Thursday, 17 December 2009

Basic and fundamental knowledge

I was recently asked with some basic and fundamental question, directly related to Linux system administration job. When you are already a senior engineer, most of the time you will forget about the fundamental, or don't really care about it. But truth is, you still need to know about it, because it is so fundamental. People will judge you with this basic information to know that if you are really knowledgeable and your level is what you really say you are at. I'm not able to answer all the question satisfactorily, and feel somewhat ashamed about it.

Let's learn it together.

1. What is the difference between a network hub and a network switch? 

When I search Google for the answer, I believe this site put it very nicely. Take a look at . In essence:
  • Hub repeats the packet it receive on one port to the other port available
  • The bandwidth is shared across all the ports. If the hub is 10Mbps, with 5 ports, then each port can only transfer at max 2Mbps
  • Switch divides the network into multiple segment thus a pair of ports can communicate without affecting other pair of ports
  • Switch maintains a table of destination address and its port, so when a packet arrives, it will send the packet to the correct port
  • The bandwidth of the port is dedicated. If the switch is 10Mbps, with 5 ports, when port 1 connect to port 2, the bandwidth is 10Mbps for that instance, and when port 3 connect to port 4, the bandwidth is also 10Mbps for that instance

2. How many bit are there in a MAC address?

I cannot answer this question correctly. The answer is 48 bit. This site provides the information: .  In essence:
  • MAC address have 12 digit hexadecimal number
  • 1 hex = 4 bit, thus 12 hex = 48 bit
  • Hex symbol is 0123456789ABCDEF
  • The first 6 hex digit represent the manufacturer

3. What is the difference between TCP and UDP?

Wikipedia have the answer:
  • TCP is Transmission Control Protocol
  • TCP is connection oriented link
  • When a machine send a TCP packet, the receiving machine have to send back acknowledgment packet when it arrive
  • If sending machine fail to get the acknowledgment after certain time period, the packet will be resend again.
  • Example of TCP usage is between web server and web browser
  • UDP is User Datagram Protocol
  • UDP never guarantees that a packet will arrive at destination
  • When a machine send a UDP packet, it is not expecting acknowledgement from the receiving machine.
  • Example of UDP usage is audio streaming, and DNS

3. What information are available in TCP packet? Name some of them.
  • Source address
  • Destination address
  • Checksum

4. What flag are available in TCP packet? Name some of them.

I also cannot answer this question correctly. The answer are:
  • SYN
  • ACK

5. When you execute "uptime" command, there are 3 numbers at the end of the line. What are they?

Answer here: . The 3 numbers are load averages for the past 1, 5, and 15 minutes.

6. What is the meaning of the load average number?

Here is the answer: . It means "the average sum of the number of processes waiting in the run-queue plus the number currently executing over 1, 5, and 15 minute time periods."

7. How do you know that the server is busy from the number?

The best answer probably from this site: . For this question, I answered if the number is 2 or bigger, then the server is busy or under heavy load. This is relative, and you will know from experience handling Linux or Unix machines.

8. What happen when the machine is busy?

I answered, the most noticeable clue is that you have trouble accessing the server remotely. When you SSH to the server, it will take a while before you are able to login. This is because the SSH connection is encrypted and the server will need to decrypt the data before able to give you access. Encryption and decryption takes big amount of CPU cycle, and if the machine is already busy, you will see it will take some time before you are able to login.

Other than that, if the machine have small amount of memory, you will see a lot of disk activity, because the OS is swapping the application that resides in memory, but is not executed, to the disk to make way for application that have higher priority.

9. How do you list processes running in the machine?

Use the command "ps"

10. How do you terminate a misbehaving application?

Use the command "kill -9 <appname>". The number 9 is sending the SIGKILL signal. To terminate application with the same name, use "pkill <appname>"

11. What other signal available?

This site summarize it: . Other signal available is SIGHUP and SIGTERM. I answered SIGHUP is to restart an application, but most information in Internet said that SIGHUP is to re-read configuration file or to stop an application. You might need to search for more concrete answer.

SIGTERM is terminate signal sent to application to stop it gracefully. When application receive a SIGTERM, it will do the necessary process to make sure it is stopped cleanly.

12. What is the difference between SIGKILL and SIGTERM?

SIGTERM is a graceful termination signal. The application that receive the signal will try its best to stop or notify any dependency, and then terminating itself. For example, if a parent process got SIGTERM, and it has few child process, the parent process will notify the child process that the parent is being terminated, and the parent might also send SIGTERM to the child to terminate them before terminating itself. SIGTERM can be ignored if the application was programmed to do so.

For SIGKILL, the application will be directly terminated by the OS. No information will be sent to any dependencies of the program. If a program have a child process, that child process might become orphan or zombie because its parent has been killed and it has no clue on what to do next. This kind of issue might cause further instability to the server if the server is already have some issue.

