Download Python

Python
Testing
Marcin Młotkowski
10th April, 2013
Threads
Shared resources
1
Threads
2
Shared resources
Marcin Młotkowski
Python
Threads
Shared resources
Introduction
From Wikipedia
A thread of execution is the smallest unit of processing that can be
scheduled by an operating system.
Threads of the same process share common code and data, but
have independent stacks.
Marcin Młotkowski
Python
Threads
Shared resources
Why use threads
to deal with slow I/O operations simultaneously (for example:
downloading files);
simultaneous requests processing, e.g. Web servers.
Marcin Młotkowski
Python
Threads
Shared resources
An example of two-threaded computation
1st example
1st thread
i=i+1
print i
i=i+1
print i
i=i+1
print i
2nd example
2nd thread
1st thread
i=i+1
2nd thread
i=i+1
print i
print i
i=i+1
i=i+1
i=i+1
print i
i=i+1
print i
i=i+1
print i
Marcin Młotkowski
print i
print i
i=i+1
i=i+1
print i
print i
Python
Threads
Shared resources
Threads in Python
thread (3.0: _thread): low-level library
threading: high-level library
Marcin Młotkowski
Python
Threads
Shared resources
Threads in Python
thread (3.0: _thread): low-level library
threading: high-level library
dummy_thread
dummy_threading
multiprocessing
Marcin Młotkowski
Python
Threads
Shared resources
How to use threads
threading module
class Thread:
def run(self):
"""Code executed in thread"""
def start(self):
"""Start a thread"""
Marcin Młotkowski
Python
Threads
Shared resources
Example task
Simulate a race, e.g. a marathon.
Marcin Młotkowski
Python
Threads
Shared resources
Runners implementation
import threading
total_distance = 0
class runner(threading.Thread):
def __init__(self, nr_startowy):
self.numer = nr_startowy
threading.Thread.__init__(self)
Marcin Młotkowski
Python
Threads
Shared resources
Race implementation
class runner, cd
def run(self):
global total_distance
dystans = 42195
while dystans > 0:
distance = distance - 1
total_distance = total_distance + 1
if distance % 10000 == 0:
print ’Runner no %i’ % self.numer
print ’Runner %i at finishing-line’ % self.numer
Marcin Młotkowski
Python
Threads
Shared resources
Run a race
r1 = runner(1)
r2 = runner(2)
r1.start()
r2.start()
r1.join()
r2.join()
print ’End of race, distance’, total_distance
Marcin Młotkowski
Python
Threads
Shared resources
What is .join
The main program is also a thread, so after
r1.start()
there are two threads
r1.join() means, that "primary" thread will wait until r1
finishes.
Marcin Młotkowski
Python
Threads
Shared resources
examples of run
Runner no 1
Runner no 2
Runner no 2
Runner no 2
Runner no 1
Runner no 1
Runner no 2Runner no 1
Runner
Runner
Runner
Runner
no 1
1 at finishing line
no 2
2 at finishing line
Marcin Młotkowski
Runner no 1Runner no 2
Runner no 1
Runner no 1
Runner no 2
Runner no 2
Runner no 1
Runner no 1
Runner no 2
Runner no 2
Runner 2 at finishing line
Runner 1 at finishing line
Python
Threads
Shared resources
Common variables
Reminder
total_distance = 0
class runner(threading.Thread):
...
total_distance = total_distance + 1
print total_distance
Marcin Młotkowski
Python
Threads
Shared resources
A puzzle
What is the value of the variable total_distance?
Marcin Młotkowski
Python
Threads
Shared resources
A puzzle
What is the value of the variable total_distance?
Theory
2 * 42195 = 84390
Marcin Młotkowski
Python
Threads
Shared resources
A puzzle
What is the value of the variable total_distance?
Theory
2 * 42195 = 84390
Experience
54390
74390
83464
...
Marcin Młotkowski
Python
Threads
Shared resources
Atomic operations?
i=i+1
LOADFAST 0
LOAD_CONST 1
BINARY_ADD
STORE_FAST 0
Marcin Młotkowski
Python
Threads
Shared resources
Atomic operations?
i=i+1
LOADFAST 0
LOAD_CONST 1
BINARY_ADD
STORE_FAST 0
i=i+1
LOADFAST 0
LOAD_CONST 1
BINARY_ADD
STORE_FAST 0
Marcin Młotkowski
Python
Threads
Shared resources
Locks
A class Lock
lock = Lock()
def run(self):
global lock
...
lock.acquire()
total_distance = total_distance + 1
lock.release()
Marcin Młotkowski
Python
Threads
Shared resources
Other locks
RLock
A thread may lock any number of times, and as many times release
it. It slows down computation dramatically.
Semaphore
Lock a fixed number of times:
sem = Semaphore(3)
sem.acquire()
sem.acquire()
sem.acquire()
sem.acquire() # lock
Marcin Młotkowski
Python
Threads
Shared resources
Waiting for resource
One thread — a bartender – is pouring a glass of milk, the second
— a customer — is waiting until the glass is full and drink.
Marcin Młotkowski
Python
Threads
Shared resources
Implementation
lck = Lock()
Pouring
lck.acquire()
for i in range(5):
glass_of_milk = glass_of_milk + 1
lck.release()
Drinking
while glass_of_milk != 5: pass
lck.acquire()
while glass_of_milk > 0:
glass_of_milk = glass_of_milk - 1
lck.release()
Marcin Młotkowski
Python
Threads
Shared resources
Implementation
lck = Lock()
Pouring
lck.acquire()
for i in range(5):
glass_of_milk = glass_of_milk + 1
lck.release()
Drinking
while glass_of_milk != 5: pass
lck.acquire()
while glass_of_milk > 0:
glass_of_milk = glass_of_milk - 1
lck.release()
Marcin Młotkowski
Python
Threads
Shared resources
Conditional variables
Conditional variables allow to put to sleep and wake threads.
Marcin Młotkowski
Python
Threads
Shared resources
Implementation
lck = threading.Condition()
Consume
lck.acquire()
while glass_of_milk != 5:
lck.wait()
while glass_of_milk > 0: glass_of_milk = glass_of_milk - 1
lck.release()
Pouring
lck.acquire()
for i in range(5):
glass_of_milk = glass_of_milk + 1
lck.notify()
lck.release()
Marcin Młotkowski
Python
Threads
Shared resources
Conditional variables
Conditional variables act as locks (acquire(), release());
a method wait() releases a lock and put to sleep a current
thread;
a method notify() wakes one of sleeping thread (related to
this cond. var.), notifyAll() wakes all sleeping threads.
Marcin Młotkowski
Python
Threads
Shared resources
Disadvantages
there is only one glass of milk, which can be either poured or
someone drinks
the bartender cannot pour more glasses of milk and go home
Marcin Młotkowski
Python
Threads
Shared resources
Data structures for multi-threaded programs
A class Queue:
It is a queue FIFO, thread–safe;
Constructor: Queue(size )
get an element and remove: .get(); if the queue is empty
raises an exception Empty
.get(True): if the queue is empty, it is put to sleep;
insert an element: .put(element ), if the queue is full an
exception Full is raised;
insert an element: .put(element, True), if the queue is full
the thread is put to sleep;
.full(), .empty()
Marcin Młotkowski
Python
Threads
Shared resources
Variants of Queue
LifoQueue
PriorityQueue
Marcin Młotkowski
Python