Editing Web server (section)

=== Kernel-mode and user-mode web servers ===
A web server software can be either incorporated into the [[Operating system|OS]] and executed in [[kernel (operating system)|kernel]] space, or it can be executed in [[user space]] (like other regular applications).

Web servers that run in [[Supervisor mode|kernel mode]] (usually called [[In-kernel web server|kernel space web servers]]) can have direct access to kernel resources and so they can be, in theory, faster than those running in user mode; anyway there are disadvantages in running a web server in kernel mode, e.g.: difficulties in developing ([[debugging]]) software whereas [[Runtime (program lifecycle phase)|run-time]] [[critical error]]s may lead to serious problems in OS kernel.

Web servers that run in [[user-mode]] have to ask the system for permission to use more memory or more [[Central processing unit|CPU]] resources. Not only do these requests to the kernel take time, but they might not always be satisfied because the system reserves resources for its own usage and has the responsibility to share hardware resources with all the other running applications. Executing in user mode can also mean using more buffer/data copies (between user-space and kernel-space) which can lead to a decrease in the performance of a user-mode web server.

Nowadays almost all web server software is executed in user mode (because many of the aforementioned small disadvantages have been overcome by faster hardware, new OS versions, much faster OS [[system calls]] and new optimized web server software).  See also [[comparison of web server software]] to discover which of them run in kernel mode or in user mode (also referred as kernel space or user space).