What are the four undefs good for?

They cause these loops to terminate:

while( my $number = $Q->dequeue ) {
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P.S.: But my box is faster than yours ;-)

No surprise there. My Q6600 is over 7 years old.

Mind you, I make better use of it than most people do of their Haswells :)

"Mind you, I make better use of it than most people do of their Haswells"

Indeed.

"They cause these loops to terminate"

I guess it works like this:

| 1000  1001  1002  1003 |
| 4 threads ------------ |

| 1004  1005  1006  1007 |
| 4 threads ------------ |

... until they meet undef

| undef undef undef undef|
| 4 threads ------------ |
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Best regards, Karl (still locked/blocked)

«The Crux of the Biscuit is the Apostrophe»

Your diagram (whilst probably correct for this case) is too prescriptive. It implies that the work is dolled out like kids with Smarties: 1 for you, 1 for you...

But imagine (or try) that the list of numbers was 1000 randomly chosen from 1 .. 2000, and in any order.

Then you'd see that one thread might successively happen to get runs of small (say 1 or 2 digit) numbers that take little time; whilst another thread might happen to always get 4-digit numbers that take longer; with the result that by the end, the first thread may have processed many more items than the second.

But they will both have processed nearly the same number of clock cycles and will end within a few milliseconds or so of each other.

That's one of the beauties of the queue architecture is that it is inherently self-balancing:

<Reveal this spoiler or all in this thread>

Outputs:

C:\test>1126584-q
[ 4] terminating at 1431869328.993618; having processed 280 values
[ 1] terminating at 1431869329.076621; having processed 305 values
[ 3] terminating at 1431869329.094198; having processed 190 values
[ 2] terminating at 1431869329.102986; having processed 225 values
Took 17.553564 seconds
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The only guarantee is that eventually all the work items are processed and each of the 4 threads will dequeue an undef and so end its loop.

It does mean that work items cannot themselves be undef (or any non-true value unless you modify the loop to: while( defined( my $number = $Q->dequeue ) ){).

And later versions of Thread::Queue acquired, (amongst a lot of dross), an end() method which can be used as an alternative to queueing undef's, but the implementation leaves a lot to be desired; and old (tried and tested) programming habits die hard :)

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