In the gwan.ch/en_timeline.html stated that,
renaming gwan to gwan_1 is to start gwan with one worker.
what is the difference between gwan_1 and gwan -w 1 ?
As seen in the documentation, there is no difference in functionality.
One way (renaming the executable) allows a permanent choice while the command-line switch allows quick tests.
Historically, these features were introduced for G-WAN tests on machines with thousands of CPU Cores. Those tests led to rewrite of some portions of the G-WAN kernel to get higher performance. The result is G-WAN v3.8.