TCL threads not running simultaneously? - multithreading

Question

I am working on a task, which must be solved in TCL. Because this task involves communication with several IO devices in parallel, I thought to solve it with threads. Because I observed problems, I wrote this TCL script, to test the threading behavior:
package require Thread
set t1 [thread::create {
for {set i 0} {$i < 10} {incr i} {
puts "AAA $i"
after 50
}
}]
puts "Created thread t1"
set t2 [thread::create {
for {set i 0} {$i < 10} {incr i} {
puts "BBB $i"
after 50
}
}]
puts "Created thread t2"
# ---------------------------
after 1000
proc threadprocSync {msg n} {
for {set i 0} {$i < $n} {incr i} {
puts "$msg $i"
after 50
}
}
set t3 [thread::create [threadprocSync "CCC" 10]]
puts "Created thread t3"
set t4 [thread::create [threadprocSync "DDD" 10]]
puts "Created thread t4"
# ---------------------------
after 1000
proc outproc {msg i} {
puts "$msg $i"
}
proc threadprocAsync {t msg n} {
for {set i 0} {$i < $n} {incr i} {
thread::send -async $t [outproc $msg $i]
after 50
}
}
set tWorker [thread::create -preserved]
puts "Created thread tWorker"
# ---------------------------
set t5 [thread::create [threadprocAsync $tWorker "EEE" 10]]
puts "Created thread t5"
set t6 [thread::create [threadprocAsync $tWorker "FFF" 10]]
puts "Created thread t6"
# ---------------------------
after 1000
thread::release $tWorker
The code contains delays of 50 ms to make the execution slower, so that the threads needs significantly more time, than the initialization of the environment. But I observe here, that only the first pair of threads, which print AAA and BBB runs simultaneously. I expected the same behavior also for the threads with CCC and DDD and for the threads with EEE and FFF. What am I doing wrong?
The above script produces the following output:
C:\Projects\tcltest>tclsh threads.tcl
Created thread t1
AAA 0
Created thread t2
BBB 0
AAA 1
BBB 1
AAA 2
BBB 2
AAA 3
BBB 3
BBB 4
AAA 4
AAA 5
BBB 5
AAA 6
BBB 6
BBB 7
AAA 7
AAA 8
BBB 8
AAA 9
BBB 9
CCC 0
CCC 1
CCC 2
CCC 3
CCC 4
CCC 5
CCC 6
CCC 7
CCC 8
CCC 9
Created thread t3
DDD 0
DDD 1
DDD 2
DDD 3
DDD 4
DDD 5
DDD 6
DDD 7
DDD 8
DDD 9
Created thread t4
Created thread tWorker
EEE 0
EEE 1
EEE 2
EEE 3
EEE 4
EEE 5
EEE 6
EEE 7
EEE 8
EEE 9
Created thread t5
FFF 0
FFF 1
FFF 2
FFF 3
FFF 4
FFF 5
FFF 6
FFF 7
FFF 8
FFF 9
Created thread t6
C:\Projects\tcltest>