Threads and synchronized access to an array

Ruby
1

Hi everyone!
A n00b question - I have a following ruby code:

@arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]

def iterate(thread_number)
@arr.each do |line|
puts “#{line} - I am thread number #{thread_number}”
end
end

number_of_threads = 4

threads = []
number_of_threads.times do |t|
threads[t] = Thread.new do
iterate(t)
end
end

threads.each {|t| t.join}

The output for this code is:

1 - I am thread number 0
2 - I am thread number 0
3 - I am thread number 0
4 - I am thread number 0
5 - I am thread number 0
6 - I am thread number 0
7 - I am thread number 0
8 - I am thread number 0
1 - I am thread number 1
2 - I am thread number 1
3 - I am thread number 1
4 - I am thread number 1
5 - I am thread number 1
6 - I am thread number 1
7 - I am thread number 1
8 - I am thread number 1
1 - I am thread number 2
2 - I am thread number 2
3 - I am thread number 2
4 - I am thread number 2
5 - I am thread number 2
6 - I am thread number 2
7 - I am thread number 2
8 - I am thread number 2
1 - I am thread number 3
2 - I am thread number 3
3 - I am thread number 3
4 - I am thread number 3
5 - I am thread number 3
6 - I am thread number 3
7 - I am thread number 3
8 - I am thread number 3

What should be changed in this code to make the output looks like this?

1 - I am thread number #{thread_number}
2 - I am thread number #{thread_number}
3 - I am thread number #{thread_number}
4 - I am thread number #{thread_number}
5 - I am thread number #{thread_number}
6 - I am thread number #{thread_number}
7 - I am thread number #{thread_number}
8 - I am thread number #{thread_number}

I want to iterate the array values only one time by some number of
threads, how should I control the access to array values? Each thread
must be prohobited from using the array values, which have been already
used by other threads.

I was trying to change the code like this:
@mutex = Mutex.new
@arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]

@mutex.synchronize {
def iterate(thread_number)
@arr.each do |line|
puts “#{line} - I am thread number #{thread_number}”
end
end
}

number_of_threads = 4

threads = []
number_of_threads.times do |t|
threads[t] = Thread.new do
iterate(t)
end
end

threads.each {|t| t.join}

and this:
@mutex = Mutex.new
@arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]

def iterate(thread_number)
@arr.each do |line|
puts “#{line} - I am thread number #{thread_number}”
end
end

number_of_threads = 4

threads = []
number_of_threads.times do |t|
threads[t] = Thread.new do
@mutex.synchronize {
iterate(t)
}
end
end

threads.each {|t| t.join}

2

On Jun 18, 2008, at 5:23 PM, Bob Bobrov wrote:

================================================
6 - I am thread number 0
1 - I am thread number 2
4 - I am thread number 3
1 - I am thread number #{thread_number}
threads, how should I control the access to array values? Each thread
@arr.each do |line|
iterate(t)
def iterate(thread_number)
threads[t] = Thread.new do
@mutex.synchronize {
iterate(t)
}
end
end

threads.each {|t| t.join}

Perhaps:

require ‘thread’

@mutex = Mutex.new
@arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]

def iterate(thread_number)
while @arr.length > 0 do
sleep 0.01
@mutex.synchronize {
if line = @arr.shift
puts “#{line} - I am thread number #{thread_number}”
end
}
end
end

number_of_threads = 4

threads = []
number_of_threads.times do |t|
threads[t] = Thread.new do
iterate(t)
end
end

threads.each {|t| t.join}

END

without the sleep, the first thread started grabs them all before the
next one gets a chance. You never did anything to alter the @arr that
all threads saw.

I haven’t played with threads for a long time, but unless you change
something about @arr, every iterate() will operate once for each
element. You run into trouble with the Array#each if you modify the
contents of the array inside the block.

-Rob

Rob B. http://agileconsultingllc.com
[email protected]

3

Rob B. wrote:

def iterate(thread_number)
while @arr.length > 0 do
sleep 0.01
@mutex.synchronize {
if line = @arr.shift
puts “#{line} - I am thread number #{thread_number}”
end
}
end
end

Thank you, exactly what I need!

4

On Jun 18, 2008, at 6:11 PM, Rob B. wrote:

On Jun 18, 2008, at 5:23 PM, Bob Bobrov wrote:

Hi everyone!
A n00b question - I have a following ruby code:

snip

I want to iterate the array values only one time by some number of
threads, how should I control the access to array values? Each thread
must be prohobited from using the array values, which have been
already
used by other threads.

I was trying to change the code like this:

snip

@mutex.synchronize {
number_of_threads.times do |t|
the next one gets a chance. You never did anything to alter the
@arr that all threads saw.

I haven’t played with threads for a long time, but unless you change
something about @arr, every iterate() will operate once for each
element. You run into trouble with the Array#each if you modify the
contents of the array inside the block.

-Rob

If you remove the sleep and initialize with @arr=(1…10000).to_a you
see the kind of result you probably want where the @mutex.synchronize
block is really controlling the access.

-Rob

Rob B. http://agileconsultingllc.com
[email protected]