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Parallel and Concurrent Programming: Processes and Threads | CMPSCI 691, Study notes of Computer Science

University of Massachusetts - AmherstComputer Science

Material Type: Notes; Class: S-Gen Purpose Cmptn-GPU; Subject: Computer Science; University: University of Massachusetts - Amherst; Term: Spring 2006;

Typology: Study notes

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Uploaded on 08/18/2009

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U
UNIVERSITY OF
NIVERSITY OF M
MASSACHUSETTS
ASSACHUSETTS, A
, AMHERST
MHERST •
•Department of Computer Science
Department of Computer Science
Parallel & Concurrent Programming:
Processes & Threads
Emery Berger
CMPSCI 691W - Spring 2006
U
UNIVERSITY OF
NIVERSITY OF M
MASSACHUSETTS
ASSACHUSETTS, A
, AMHERST
MHERST •
•Department of Computer Science
Department of Computer Science 2
Outline
Processes
Threads
Basic synchronization
Bake-off
pf3
pf4
pf5
pf8
pf9
pfa

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UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science

Parallel & Concurrent Programming:

Processes & Threads

Emery Berger

CMPSCI 691W - Spring 2006

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 2

Outline

„ Processes

„ Threads

„ Basic synchronization

„ Bake-off

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 3

Processes vs. Threads…

„ Both useful for parallel programming

& concurrency

„ Hide latency

„ Maximize CPU utilization

„ Handle multiple, asynchronous events

„ But: different programming styles,

performance characteristics, and more

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 4

Processes

„ Process:

execution

context (PC,

registers) +

address space,

files, etc.

„ Basic unit of

execution

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 7

Processes Example

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 8

Communication

„ Processes:

„ Input = state before fork()

„ Output = return value

„ argument to exit()

„ But: how can processes communicate

during execution?

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 9

IPC

„ signals

„ Send & receive ints

„ Not terribly useful for parallel or

concurrent programming

„ pipes

„ Communication channels – easy & fast

„ Just like UNIX command line

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 10

Pipe example

int main() {

int pfds[2];

pipe(pfds);

if (!fork()) {

close(1); /* close normal stdout */

dup(pfds[1]); /* make stdout same as pfds[1] */

close(pfds[0]); /* we don't need this */

execlp("ls", "ls", NULL);

} else {

close(0); /* close normal stdin */

dup(pfds[0]); /* make stdin same as pfds[0] */

close(pfds[1]); /* we don't need this */

execlp("wc", "wc", "-l", NULL);

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 13

Threads API

„ UNIX (POSIX):

„ pthread_create() – start separate

thread executing function

„ pthread_join() – wait for thread to

complete

„ Windows:

„ CreateThread (…)

„ only 6 arguments!

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 14

Threads example

#include <pthread.h>

void * run (void * d) {

int q = ((int) d);

int v = 0;

for (int i = 0; i < q; i++) {

v = v + expensiveComputation(i);

return (void *) v;

main() {

pthread_t t1, t2;

int r1, r2;

pthread_create (&t1, run, 100);

pthread_create (&t2, run, 100);

pthread_wait (&t1, (void *) &r1);

pthread_wait (&t2, (void *) &r2);

printf (“r1 = %d, r2 = %d\n”, r1, r2);

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 15

Communication

„ In threads, everything shared except:

stacks, registers & thread-specific data

„ Old way:

„ pthread_setspecific

„ pthread_getspecific

„ New way: __thread

„ static __thread int x;

„ Easier in Java…

„ Updates of shared state must be

synchronized

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 16

Basic synchronization

„ Mutual exclusion locks

„ Only one thread in critical section

pthread_mutex_lock (&l);

update data; /* critical section */

pthread_mutex_unlock (&l);

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 19

Scheduling

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 20

Context Switch Cost

„ Threads – much cheaper

„ Stash registers, PC (“IP”), stack pointer

„ Processes:

„ Same as threads plus –

„ Process context

„ TLB shootdown

ÖProcess switches more expensive, or

require long quanta

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 21

Flexibility / Ease-of-use

„ Processes – more flexible

+ Easy to spawn remotely

+ Can communicate via sockets = can be

distributed across cluster / Internet

- Requires explicit communication or risky

hackery

„ Threads

„ Communicate through memory – must

be on same machine

- Require thread-safe code

UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 22

Robustness

„ Processes – far more robust

„ Processes isolated from other processes

„ Process dies ) no effect

„ Apache 1.x

„ Threads:

„ If one thread crashes (e.g., derefs

NULL), whole process terminates

„ Then there’s the stack size problem

„ Apache 2.x…