How to use multithreads in subthread?

General Usage
,
1 
# start with -t 6
function test_threads()
   Threads.@threads for i in 1:100
       @show Threads.threadid()
   end
end
Threads.@spawn test_threads()

output as following:
Threads.threadid() = 2
Threads.threadid() = 2
Threads.threadid() = 2
Threads.threadid() = 2
Threads.threadid() = 2
Threads.threadid() = 2
Threads.threadid() = 2
Threads.threadid() = 2
Threads.threadid() = 2
…