'''[http://www.tcl.tk/man/tcl/ThreadCmd/tpool.htm%|%tpool]''' — Part of the Tcl threading extension implementing pools of worker threads
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[paulwal] 2020-05-30
In trying to answer [MHo]'s question below, I dug into the code a bit (threadPoolCmd.c) and did some experimenting. I believe I've found the magic formula to make -maxworkers function correctly. 

First of all, you can't use the -nowait option. It breaks the -maxworkers functionality. I experience no waiting anyways with or without -nowait, so I'm not sure what the point of this option is. 

Secondly, you need a slight pause between posting jobs. I haven't quite wrapped my head around why that is, but through testing this seems to be the case. In the code below, I add a one second pause between jobs, and each job takes 30 seconds. After 30 seconds, you should see a job complete every one second. In testing, I was able to reduce the pause down to 1 millisecond. 

======
package require Thread
set pool1 [tpool::create -minworkers 5 -maxworkers 20 -idletime 40]
puts start:[clock seconds]\n----
for {set i 1} {$i <= 20} {incr i} {
    tpool::post $pool1 {after 30000; puts [clock seconds]}
    after 1000
}
======

Result:
======
start:1590816680
----
1590816710
1590816711
1590816712
1590816713
1590816714
1590816715
1590816716
1590816717
1590816718
1590816719
1590816720
1590816721
1590816722
1590816723
1590816724
1590816725
1590816726
1590816727
1590816728
1590816729
======


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[MHo] 2017-07-31 There's something that I don't understand:

Why ist the runtime of
======
set pool1 [tpool::create -minworkers 10 -maxworkers 20 -idletime 10]
for {set i 1} {$i <= 20} {incr i} {
    lappend jobs [tpool::post $pool1 {after 10000}
}
while {[llength $jobs]} {
      set ready [tpool::wait $pool1 $jobs jobs]
}
======
double the time of
======
set pool1 [tpool::create -minworkers 20 -maxworkers 20 -idletime 10]
for {set i 1} {$i <= 20} {incr i} {
    lappend jobs [tpool::post $pool1 {after 10000}
}
while {[llength $jobs]} {
      set ready [tpool::wait $pool1 $jobs jobs]
}
======
The first call runs ~20,xs, the second one, as expected, ~10,xs.

The documentation states, the only difference is the time at which the workers are created - beforehand, or "on demand" when a post is done.
As there are enough -maxworkers defined, what is limiting the operation then...?

I also asked this on that page: [Ask, and it shall be given # 12]

I've looked at several examples that I've found here and there, and it seems that almost everyone sets -min == -max..... There must be something misterious around this values. I can also specify -minworkers 40 -maxworkers 20, no problem. 40 Workers are created.... Ok, this last fact becomes clear after looking at the c-source:
======
    if (minw > maxw) {
        maxw = minw;
    }
======
Even more mystery....it works as expected with this little variation:
======
set pool1 [tpool::create -minworkers 1 -idletime 10]
for {set i 1} {$i <= 20} {incr i} {
    lappend jobs [tpool::post $pool1 {after 10000}
    puts $jobs; # <--------- mystery
}
while {[llength $jobs]} {
      set ready [tpool::wait $pool1 $jobs jobs]
}
======
So, I think the answer to myself is for now:

Posting jobs one after another without doing something between each iteration blocks the start of new threads. Btw, it's not clear, what ''something'' exactly is, and why it helps....