Page 295 - DCAP101_BASIC_COMPUTER_SKILLS

Basic Computer Skills

Notes GET /path/file.html HTTP/1.1
Host: www.example.com
The Web server on www.example.com will append the given path to the path of its root
directory. On an Apache server, this is commonly /home/www (On UNIX machines, usually/
var/www). The result is the local file system resource:
/home/www/path/file.html
The Web server then reads the file, if it exists and sends a response to the client’s Web
browser. The response will describe the content of the file and contain the file itself or an
error message will return saying that the file does not exist or is unavailable.

14.1.3 Load Limits
A Web server (program) has defined load limits, because it can handle only a limited
number of concurrent client connections (usually between 2 and 80,000, by default between
500 and 1,000) per IP address (and TCP port) and it can serve only a certain maximum
number of requests per second depending on:
its own settings;
the HTTP request type;
content origin (static or dynamic);
the fact that the served content is or is not cached;
the hardware and software limitations of the OS where it is working;
When a Web server is near to or over its limits, it becomes unresponsive.

14.1.4 Kernel-mode and User-mode Web Servers
A Web server can be either implemented into the OS kernel, or in user space (like other
regular applications).
An in-kernel Web server (like TUX on GNU/Linux or Microsoft IIS on Windows) will
usually work faster, because, as part of the system, it can directly use all the hardware
resources it needs, such as non-paged memory, CPU time-slices, network adapters, or
buffers.
Web servers that run in user-mode have to ask the system the permission to use more
memory or more CPU resources. Not only do these requests to the kernel take time, but
they are not always satisfied because the system reserves resources for its own usage and
has the responsibility to share hardware resources with all the other running applications.
Also, applications cannot access the system’s internal buffers, which causes useless buffer
copies that create another handicap for user-mode web servers. As a consequence, the only
way for a user-mode web server to match kernel-mode performance is to raise the quality
of its code to much higher standards, similar to that of the code used in web servers that
run in the kernel. This is a significant issue under Windows, where the user-mode overhead
is about six times greater than that under Linux.

14.1.5 Overload Causes
At any time web servers can be overloaded because of:
Too much legitimate web traffic. Thousands or even millions of clients connecting to
the web site in a short interval, e.g., Slashdot effect;
Distributed Denial of Service attacks;
Computer worms that sometimes cause abnormal traffic because of millions of infected
computers (not coordinated among them);

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