13. Have you experienced application that will not terminate even after you send SIGKILL signal? How do you terminate such application?

More info here: . That application is called zombie application. To find zombie application, use command "ps aux | grep Z", where the zombie application will have Z as its status. You cannot killed a zombie application because it is already dead. What I usually do is, if the zombie process have a parent process, I will terminate the parent process, where most of the time the zombie process will terminate because its dependency to its parent has been terminated.

But, there are cases that, when you terminate the parent process of the zombie, the zombie will then use process with PID 1 as its parent. Process with PID 1 is the init process, and it is the first process to run when the server starts. If this happen, you have no other way to kill the zombie other than rebooting the server.

14. What actually pkill command do?

pkill will list the PID of the process that have the name as specified, then will terminate the application one by one. The default signal sent is SIGTERM.

That's all. I have learned quite a lot after this event. Hopefully this post will be helpful to someone out there.
Got comment? Let me know. :)

Tuesday, 10 November 2009

Tracing bitmap in Inkscape

During BOF session in MyGOSSCON 2009, Nuhaa a.k.a cawanpink representing, was presenting on how to use Inkscape to produce graphic images. After her presentation, I inform her that Inkscape has the capability to trace bitmap into vector graphic, so that if you want to scale the image, it will not become blur (when scale down) or become boxy (when scale up). I guide her the step to produce the vector image. I believe a lot of Inkscape user also did not know much about this capability. This post will explain the step.

  1. Search Google Images for the image that you want to trace. Usually you would like to trace logo to be use somewhere else. When you search for image, try to find big or medium size image. In this example, I want to search for 1Malaysia logo.

  2. Download the logo to your computer. Here is the logo that I got.

  3. Open the bitmap image in Inkscape.

  4. Press Ctrl+A to select all the image. Then select the menu Path > Trace Bitmaps

  5. Dialog option will be displayed. Select the option Colors, Stack scans and Remove background. After that, click the Update button. You will see a preview of the trace. If you are happy with the preview, click OK

  6. The vector image will be available on top of the bitmap image. If you select the logo, you can move it around. In the picture below, I move the vector image to the right.

  7. You can delete the bitmap image then move back the vector image to the canvas. After that you can do anything with the vector image. Here, I made a clone of the image, then scale it down. The image is still sharp because it is a vector.

That's all. You can save the vector image to SVG format if you want to edit it again in the future.

Happy drawing! :)

Thursday, 5 November 2009

Programming in PostScript

I have found my new craze! It is programming in PostScript! :D

What is PostScript, you may ask? PostScript is a programming language optimized to print graphic and text. You can think it as a page description language, much like HTML is the document description language for the web.

It all started when Najmi posted to MyPenguin99 mailing list about the powerfulness of Python to generate a PDF file. It is called Simson Garfinkel's Notepaper Generator. Upon closer inspection, I found out that it is not the Python that is powerful, but it is the PostScript language that is powerful that you can program your paper document to look like what you want it to be.

The Python script is just a script that help you to either enable or disable a feature, change the owner of the paper, and generate the calendar to be put into the PostScript file. From the PostScript file, it is converted to PDF by using ps2pdf command line program.

Suddenly, a question pops in my head. How hard it is to hand-coded a Postscript file? I have never program a PostScript before and don't know the answer. But I'm going to find out.

Few years back when I'm an electronic engineering student, graph paper is one of my tools of the trade. Back then, when I'm running out of graph paper, I have to buy a graph paper pad with 10 sheet in it, even though I might only need a single sheet to complete my assignment.

So, to challenge myself for the PostScript programming, I'm planning to create a 1 mm by 1 mm grid that will occupy an A4 paper, with about 1 inch border from the edge of the paper. That should be easy, I think.

And actually, it is easy! I just learned PostScript in 2 days from information available on the Internet, and also found a PDF file from Adobe called "PostScript Language Tutorial & Cookbook". It contains all the command that you need to know about programming in PostScript. Search Google using the keyword "postscript tutorial".

PostScript is not hard. It is stack-based in the same manner as RPN calculator. And this is easy for me because I program Motorola microcontroller using stack-based instruction in assembly language during my university days :)

The result of the PostScript file can be viewed without having to print it out, by using GSview for Windows. You can also use Evince for Gnome or Okular for KDE. The PostScript file can be converted to PDF file
 by using PDFCreator in Windows, or ps2pdf command line in Unix/Linux/BSD.

Below are two screenshot of GSview, one viewing the A4 paper in whole, and the other zooming the paper to its width.

Get the source from the URL below. Now I can start to create my own GTD paper organizer. :D

Enjoy! :)