Question on threads

Hello,

count = 0
threads = []

10.times do |i|
threads[i] = Thread.new do
sleep(rand(0.1))
Thread.current[“mycount”] = count
count += 1
end
end

threads.each {|t| t.join; print t[“mycount”], ", " }

For the code above, why the output numbers are random, rather than
from 0 to 9 by increasing?

Thanks.

2010/3/9 Jean G. [email protected]:

end
end

threads.each {|t| t.join; print t[“mycount”], ", " }

For the code above, why the output numbers are random, rather than
from 0 to 9 by increasing?

Because there are no guarantees about thread scheduling. Btw, your
code is not really thread safe since you access a shared resource
without proper synchronization (although it might work on some Ruby
platforms).

Kind regards

robert

Jean G. wrote:

Hello,

count = 0
threads = []

10.times do |i|
threads[i] = Thread.new do
sleep(rand(0.1))
Thread.current[“mycount”] = count
count += 1
end
end

threads.each {|t| t.join; print t[“mycount”], ", " }

For the code above, why the output numbers are random, rather than
from 0 to 9 by increasing?

Because:

(1) each thread sleeps for a random amount of time before capturing and
incrementing the value of ‘count’; but

(2) you join each thread in the order in which they were started.

Consider, for example, that threads[0] might sleep for 0.09 seconds, but
threads[1] might sleep for 0.02 seconds. Hence threads[1] will capture a
lower value than threads[0].

As has already been pointed out, this code is not threadsafe -
occasionally, two threads may capture the same value of ‘count’. That’s
because

count += 1

is really a shorthand for

count = count + 1

which is basically:

  • read value of count
  • add one to this value
  • store this value back to count

Thread X could get as far as reading the value of ‘count’ before it is
suspended; then thread Y could run, read the same value of ‘count’, and
increment it. Then thread X will be re-scheduled, and also increment and
save back the same value.

2010/3/9 Brian C. [email protected]:

count += 1

occasionally, two threads may capture the same value of ‘count’. That’s

  • add one to this value
  • store this value back to count

Thread X could get as far as reading the value of ‘count’ before it is
suspended; then thread Y could run, read the same value of ‘count’, and
increment it. Then thread X will be re-scheduled, and also increment and
save back the same value.

Brian, thank you for taking the time to do a more elaborate explanation.

One additional thing: since Ruby’s threads can actually return a value
we can rewrite the original piece to this version, which is also
thread safe:

lock = Mutex.new
count = 0

threads = (1…10).map do |i|
Thread.new do
sleep(rand(0.1))

lock.synchronize do
  count += 1
end

end
end

threads.each do |th|
puts th.value
end

Note that #synchronize returns the value returned by the block and by
that way we return the result of incrementing as the thread’s return
value which is captured through Thread#value (which also joins the
thread).

Kind regards

robert

PS: We can make this even shorter, just for the fun of it - I don’t
really recommend that style:

lock = Mutex.new
count = 0

(1…10).map do |i|
Thread.new do
sleep(rand(0.1))

lock.synchronize do
  count += 1
end

end
end.each do |th|
puts th.value
end

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