php|architect's Guide to Web Scraping with PHP - Wind Business ...

A p p e n d i x B
M u l t i p r o c e s s i n g
When a PHP script is executed using the CLI SAPI (i.e. from a command line), that
instance of execution exists as a process in the local operating system. The Process
Control (pcntl) extension makes it possible for PHP scripts to perform what is called
a process fork. This entails the original PHP process (then called the parent process)
creating a copy of itself (appropriately called the child process) that includes everything
from the data associated with the process to the current point of execution
following the fork instruction.
Once the fork completes, both processes exist and execute independently of each
other. The parent process can fork itself multiple times in succession and retains a
limited awareness of its child processes. In particular, it is notified when any given
child process terminates.
Because each child process is a copy of its parent process, the number of child
processes that can be forked is limited by the hardware resources of the local system.
CPU will merely limit the speed at which all child processes can complete. M e m o r y,
however, can become a bottleneck if the local system’s RAM is exceeded and it has to
resort to using swap space for storage.
This restriction creates the desire to fork as many processes as possible to complete
a job in parallel without hitting any resource limits. This is generally achieved
by using a predetermined cap on the number of processes to fork. Once any given
child processes completes, however, it may be desirable to create a new child process