 21, 2002
Serguei A. Mokhov,
[email protected]
16
Basic Security (3)
• Binary privilege model is NOT enough:
– Binary in a sense: GOD vs. LAMER
– I.e. root (or superuser or monitor) vs. user
• Exploiting various system vulnerabilities a user
can become god.
– E.g. buffer overflows in trusted / privileged (falsely)
software.
– Other bugs.
• Thus, sandboxing such programs and filtering
system calls and having more fine-grained roles
and ACLs and permissions.
May 21, 2002
Serguei A. Mokhov,
[email protected]
17
Next Tutorial
• Synchronization
• Semaphores
• Your questions, as usual
May 21, 2002
Serguei A. Mokhov,
[email protected]
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