I set up my nginx with php-fpm. Each vhost has its own unix-socket and separate POOL.
But the problem is that php-fpm is creating 7 thread for each config pool that I created in /etc/php5/fpm/pool.d/
Why? And how could I define that the pool creates only when the url related to VHOST is viewed.
The number of children is controlled by the process manager configuration directives in the pool configuration file. I guess the current settings are either pm = static with pm.max_children = 6 or pm = dynamic with pm.start_servers = 6. One process would be the controlling process, the others would be children ready to handle requests. If you only have 5 not 6 children configured, then I must confess I don't know what that last process is used for. On large high-throughput sites, such a configuration with many ready-to-use children makes sense, but in a memory-constrained (probably virtual) environment where you only expects PHP scripts to execute every now and then, a different configuration might be more appropriate.
To achieve this, edit the pool config files and set pm = ondemand. Then children will be created only to handle requests. This will mean less memory consumption while no requests are active, although it might also mean more time required to create a child. The latter can be reduced by choosing a reasonable pm.process_idle_timeout, so that a sane number of php processes remain idle while users can be expected to use them again soon. That “sane number” is controlled by pm.max_children.
Note: This advice is based on my experience with Debian 7, but it should apply to Ubuntu as well.
Lots of people encourage to use pm=ondemand to save memory. However, it also means that your time to first byte (or server answer ) is going to be very big because if your web server had no recent visits, the php-fpm module will have to first create a child before answering a request. If you use pm=dynamic, it means you will always have at least 1 child ready to answer a client request. The following will load 1 child per user after restarting php-fpm:
pm=dynamic
pm.start_servers=1
pm.min_spare_servers=1
