Multi Threading


#1

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.


#2

Sriram V. wrote:

@queue = []

Create 5 threads which fetch values from the Q.

end

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


#3

2009/4/17 Sriram V. removed_email_address@domain.invalid:

@queue = []

Create 5 threads which fetch values from the Q.

end

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 == self so you do not need to change sync code

@mutex = extend MonitorMixin
@queue = s.dup
end

Then you can do external synchronization, e.g.

queue.synchronize do
if queue.empty?

finish

else
elm = deq
end
end

Much better though is this approach

require ‘thread’

use library class

queue = Queue.new

first start threads

does not really matter but if filling

the queue takes time work can

begin immediately

threads = (1…5).map do
label = Thread.current.to_s.freeze

Thread.new queue do |q|
until ( job = q.deq ) == :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


#4

On Fri, Apr 17, 2009 at 7:55 AM, Robert K.
removed_email_address@domain.invalid wrote:

the queue takes time work can

end

wait for termination

http://blog.rubybestpractices.com/

Minor nitpick - these lines should be reversed:

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

i.e.

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

Regards,
Sean


#5

Hey Robert,

That was an amazing solution!Thanks a million :slight_smile:

Thank you Michael and Sean for your time :slight_smile:


#6

2009/4/17 Sean O’Halpin removed_email_address@domain.invalid:

Minor nitpick - these lines should be reversed:

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

i.e.

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

Oh yes, absolutely! Apparently I moved the line too high. Thanks for
catching that gotcha, Sean!

Kind regards

robert