O pe ra tin g S ys te ms P ro cessesand T h re ads P ro cess in th e O ...

O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong>
Processes and Threads
dr. Tomasz Jordan Kruk
T.Kruk@ia.pw.edu.pl
Institu<strong>te</strong> of Control & Computation Engineering
Warsaw University of Technology
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 1/32
Process in the O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong>m
Process - an abst<strong>ra</strong>ction of a running prog<strong>ra</strong>m with its compu<strong>tin</strong>g
environment. Process is a basic dynamic object in the o<strong>pe</strong><strong>ra</strong><strong>tin</strong>g s<strong>ys</strong><strong>te</strong>m.
Requirements to be met by the o<strong>pe</strong><strong>ra</strong><strong>tin</strong>g s<strong>ys</strong><strong>te</strong>m with reference to processes:
√ in<strong>te</strong>rleaving the execution of multiple processes to maximize processor
utilization while providing reasonable response time,
√ alloca<strong>tin</strong>g resources to processes in conformance with a s<strong>pe</strong>cified
policy, while at the same time avoiding deadlock,
√ suppor<strong>tin</strong>g in<strong>te</strong>rprocess communication,
√ suppor<strong>tin</strong>g user creation of processes.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 2/32
Multiprog<strong>ra</strong>mming
One prog<strong>ra</strong>m coun<strong>te</strong>r
A
B
C
Process
switch
Four prog<strong>ra</strong>m coun<strong>te</strong>rs
D
C
Process
A
B C D
B
A
D
Time
(a) (b) (c)
√ process must not be prog<strong>ra</strong>mmed with built-in assumptions about
timing,
√ the difference between a process and a prog<strong>ra</strong>m,
√ processes sequential, concurrent, pa<strong>ra</strong>llel and distribu<strong>te</strong>d,
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 3/32
Process Creation and Termination
Four principal events that cause processes to be crea<strong>te</strong>d:
√ s<strong>ys</strong><strong>te</strong>m initialization,
√ execution of a process creation s<strong>ys</strong><strong>te</strong>m call by a running process,
√ a user request to crea<strong>te</strong> a new process,
√ initiation of a batch job.
Process may <strong>te</strong>rmina<strong>te</strong> due to one of the following conditions:
√ normal exit (voluntary),
√ error exit (voluntary),
√ fatal error (involuntary),
√ killed by another process (involuntary).
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 4/32

Process Sta<strong>te</strong>s
Blocked
Running
1 3
2
4
Ready
1. Process blocks for input
2. Scheduler picks another process
3. Scheduler picks this process
4. Input becomes available
The basic sta<strong>te</strong>s of a process:
√ running - actually using the CPU at that instant,
√ ready - runnable, <strong>te</strong>mpo<strong>ra</strong>rily stop<strong>pe</strong>d to let another process run,
√ blocked - unable to run until some ex<strong>te</strong>rnal event hap<strong>pe</strong>ns.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 5/32
Process Sta<strong>te</strong>s in the Unix S<strong>ys</strong><strong>te</strong>m
√ user running,
√ kernel running,
√ ready to run, in memory,
√ asleep in memory,
√ ready to run, swap<strong>pe</strong>d,
√ sleeping, swap<strong>pe</strong>d,
√ preemp<strong>te</strong>d,
√ crea<strong>te</strong>d,
√ zombie.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 6/32
Unix Process Sta<strong>te</strong> T<strong>ra</strong>nsition Diag<strong>ra</strong>m
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 7/32
Scheduler
Processes
0 1 n – 2 n – 1
Scheduler
The lowest layer of a process-structured o<strong>pe</strong><strong>ra</strong><strong>tin</strong>g s<strong>ys</strong><strong>te</strong>m handles in<strong>te</strong>rrupts
and scheduling. Above that layer there are sequential processes.
For processor allocation for particular procesess a piece of an OS kernel
called scheduler is responsible.
Process implementation:
√ the OS maintains a table (an ar<strong>ra</strong>y of structures) called the process
table, with one entry <strong>pe</strong>r process. Sometimes those entries are called
process descriptors or process control blocks, PCB.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 8/32

