GDC - program runs in one thread, DMD - in 4 threads, why?

[eugene]

On Friday, 10 September 2021 at 11:09:10 UTC, bauss wrote:
> --DRT-gcopt=parallel:2 on the command line. A value of 0 
> disables parallel marking completely.

but it does not:

make -f Makefile-dmd
dmd --DRT-gcopt=parallel:0 engine/*.d common-sm/*.d server-sm/*.d 
pool.d echo_server.d -ofecho-server
dmd --DRT-gcopt=parallel:0 engine/*.d common-sm/*.d client-sm/*.d 
pool.d echo_client.d -ofecho-client

ps xH | grep [e]cho
  5460 pts/14   Sl+    0:00 ./echo-server
  5460 pts/14   Sl+    0:00 ./echo-server
  5460 pts/14   Sl+    0:00 ./echo-server
  5460 pts/14   Sl+    0:00 ./echo-server
  5466 pts/15   Sl+    0:00 ./echo-client
  5466 pts/15   Sl+    0:00 ./echo-client
  5466 pts/15   Sl+    0:00 ./echo-client
  5466 pts/15   Sl+    0:00 ./echo-client