Multi Threading

S

Sriram Varahan

Hello,

#*******************STARTCODE

start_time = Time.now
$count = 0
class Queue
def initialize *s # splat operator allows variable length argument
list
@mutex = Mutex.new
@queue = []
s.each { |e| @queue.push e }
end

def enq v
@queue.push v
end

def deq
@mutex.synchronize {item = @queue.shift}
end

def empty?
@mutex.synchronize{@queue.length == 0}
end

def count
@mutex.synchronize do
$count += 1
end
end
end


#*****Test

queue = Queue.new
500.times do |a|
queue.enq a
end
threads = []


# Create 5 threads which fetch values from the Q.
5.times do
threads << Thread.new {
until queue.empty?
queue.count
puts "Thread ID: #{Thread.current}.Job started"
puts "#{queue.deq}"
#sleep 0.0001
puts "Thread ID: #{Thread.current}.Job complete"
end
}
end


threads.each {|t| t.join }
puts "Count"
puts $count
puts "timeTaken:"
puts Time.now - start_time

# *************CODE ENDS******************


I have five threads which fetch values from a queue. The above code
works perfectly well in case of a single thread. But the issue arises
when there are more threads.

In case of 5 threads the number of times the block is executed is 503
where it should have been 500.

I know the reason why?
The "deq" and "empty?" methods are not synchronized.
So when the final item is removed from the thread, other threads access
the empty? method before the @queue.length becomes 0.

Hence the difference in count.

If the sleep is activated this problem is solved.

Any suggestion on how to get this working without the sleep?

Thanks.
 
M

Michael Neumann

Sriram said:
Hello,

#*******************STARTCODE

start_time = Time.now
$count = 0
class Queue
def initialize *s # splat operator allows variable length argument
list
@mutex = Mutex.new
@queue = []
s.each { |e| @queue.push e }
end

def enq v
@queue.push v
end

def deq
@mutex.synchronize {item = @queue.shift}
end

def empty?
@mutex.synchronize{@queue.length == 0}
end

def count
@mutex.synchronize do
$count += 1
end
end
end


#*****Test

queue = Queue.new
500.times do |a|
queue.enq a
end
threads = []


# Create 5 threads which fetch values from the Q.
5.times do
threads << Thread.new {
until queue.empty?
queue.count
puts "Thread ID: #{Thread.current}.Job started"
puts "#{queue.deq}"
#sleep 0.0001
puts "Thread ID: #{Thread.current}.Job complete"
end
}
end


threads.each {|t| t.join }
puts "Count"
puts $count
puts "timeTaken:"
puts Time.now - start_time

# *************CODE ENDS******************


I have five threads which fetch values from a queue. The above code
works perfectly well in case of a single thread. But the issue arises
when there are more threads.

In case of 5 threads the number of times the block is executed is 503
where it should have been 500.

I know the reason why?
The "deq" and "empty?" methods are not synchronized.
So when the final item is removed from the thread, other threads access
the empty? method before the @queue.length becomes 0.

Hence the difference in count.

If the sleep is activated this problem is solved.

Any suggestion on how to get this working without the sleep?

You should also synchronize the enque operation (Queue#enq). Btw, there is
an existing Queue class that does this thread-safe:

require 'thread'
q = Queue.new
q.push 1
x = q.pop
q.pop # => would block the thread until a new element is available

q2 = SizedQueue.new(10) # bounded queue, which blocks when size > 10

Regards,

Michael
 
R

Robert Klemme

2009/4/17 Sriram Varahan said:
Hello,

#*******************STARTCODE

start_time =3D Time.now
$count =3D 0
class Queue
=A0def initialize *s # splat operator allows variable length argument
list
=A0 =A0@mutex =3D Mutex.new
=A0 =A0@queue =3D []
=A0 =A0s.each { |e| @queue.push e }
=A0end

