Why file descriptor metrics are important in performance testing

3 May

A file descriptor is an object that a process uses to read or write to an open file and to open network sockets. An Operating System places limits on the number of file descriptors that a process may open. A lack of available file descriptors can cause a wide variety of symptoms which are not always easily traced back to. The Open Files Descriptors (OFD) provides a count of the total number of file descriptors that are currently allocated and open for processing. The percentage of the total number of open file descriptors with respect to the maximum allowed count of descriptors for processing is a good metric for evaluating the health of a web application
A file descriptor is an opaque handle that is used in the interface between user and kernel space to identify file/socket resources. Therefore, when you use open() or socket() (system calls to interface to the kernel), you are given a file descriptor, which is an integer (it is actually an index into the processes u structure – but that is not important). Therefore, if you want to interface directly with the kernel, using system calls to read(), write(), close() etc. the handle you use is a file descriptor.

There is a layer of abstraction overlaid on the system calls, which is the stdio interface. This provides more functionality/features than the basic system calls do. For this interface, the opaque handle you get is a FILE*, which is returned by the fopen() call. There are many many functions that use the stdio interface fprintf(), fscanf(), fclose(), which are there to make your life easier. In C, stdin, stdout, and stderr are FILE*, which in UNIX respectively map to file descriptors 0, 1 and 2.



Step # 1 Find Out PID

To find out PID for mysqld process, enter:
# ps aux | grep mysqld
# pidof mysqld


Step # 2 List File Opened By a PID # 28290

Use the lsof command or /proc/$PID/ file system to display open fds (file descriptors), run:
# lsof -p 28290
# lsof -a -p 28290

# cd /proc/28290/fd
# ls -l | less

You can count open file, enter:
# ls -l | wc -l

Tip: Count All Open File Handles

To count the number of open file handles of any sort, type the following command:
# lsof | wc -l
Sample outputs:


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