Hi Folks:
I feel that channel preferences are little explained. Based on the earlier
conversations about channel preference, I have written an initial channel
preferences document. Please review the document and give comments. When the
document is correct, I will draw diagrammes and it add to the Wiki.
Channel Preferences
1.0 Introduction:
The purpose of the channel preference is to provide a form of scheduling
priority for tasklets. The channel preference is a channel object attribute.
The channel preference is used as follows:
ch = stack.channel()
ch.preference = PREFERENCE
PREFERENCE is an integer in the range {-1, 0, 1} where
-1 Receiver preference
0 no preference
1 Sender preference
receiver preference (-1) is the default value.
1.1 Review of The Scheduling Algorithm
In order to understand channel preferences, a review of the scheduling
algorithm is required. Stackless documentation describes the scheduling
algorithm as 'round-robin.' Tasklets than can be scheduled (or executed) are
held on the runnables list. The runnable list is a circular doubly
linked. The stackless.getcurrent() method returns the head or currently
executing tasklet.
<Illustration>
When a runnable tasklet yields (i.e., calls stackless.schedule), it is
placed at the end of the runnable list. Then the next runnable tasklet is
scheduled. If the currently executing tasklet performs a channel operation that
will cause it to block, it is taken off the runnable list and transferred to
the channel's queue.
<Illustration>
When there are tasklets in a channel's queue, channel preferences come into
play.
2.0 Channel Preference Explained
To illustrate how channel preferences work, four scenarios will be examined.
Note: a tasklet performing a channel.send() will be called the sender. A
tasklet performing a channel.receive() will be called a receiver.
2.2 Current tasklet is a Sender, blocked Receiver, Sender Preference
Data is transferred to the receiver. The receiver is unblocked and placed
at the end of the runnables' list. The sender continues executing.
2.3 Current tasklet is a Sender, blocked Receiver, Receiver Preference
Data is transfered to the receiver. The receiver is unblocked and made the
current tasklet. The receiver is scheduled.
2.4 Current Tasklet is a Receiver, blocked Sender, Sender Preference
Data is transferred to the receiver. The sender is unblocked and made the
current tasklet. The sender is scheduled.
2.5 Current Tasklet is a Receiver, blocked Sender, Receiver Preference
Data is transferred to the receiver. The sender is placed to the end of the
runnable list. The receiver continues executing.
3.0 Conclusion
Why do channel preferences matter? The choice of channel preference will
affect various scheduling algorthm metrics. For instance response time (the
time between an IO request is sent and a response is received). Depending on
the application, the developer may wish to optimize a particular metric.
This tutorial will end with the analysis of a programme, preferences.py.
Preferences.py consists of a single sender, five clients and receiver
preference. When executing the scheduling sequence was:
C0,C1,C2,C3,C4,S,C1,S,C2,S,C3,S,C4,S
execution time 2.53314614296 average response: 1.00204200745
With a single sender, five clients and sender preference, the scheduling
sequence is
C0,C1,C2,C3,C4,S,C0,C1,C2,C3,C4
execution time 2.49860596657 average response: 2.49821634293
Over several runs, the execution time of both configurations will be close.
However the average response time greatly differ. The reason? With receiver
preference, a receiver is scheduled the moment data is available. With sender
preference, receivers are placed at the end the runnable list before being
scheduled. Once the sender is finished, the receivers will execute.
End.
"""
preferences.py
The purpose of preferences.py is to demonstrate how channel preferences
can change scheduling metrics, i.e., average response time
For this example, there is one producer and multiple consumers
usage:
preferences.py preference number_of_consumer_taskets
-1 prefer receiver
1 prefer sender
"""
import sys
import string
import stackless
import time
OUTPUT = True
rSum = 0.0
def producer(channel):
pStart = time.time()
for i in range(0, MAX):
if OUTPUT:
print "producer about to send"
channel.send('a')
time.sleep(.5)
if OUTPUT:
print "producer sent"
pFinish = time.time() - pStart
print "%s: %f" % ("producer finished", pFinish)
def consumer(channel, n):
global rSum
cStart = time.time()
if OUTPUT:
print "consumer ", n, "about to receive"
channel.receive()
cFinish = time.time() - cStart
rSum = rSum + cFinish
if OUTPUT:
print "consumer ", n, "received", "time: ", cFinish
if __name__ == "__main__":
name = {}
channel = stackless.channel()
channel.preference = string.atoi(sys.argv[1])
MAX = string.atoi(sys.argv[2])
start = time.time()
for i in range(0,MAX):
t = stackless.tasklet(consumer)(channel, i)
name[t] = i
t = stackless.tasklet(producer)(channel)
name[t] = "producer"
stackless.run()
print "execution time", time.time() - start, "average response: ",rSum/MAX
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