Free up that memory

Recently came across this cool interface in the Linux kernel. Typically, the memory might be over-provisioned on the system. Instead of wasting the memory, the kernel normally utilizes a lot of it for page cache, dentry cache and inodes. These caches speed I/O operations and improves performance. However, there are cases when large amount of memory might actually be needed by the apps. While most of the cache pages could be easily reclaimed, there is obviously some overhead involved (the pages could be dirty and might have to written back to the disk, thus incurring disk write latency). Linux has a neat kernel.. so now, while it uses its smart to utilize the memory well, it also provides a method for people to indicate that they do not want the kernel to use its smarts ;-)

To free memory, just do the following:

# echo 1 > /proc/sys/vm/drop_caches

(the above frees only page cache)

# echo 2 > /proc/sys/vm/drop_caches

(for freeing dentry caches and inodes)

# echo 3 > /proc/sys/vm/drop_caches

(for freeing all of the above)

It would be advisable to first do a 'sync' before dropping the caches, so that all the dirty pages could be acted upon.

Simplifying GCC

GCC is the GNU Compiler Collection which provides C, C++ etc compilers. These compilers are used by default in all *nixes .

Here i provide simple command line options which can prove to be quite useful.

1. The simplest way to use GCC to compile a C source file is
   
   $ gcc -o test test1.c test2.c
   
   gcc is the C compiler, test1.c and test2.c are the input C source files and -o lets us specify the name of the output file. Here it is "test". Without the -o option, "a.out" is the default executable that gets created.
   
   
2. The preprocessor:
   
   $ gcc -E test.c > test.out
   
   This option, ensures the compilation process stops after the pre-processor has run. This helps us in figuring out issues/problems in macros.
   
   
   
3. The Compiler:
   
   $ gcc -c test.c -o test.o
   
   This option ensures the compilation process completes but doesn't invoke the linker/loader. This is useful if you want to just remove compilation warnings and errors.
   
   
   
4. Header Files:
   
   $gcc -c test.c -I /location/of/header/files -o test
   
   Many a times the headers files you want to use, is located is some other directory. A "bad" practice followed is to include the direct path of the header files in the C src file.
   Instead use this option. It tells the compiler which directories to look in for the mentioned header files. The -I options can be used multiple times for multiple directories where header files are located.
   
   
5. Library Files:
   
   $ gcc -c test.c -lpthread -L /usr/lib/libpthread
   
   
   Another requirement that is frequently required is using standard libraries ( NPTL Threads etc) or non-standard ones (expat etc). '-l' option tells which library to use while linking while '-L' tells where the find this library. In the above example during linking, it will search for pthread library in the dir /usr/lib/libpthread.
   
   
6. Warnings, Errors, etc:
   
   $gcc test.c -o test -Wall -Werror
   
   
   -Wall options shows all warnings that are typically not shown during regular compilation. These errors are easy fixable like "Unused varniables", "implicit function declaration" etc. -Werror options tells the compiler to treat all warnings as errors and stop compilation instantly.
   Sometimes -Werror can be too strict for our purpose. Instead you can treat only certain warnings are errors.
   eg. -Werror-implicit-function-declaration: Treat only implicit function declaration warnings as errors. For more such options check the gcc man pages.
   
   
7. Debugging:
   
   $gcc -g test.c -o test
   
   This option activates all the debugging symbols. This is required if one plans to use gdb for debugging (which is mostly the case).
   
   
8. Optimizations:
   $gcc -O2 test.c -o test
   
   This option lets the compiler optimize the code . -O can take 0,1,2 levels of optimizations.
   More info is available in the man pages of gcc.
   

These options are the ones that are most frequently used. Obviously there are many more options available . Use them as per your needs and refer the man pages for the exhaustive list of options.

Fix Thinkpad Function Keys/Hot Keys in kde 4.2 (Jaunty) - patch to display brightness levels.

A small patch for the script to display the brightness level of your Thinkpad display with the brightness Increase/Decrease keys.