Some Fields of a Typical Process Table Entry
Process management Memory management
Regis<strong>te</strong>rs Poin<strong>te</strong>r to <strong>te</strong>xt segment
Prog<strong>ra</strong>m coun<strong>te</strong>r Poin<strong>te</strong>r to data segment
Prog<strong>ra</strong>m Status Word Poin<strong>te</strong>r to stack segment
Stack poin<strong>te</strong>r
Process sta<strong>te</strong>
Priority
Scheduling pa<strong>ra</strong>me<strong>te</strong>rs File management
Process ID root directory
Parent proces Working directory
Process group File descriptors
Signals User ID
Time when process star<strong>te</strong>d Group ID
CPU time used
Time of next alarm
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 9/32
In<strong>te</strong>rrupt Revisi<strong>te</strong>d
√ when an I/O device has finished the work given to it, it causes an in<strong>te</strong>rrupt by
asser<strong>tin</strong>g a signal on a bus line it has been assigned,
√ the signal de<strong>te</strong>c<strong>te</strong>d by the in<strong>te</strong>rrupt controller chip on the motherboard,
√ if no in<strong>te</strong>rrupts <strong>pe</strong>nding, the in<strong>te</strong>rrupt controller processes the in<strong>te</strong>rrupt
immedia<strong>te</strong>ly - it puts a number on the address lines s<strong>pe</strong>cifying which device
wants at<strong>te</strong>ntion and asserts a signal that in<strong>te</strong>rrupts the CPU,
√ the CPU stops current work and uses the number on the address lines as an
index into a table called the in<strong>te</strong>rrupt vector to fetch a new prog<strong>ra</strong>m coun<strong>te</strong>r,
√ the coun<strong>te</strong>r points to the start of the corresponding in<strong>te</strong>rrupt service procedure,
√ shortly af<strong>te</strong>r star<strong>tin</strong>g running, the in<strong>te</strong>rrupt service procedure acknowledges the
in<strong>te</strong>rrupt by wri<strong>tin</strong>g a certain value to one of the in<strong>te</strong>rrupt controller’s I/O ports -
the controller is now free to issue another in<strong>te</strong>rrupt,
√ the hardware alwa<strong>ys</strong> saves certain information before star<strong>tin</strong>g the service
procedure, at least the prog<strong>ra</strong>m coun<strong>te</strong>r but in some archi<strong>te</strong>ctures all the visible
regis<strong>te</strong>rs and a large number of in<strong>te</strong>rnal ones.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 10/32
Activities of the OS When an In<strong>te</strong>rrupt Occurs
1. Save any regis<strong>te</strong>rs (including the PSW) that have not already been saved by the
in<strong>te</strong>rrupt handler.
2. Set up a con<strong>te</strong>xt for the in<strong>te</strong>rrupt service procedure. Doing this may involve
set<strong>tin</strong>g up the TLB, MMU and a page table.
3. Set up a stack for the in<strong>te</strong>rrupt service procedure.
4. Acknowledge the in<strong>te</strong>rrupt controller. If there is no cent<strong>ra</strong>lised in<strong>te</strong>rrupt
controller, reenable in<strong>te</strong>rrupts.
5. Copy the regis<strong>te</strong>rs from where they were saved (possibly some stack) to the
process table.
6. Run the in<strong>te</strong>rrupt service procedure. It will ext<strong>ra</strong>ct information from the
in<strong>te</strong>rrup<strong>tin</strong>g device controller’s regis<strong>te</strong>rs.
7. Choose which process to run next.
8. Set up the MMU con<strong>te</strong>xt for the process to run next. Some TLB setup may also
be needed.
9. Load the new process’ regis<strong>te</strong>rs, including the PSW.
10. Start running the new process.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 11/32
Threads of Execution
When there is a need for concurrent threads of execution organized as a
group of processes, having sepa<strong>ra</strong><strong>te</strong>d pro<strong>te</strong>c<strong>te</strong>d address spaces means:
√ from the point of view of pro<strong>te</strong>ction: an advantage, but here we pro<strong>te</strong>ct our
processes against our processes,
√ from the point of view of communication: a d<strong>ra</strong>wback,
√ from the point of view of the level of simplicity in sharing resources: a d<strong>ra</strong>wback,
√ from the point of view of <strong>pe</strong>rformance: a d<strong>ra</strong>wback, at least if processes not
pa<strong>ra</strong>llel,
Thus, maybe we should consider put<strong>tin</strong>g together coo<strong>pe</strong><strong>ra</strong><strong>tin</strong>g threads of
execution into one shared address space, and this would meant:
√ from the point of view of pro<strong>te</strong>ction: a d<strong>ra</strong>wback, but we are the author of the
coo<strong>pe</strong><strong>ra</strong><strong>tin</strong>g threads codes and we should know what we do,
√ from the point of view of communication: an advantage,
√ from the point of view of the level of simplicity in sharing resources: an
advantage.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 12/32

