It is suggested that the command 'xargs' - that is often found on unix-like systems by default - does exactly the same as PPSS does when used with the -p option.
In it's most basic form, this is true to some extend. Xargs processes items and keeps an x number of jobs running in parallel. I think that there may be cases that xargs is sufficient for your task at hand. A simple example that demonstrates how xargs can be used: