php-fpm use a lot of cpu

If you have only two CPU and has 30 children in parallel, are carrying on the number of processes that your CPU supports. When there are several children trying to work, each has less CPU, and the result is that everything goes slower, plus the overhead of CPU. This is really important in case like WordPress who use a lot of CPU for each request.

Is better reduce the number of maximum to 2 or 3 children, and nginx which is responsible for managing the queue of connections, queries are not lost.

When FPM is "ondemand" you only need to define pm.max_children, in this case:

pm.max_children = 2 

I hope you find it useful.

This is an old question, but it was bumped today by someone editing the post.

While installing a Wordpress caching plugin can be effective, it still means PHP is executed, which is relatively slow. If some of your website visitors are anonymous (ie not logged in) you can do page caching (fastcgi_cache) in Nginx that will avoid calling PHP at all, which dramatically lowers CPU usage and improves response times. The page can be cached for as little as 1 second to be valuable, or if the site is quite static you can take that out to a day or more. The tricky part is clearing the cache when you publish changes, but I have a solution to that - see below.

Have a read of the tutorial I wrote on LEMP. This first part gives you config files to download, they're well commented and the later parts of the tutorial explain them. There's also a great article by Nginx on microcaching.