Linux scheduler

The most fundamental part of any operating system is its scheduler which manages all the processes (tasks) in the OS.
If you look into /usr/src/linux-headers/include/linux ,you will find all the system files which makes up the kernel .Here search for sched.h ;This would be our scheduler and if you open it in a text editor,you will find a task_struct structure.

 This represents the process control block(PCB) which is a datastructure holding all essential details of a process

you can view a partial PCB of a process by
$ cat /proc/pid/status  

here pid is the process id of the process ,whose status you want to view.

PCB is composed of all details needed for running a process along with details of resources which a process has access to,and the scheduler uses it for context switching or switching between process for CPU access .


Kernel keeps a list of process descriptors wherin a doubly linked circular list is maintained ,this linked list is composed of all process descriptors including the init.

Here each block is a PCB for that task, Now the scheduler needs to make a list of tasks which are ready to be executed.The ready queue is implemented in this circular linked list itself .

Here the scheduler just have to know the pointer to the beginning of the queue and to the end.The red arrows indicate the pointer from 1 PCB to the next in the ready queue.Similarly a wait queue can be maintained just by managing pointers in the PCB.The next process to be run is determined  by Struct task_struct *next_run; 


Source codes for scheduling algorithms using pthreads in c are available here

Making a script run as an app in ubuntu launcher

Don't we all love to execute scripts without the geeky looking terminal ?, say we have a shell or a python or a tcl-tk script ,we want now to run this as any app with an icon, Well there is a way.

Install gnome panel by using
$ sudo apt-get install gnome-panel


gnome-panel is just a taskbar application in desktop for ubuntu.

Once the installation is done type
$ gnome-desktop-item-edit --create-new ~/Desktop

you will then observe a create launcher applet , select type as Application
Give any funky name you want for your application.
And in the command field, provide the command which you use to run your script in the terminal.

so to run a tcl-tk application , use wish path_name/file_name.tcl

 You can now click the icon to run your script.

You can also edit your application icon , just open it in gedit , the icon is saved as name.desktop

in the above case someapp.desktop



#!/usr/bin/env xdg-open

[Desktop Entry]
Version=1.0
Type=Application
Terminal=false
Icon[en_IN]=python
Exec=wish path_name/file_name.tcl
Name[en_IN]=someapp
Name=someapp

extracting emotion from text

Here we shall look at a simple procedure to perform a sentiment analysis of the given text,

We proceed by making a list of words and associating with a value indicating the degree of emotion conveyed by the same.

something like :

awesome 4
bad -3
sad -3
cool 2

etc

Here the positive value indicate a positive emotion ,and the negative with negative emotion conveying words.The value indicate the degree of emotion conveyed by that word.
Now all we have to do is demarcate the text into constituent words and individually look up in this list,and sum up or take the average of the corresponding value to get the emotion value conveyed by the text.You can be wise enough to avoid usage of "stop words" in the list as they do not convey any meaning. 

Personally i adore python for the awesome text processing capabilities , you can use the text as key and look it up as a dictionary ,have a look here.

 A front end browser with a backend python script computing the emotion conveyed.

You can also use twitter to get general opinion on a particular topic

use http://search.twitter.com/search.json?q=topic_name

 (all spaces in topic_name is to be replaced with +)

Twitter returns data in json format which you have to parse to extract the texts and use your python script to compute the emotion values which you can add or average as a whole to get general opinion

Using google map service without API key

To use google map service, you are instructed by the developer manual to generate an api key using your google account.You may have to cough up money to google if the number of hits to your site increase beyond a certain number.

But can we do the same sans api key ? +

Yes, well at-least for the geocoding and reverse geocoding service.

All you have to do is to ping :  https://maps.google.com/maps?q="location name"

example  https://maps.google.com/maps?q=washington or https://maps.google.com/maps?q=new+york

see how i replace spaces with + in the url.

To use it locally request data in json format, something like https://maps.google.com/maps?q="location name"&output=json

All you have to do is parse it for the required fields and there you have it,your data with no api key. Don't we love free stuff !!

PHP FAQ

How to secure get parameters in php ?

Try using

sending end

$a="some crap";
$key = 'pass';// can use your own key here
$string = " ".$a." ";
$encrypteda = base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_256, md5($key), $string, MCRYPT_MODE_CBC, md5(md5($key))));



receiving end

$key = 'pass';
$rand=($_GET['parameter']);
$rand = str_replace(" ", "+", $rand);
$rand = rtrim(mcrypt_decrypt(MCRYPT_RIJNDAEL_256, md5($key), base64_decode($rand), MCRYPT_MODE_CBC, md5(md5($key))), "\0");
$rand = str_replace(" ", "", $rand);



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 How to download multiple files in zipped form ?
Want to download files gmail style ? , well you can do it by using simple php script
we shall be using the following approach

open a ziparchive

add files to the ziparchive (you can add multiple files here )

close it after adding

Now just pass a header information containing the name of zip archive

finally read the archive file


That would be -
a reference to archive (inspired by java i guess!)

$ziparch= new ZipArchive()

open an archive
$ziparch->open($somename);

Then add files by

A loop
{
$ziparch->addFile($filename);
 }

 Close once done
$ziparch->close();

followed by provision of header information indicating the ziparchive name

header("Content-type: application/zip");
    header("Content-Disposition: attachment; filename=$somename");
    header("Pragma: no-cache");
    header("Expires: 0");


finally

readfile("$somename");



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How to pass data between php pages ?
If you intend to pass data and yet find the popular post and get method to be incompatible use session .

all you got to do is start session at the beginning of the script

session_start()


Add data to session variables
(say in page 1)

$_SESSION['identifier']=$data;


Retrieve data by
(say in page 2)

$get_data=$_SESSION['identifier'];




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How to periodically execute php scripts ?
Without any user pinging the server, you can use cron service to schedule execution of scripts.Check out crontab in your favorite search engine.But there is a catch ,you have to incorporate this service onto the server and in some of the cases it is non trivial.
another method is to use the php's time() which will give the unix time stamp. well You can use just this.

Consider a time() instant   1358185093   ,get the ,say last 5 numbers   or  85093
now in your index.php or in your php script use this

if($last_5_digit == 50000)
{
 execute a periodic script.
}

NOTE : this works good for servers which get heavy traffic,since the users are constantly pinging the server (even when time()(last 5) == 5000)

The best ,easier method which i generally use is to add a time field in a table and observe the current time .Then delete all rows whose time field and the current time difference is beyond a threshold.



------------------------------------------------------------------------------------------------------------------------------

How to ping urls when your server is behind a proxy ?


use

$acontext = array(
    'http' => array(
        'proxy' => 'tcp://proxy_server:port',          // example tcp://10.10.10.10//3128
        'request_fulluri' => true,
    ),
);

$cxContext = stream_context_create($acontext);
 $geturlcontents = file_get_contents("$url", False, $cxContext);





Incidentally for getting web resource in python

use

html=urllib2.urlopen('http://python.org/').read()





but ensure in case of proxy server,to set your environment variable http_proxy

use            $ export http_proxy=http://proxy_server:port


$ echo $http_proxy            should echo something like http://proxy_server:port/