Yes, threads from the Thread extension are real OS threads, which have a Tcl interp in them. Since they are separate interpreters then there's no impact from any kind of "per-interpreter lock".
Passing messages between Threads can be done asynchronously which does not block either thread.
Example Tcl program with threads
package require Thread
set tid [thread::create]
thread::send -async $tid {
while true { after 200; puts Jazz }
}
while true { after 100; puts Party }
This creates one thread which is in an infinite loop printing "Jazz" to stdout every 200ms, and the "main" thread which is in an infinite loop printing "Party" to stdout every 100ms.
Passing messages between Threads can be done asynchronously which does not block either thread.
Example Tcl program with threads
This creates one thread which is in an infinite loop printing "Jazz" to stdout every 200ms, and the "main" thread which is in an infinite loop printing "Party" to stdout every 100ms.