Processes and Threads Attribu<strong>te</strong>s
Per process i<strong>te</strong><strong>ms</strong> Per thread i<strong>te</strong><strong>ms</strong>
Address space Prog<strong>ra</strong>m coun<strong>te</strong>r
Global variables Regis<strong>te</strong>rs
O<strong>pe</strong>n files Stack
Child processes Sta<strong>te</strong>
Pending alar<strong>ms</strong>
Signals and signal handlers
Accoun<strong>tin</strong>g information
Threads of the same process may exchange information with usage of global
variables of the process.
√ what with threads as<strong>pe</strong>cts when the subprocess is crea<strong>te</strong>d?
√ what is the correct layer for servicing signals?
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 13/32
Threads Stack
Thread 2
Thread 1
Thread 3
Process
Thread 1's
stack
Thread 3's stack
Kernel
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 14/32
Multithreaded Server
Web server process
Dispatcher thread
Worker thread
User
space
Web page cache
Kernel
Kernel
space
Network
connection
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 15/32
An Outline of the Multithreaded Server Algorithm
while (TRUE) { while (TRUE) {
get_next_request(&buf); wait_for_work(&buf)
handoff_work(&buf); look_for_page_in_cache(&buf, &page);
} if (page_not_in_cache(&page))
read_page_from_disk(&buf, &page);
return_page(&page);
}
(a) (b)
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 16/32

Methods of Server Construction
Three wa<strong>ys</strong> to construct a server
√ threads - pa<strong>ra</strong>llelism, blocking s<strong>ys</strong><strong>te</strong>m calls,
√ single-threaded process - no pa<strong>ra</strong>llelism, blocking s<strong>ys</strong><strong>te</strong>m calls,
√ fini<strong>te</strong>-sta<strong>te</strong> machine - pa<strong>ra</strong>llelism, nonblocking s<strong>ys</strong><strong>te</strong>m calls, in<strong>te</strong>rrupts.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 17/32
Kernel-level threads and User-level Threads
Process Thread Process Thread
User
space
Kernel
space
Kernel
Kernel
Run-time
s<strong>ys</strong><strong>te</strong>m
Thread
table
Process
table
Process
table
Thread
table
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 18/32
Threads – Hybrid Solutions
Multiple user threads
on a kernel thread
User
space
Kernel
Kernel thread
Kernel
space
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 19/32
Multithreaded Archi<strong>te</strong>cture under Solaris OS
Solaris makes use of four sepa<strong>ra</strong><strong>te</strong> thread-rela<strong>te</strong>d concepts:
√ Process - the normal Unix process,
√ User-level threads - ULTs, implemen<strong>te</strong>d through a threads lib<strong>ra</strong>ry in the
address space of a process,
⋆ invisible to the o<strong>pe</strong><strong>ra</strong><strong>tin</strong>g s<strong>ys</strong><strong>te</strong>m,
⋆ in<strong>te</strong>rface for application pa<strong>ra</strong>llelism.
√ Lightweight processes - LWPs, a mapping between ULTs and kernel
threads,
⋆ each LWP supports one or more ULTs and maps to one kernel thread,
⋆ LWPs are scheduled by the kernel inde<strong>pe</strong>ndently,
⋆ LWPs may execu<strong>te</strong> in pa<strong>ra</strong>llel on multiprocessors.
√ Kernel threads fundamental entities that can be scheduled and
dispatched to run on one of the s<strong>ys</strong><strong>te</strong>m processors.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 20/32

Threads under Solaris – an Example
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 21/32
Mig<strong>ra</strong>tion to the Multithreaded Code
Thread 1 Thread 2
Access (errno set)
O<strong>pe</strong>n (errno overwrit<strong>te</strong>n)
Errno ins<strong>pe</strong>c<strong>te</strong>d
Time
ex<strong>te</strong>rn int *___errno();
#define errno (*(___errno()))
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 22/32
(a)
(b)
Priva<strong>te</strong> Global Variables
Thread 1's
code
Thread 2's
code
Thread 1's
stack
Thread 2's
stack
Thread 1's
globals
Thread 2's
globals
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 23/32
Processor-bound and I/O-bound Processes
Long CPU burst
Short CPU burst
Wai<strong>tin</strong>g for I/O
Time
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 24/32

