Examples&Exercises:

• Compile and run the code.
• Analyze the code and output.

1. Optimize file i/o performance; a program to show the effect of the buffer. http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code53.c code53.c (do not forget to download also the file, http://siber.cankaya.edu.tr/OperatingSystems/cfiles/ourhdr.hourhdr.h)
   • Many applications assume that standard input is file descriptor 0 and standard output is file descriptor 1. In this code we use the two defined names STDIN_FILENO and STDOUT_FILENO from $<unistd.h>$.
   • The program does not close the input file or output file. Instead it uses the fact that whenever a process terminates, all open file descriptors are closed.
   • This program works for both text file and binary files, since there is no difference between the two to the kernel.
   • First create an input file by

     $dd if=/dev/zero of=inputfile bs=8K count=100
     

     • The output is

       100+0 records in
       100+0 records out
       819200 bytes (819 kB) copied, 0.010789 seconds, 75.9 MB/s
       

     • timing for the writing speed is given as the last item (as 75.9 MB/s),
     • what are the parameters bs and count? (man dd)
     • observe the writing speed by changing the value of bs,
     • observe the writing speed by changing the value of count,
   • Now, execute the program as

     $ time code53 < inputfile > outputfile
     

     • Run the program using the different values for the BUFFSIZE. Fill the following table (inputfile size as 100 MB);
       

       BUFFSIZE (byte)	User CPU (sec)	System CPU (sec)	Clock Time (sec)	# of loops
       1
       2
       4
       8
       16
       32
       64
       128
       256
       512
       1024
       2048
       4096
       8192
       16384
       32768
       65536
       131072

   • Double the input file size and repeat the tests for some selected buffer sizes.
   • Interpret your results.
2. Memory layout of devices, all the devices in the linux are represented as file descriptors and these file descriptors have addresses in the memory;
   • Study the command

     $ cat /proc/ioports
     

     • How it looks like? Try to understand the devices and describe the order of appearance.
     • Each entry specifies (in hexadecimal) a range of ports locked by a driver or owned by a hardware device.
   • Study the command

     $ cat /proc/iomem
     

     • Similar to what happens for I/O ports, I/O memory information is available.
     • Once again, the values shown are hexadecimal ranges, and the string after the colon is the name of the "owner" of the I/O region.
   • As far as driver (device driver) writing is concerned, the registry for I/O memory is accessed in the same way as for I/O ports, since they are actually based on the same internal mechanism.