xargs to run commands in parallel

I recently found out xargs had options to parallelize what it is working on. I finally had a good reason to try it. I'm processing log files for the last year. Each day is it's own unique standalone task. My workstation is has 1 CPU with 6 cores that are hyperthreaded to give 12 logical cores. So... I asked xargs to run the processing script with 6 day log files and to run 10 processes in parallel. Zoom!

ls 2012*.tar | xargs -n 6 -P 10 process_log_files.py

The script takes a tar for the day and outputs a csv. So I am running watch to count the number of csv files I have as a watch to track progress.

watch -n "ls 2012*.csv | wc -l"

This will only work for certain limited types of operations, but in the case of my log files, it is a massive speedup of what I am doing. What used to take hours, now runs in about 20 minutes.