Is it possible to build nodejs server with master/slave mode or cluster mode with only one CPU core, so that the others could be up once the current thread is down?
Yes, while using the core cluster module, you can spawn more children than cores. It is not recommended for regular use due context switching and the overhead incurred with new node processes.
However, if this is to load new code, an overall different approach is required. There are some existing modules that can assist with zero downtime reloads and they mostly proxy requests to new instances to perform the switch.