Processes Scheduling
There are two basic scheduling <strong>te</strong>chniques:
√ nonpreemptive scheduling,
√ preemptive scheduling.
There are different requirements for different environments: batch s<strong>ys</strong><strong>te</strong><strong>ms</strong>,
in<strong>te</strong><strong>ra</strong>ctive s<strong>ys</strong><strong>te</strong><strong>ms</strong>, real-time s<strong>ys</strong><strong>te</strong><strong>ms</strong>.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 25/32
Features of the Good Scheduling Algorithm
All s<strong>ys</strong><strong>te</strong><strong>ms</strong>
√ fairness - giving each process a fair share of the CPU,
√ policy enforcement - seeing that sta<strong>te</strong>d policy is carried out,
√ balance - keeping all parts of the s<strong>ys</strong><strong>te</strong>m busy.
Batch s<strong>ys</strong><strong>te</strong><strong>ms</strong>
√ throughput - maximize jobs <strong>pe</strong>r hour,
√ turnaround time - minimize time between submisission and <strong>te</strong>rminantion,
√ CPU utilization - keep the CPU busy all the time.
In<strong>te</strong><strong>ra</strong>ctive s<strong>ys</strong><strong>te</strong><strong>ms</strong>
√ response time - respond to requests quickly,
√ proportionality - meet users’ ex<strong>pe</strong>ctations.
Real-time s<strong>ys</strong><strong>te</strong><strong>ms</strong>
√ mee<strong>tin</strong>g deadlines - avoid losing data,
√ predictability - avoid quality deg<strong>ra</strong>dation in mulimedia s<strong>ys</strong><strong>te</strong><strong>ms</strong>.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 26/32
Scheduling in Batch S<strong>ys</strong><strong>te</strong><strong>ms</strong>
8 4 4 4
4 4 4 8
A B C D
B C D A
(a)
Shor<strong>te</strong>st Job First Scheduling
(b)
Scheduling in batch s<strong>ys</strong><strong>te</strong><strong>ms</strong>
√ FCFS, First-Come First-Served,
√ SJF, Shor<strong>te</strong>st Job First,
√ SRTN, Shor<strong>te</strong>st Remaining Time Next,
√ Three-Level Scheduling.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 27/32
Three-Level Scheduling
CPU
CPU scheduler
Arriving
job
Input
queue
Main
Memory
Admission
scheduler
Memory
scheduler
Disk
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 28/32

Scheduling in In<strong>te</strong><strong>ra</strong>ctive S<strong>ys</strong><strong>te</strong><strong>ms</strong>
Current
process
Next
process
Current
process
B F D G A
F D G A B
(a)
Round-Robin scheduling
(b)
Scheduling in in<strong>te</strong><strong>ra</strong>ctive s<strong>ys</strong><strong>te</strong><strong>ms</strong>
√ Round-Robin algorithm,
√ priority scheduling,
√ shor<strong>te</strong>st process next (estimation).
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 29/32
Scheduling with Classes of Priorities
Queue
headers
Runable processes
Priority 4
(Highest priority)
Priority 3
Priority 2
Priority 1
(Lowest priority)
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 30/32
Scheduling in Real-time S<strong>ys</strong><strong>te</strong><strong>ms</strong>
√ s<strong>ys</strong><strong>te</strong><strong>ms</strong> with soft and hard requirements,
√ <strong>pe</strong>riodic and a<strong>pe</strong>riodic events,
m∑
Ci
≤ 1
i=1
Pi
A real-time s<strong>ys</strong><strong>te</strong>m that meets this cri<strong>te</strong>ria is said to be schedulable.
√ C i time of one service of <strong>pe</strong>riodic event,
√ P i <strong>pe</strong>riod of <strong>pe</strong>riodic event occurence.
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 31/32
Threads Scheduling
Order in which
threads run
Process A Process B Process A Process B
2. Runtime
s<strong>ys</strong><strong>te</strong>m
picks a
thread
1 2 3 1 3 2
1. Kernel picks a process 1. Kernel picks a thread
Possible: A1, A2, A3, A1, A2, A3
Not possible: A1, B1, A2, B2, A3, B3
√ a. for user-level threads,
√ b. for kernel-level threads.
Possible: A1, A2, A3, A1, A2, A3
Also possible: A1, B1, A2, B2, A3, B3
(a) (b)
Faculty of E&IT, Warsaw University of Technology O<strong>pe</strong><strong>ra</strong><strong>tin</strong>g S<strong>ys</strong><strong>te</strong><strong>ms</strong> / Processes and Threads – p. 32/32