=A0def enq v
=A0 =A0 [email protected] v
=A0end

=A0def deq
=A0 [email protected] {item =3D @queue.shift}
=A0end

=A0def empty?
=A0 [email protected]{@queue.length =3D=3D 0}
=A0end

=A0def count
=A0 @mutex.synchronize do
=A0 =A0 $count +=3D 1
=A0 end
=A0end
end


#*****Test

queue =3D Queue.new
500.times do |a|
=A0queue.enq a
end
threads =3D []


# Create 5 threads which fetch values from the Q.
=A05.times do
=A0 =A0threads << Thread.new {
=A0 =A0until queue.empty?
=A0 =A0 =A0queue.count
=A0 =A0 =A0puts "Thread ID: #{Thread.current}.Job =A0started"
=A0 =A0 =A0puts "#{queue.deq}"
=A0 =A0 =A0#sleep 0.0001
=A0 =A0 =A0puts "Thread ID: #{Thread.current}.Job =A0complete"
=A0 =A0end
=A0 }
=A0end


threads.each {|t| t.join }
puts "Count"
puts $count
puts "timeTaken:"
puts Time.now - start_time

# *************CODE ENDS******************


I have five threads which fetch values from a queue. The above code
works perfectly well in case of a single thread. But the issue arises
when there are more threads.

In case of 5 threads the number of times the block is executed is 503
where it should have been 500.

I know the reason why?
The "deq" and "empty?" methods are not synchronized.
So when the final item is removed from the thread, other threads =A0acces= s
the empty? method before the @queue.length becomes 0.

Hence the difference in count.

If the sleep is activated this problem is solved.

Any suggestion on how to get this working without the sleep?

There are several options. You could use MonitorMixin instead of Mutex
and include it in initialize

def initialize *s
# @mutex =3D=3D self so you do not need to change sync code
@mutex =3D extend MonitorMixin
@Queue =3D s.dup
end

Then you can do external synchronization, e.g.

queue.synchronize do
if queue.empty?
# finish
else
elm =3D deq
end
end

Much better though is this approach

require 'thread'

# use library class
queue =3D Queue.new

# _first_ start threads
# does not really matter but if filling
# the queue takes time work can
# begin immediately
threads =3D (1..5).map do
label =3D Thread.current.to_s.freeze

Thread.new queue do |q|
until ( job =3D q.deq ) =3D=3D :terminate
puts "Thread ID: #{label}.Job started"
puts job
puts "Thread ID: #{label}.Job complete"
end
end
end

# fill queues
500.times do |a|
queue.enq a
end

# "close" queues
threads.size.times { queue.enq :terminate }

# wait for termination
threads.each do |th|
th.join
end

Cheers

robert

--=20
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/
 
S

Sean O'Halpin

Much better though is this approach

require 'thread'

# use library class
queue =3D Queue.new

# _first_ start threads
# does not really matter but if filling
# the queue takes time work can
# begin immediately
threads =3D (1..5).map do
=A0label =3D Thread.current.to_s.freeze

=A0Thread.new queue do |q|
=A0 =A0until ( job =3D q.deq ) =3D=3D :terminate
=A0 =A0 =A0puts "Thread ID: #{label}.Job =A0started"
=A0 =A0 =A0puts job
=A0 =A0 =A0puts "Thread ID: #{label}.Job =A0complete"
=A0 =A0end
=A0end
end

# fill queues
500.times do |a|
=A0queue.enq a
end

# "close" queues
threads.size.times { queue.enq :terminate }

# wait for termination
threads.each do |th|
=A0th.join
end

Cheers

robert

Minor nitpick - these lines should be reversed:
label =3D Thread.current.to_s.freeze
Thread.new queue do |q|
i.e.

Thread.new queue do |q|
label =3D Thread.current.to_s.freeze

Regards,
Sean
 
S

Sriram Varahan

Hey Robert,

That was an amazing solution!Thanks a million :)


Thank you Michael and Sean for your time :)
 

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