How to use xargs in parallel

A good start is to use -P 4 -n 1 to run 4 processes in parallel (-P 4), but give each instance of the command to be run just one argument (-n 1)

These are the xargs options for parallel use from the xargs manpage:

 -P, --max-procs=MAX-PROCS    run at most MAX-PROCS processes at a time

 -n, --max-args=MAX-ARGS      use at most MAX-ARGS arguments per command line

Example:

cat urls.txt | xargs -P 4 -n 1 wget

This command will run up to 4 wget processes in parallel until each of the URLs in urls.txt has been downloaded. These processes would be run in parallel

wget [URL #1]
wget [URL #2]
wget [URL #3]
wget [URL #4]

If you would use -P 4 -n 2 these processes would be run in parallel:

wget [URL #1] [URL #2]
wget [URL #3] [URL #4]
wget [URL #5] [URL #6]
wget [URL #7] [URL #8]

Using a higher value for -n might slightly increase the efficiency since fewer processes need to be initialized, but it might not work with some commands if you pass multiple arguments.