--- softkeys.py 2009-05-08 11:51:28.000000000 +0530                       
+++ softkeys-0.1.py     2009-05-19 12:43:58.000000000 +0530               
@@ -1,6 +1,7 @@                                                           
 #!/usr/bin/env python                                                    
 # -*- coding: utf-8 -*-                                                  
 # Copyright: 2009 by Tillmann Falck <tillmann _at_ falcken _dot_ de>     
+# Uttaran Dutta added the funtion for the display for the brightness for T60p <linux-hacks.blogspot.com>
 # License: GPL v2 or later                                                                             
                                                                                                        
 from Xlib.display import Display                                                                       
@@ -16,8 +17,11 @@                                                                                      
 XF86Battery = 244                                                                                      
 XF86Display = 235                                                                                      
 KeyUndock = 202                                                                                        
+XF86MonBrightnessDown = 232                                                                            
+XF86MonBrightnessUp = 233                                                                              
                                                                                                        
-keys = [XF86Sleep, XF86Standby, XF86Screensaver, XF86Battery, XF86Display, KeyUndock]                  
+                                                                                                       
+keys = [XF86Sleep, XF86Standby, XF86Screensaver, XF86Battery, XF86Display, KeyUndock, XF86MonBrightnessDown, XF86MonBrightnessUp]
                                                                                                                                 
 def callDisplay():                                                                                                              
   try:                                                                                                                          
@@ -63,6 +67,14 @@                                                                                                               
     print >>sys.stderr, 'Error while trying to change output, try full auto'                                                    
     os.system('xrandr --auto')                                                                                                  
                                                                                                                                 
+def briStats(bus):                                                                                                              
+  bri = bus.get_object( \                                                                                                       
+    'org.freedesktop.Hal', \                                                                                                    
+    '/org/freedesktop/Hal/devices/computer_backlight')                                                                          
+  briLevel = (bri.GetBrightness(dbus_interface='org.freedesktop.Hal.Device.LaptopPanel')*100)/(bri.GetProperty('laptop_panel.num_levels', dbus_interface='org.freedesktop.Hal.Device')-1)                                                                                         
+  s = 'Brightness level is %d%% <br />' % (briLevel)                                                                                    
+  return s                                                                                                                              
+                                                                                                                                        
 def acStats(bus):                                                                                                                       
   ac = bus.get_object( \                                                                                                                
     'org.freedesktop.Hal', \
@@ -91,6 +103,26 @@
     s = 'Battery %d level %d%%<br />' % (n+1, batLevel)
   return s

+def callBrightness():
+  try:
+    sessBus = dbus.SessionBus()
+    sysBus = dbus.SystemBus()
+
+    hal = dbus.Interface( \
+      sysBus.get_object('org.freedesktop.Hal', \
+        '/org/freedesktop/Hal/Manager'), \
+      dbus_interface='org.freedesktop.Hal.Manager')
+
+    s = briStats(sysBus)
+    ea = dbus.Array([], 's')
+    notify = sessBus.get_object('org.kde.VisualNotifications', '/VisualNotifications')
+    notify.Notify('brightness information',0,'brightness information','','Brightness Information', \
+      s,ea,ea,5000, \
+      dbus_interface='org.kde.VisualNotifications')
+  except:
+    print >>sys.stderr, 'No notifications or no information'
+
+
 def callBattery():
   try:
     sessBus = dbus.SessionBus()
@@ -101,6 +133,7 @@
         '/org/freedesktop/Hal/Manager'), \
       dbus_interface='org.freedesktop.Hal.Manager')

+    briStats(sysBus)
     if hal.DeviceExists('/org/freedesktop/Hal/devices/computer_power_supply_ac_adapter_AC'):
       ac = acStats(sysBus)
     else:
@@ -197,6 +230,8 @@
     callSuspend()
   elif event.detail == KeyUndock:
     callUndock()
+  elif event.detail == XF86MonBrightnessUp or event.detail == XF86MonBrightnessDown:
+    callBrightness()
   else:
     print >>sys.stderr, "Key %d not handeled" % event.detail


Note- The patch just enables the display changing for display brightness should be already working through acpi.

Fix Thinkpad Function Keys/Hot Keys in kde 4.2 (Jaunty)

Function keys doent work by default in kde 4.2. Those who migrate from gnome to kde 4 find it difficult to adjust without the 'fn' keys. The problem is with powerdevil. The new applet does not have options to configure the acpi events. This will exits till powerdevil is fixed. As of now, we can use a python script to handle these events. Make sure python-2.6, python-dbus and python-xlib are installed in your system. Download the script from here or here.
Give exe permission
#chmod +x softkeys.py

Place the script in ~/.kde/Autostart/ and restart your system. The function keys must start working now.

Create Desktop Shortcuts Using Cmd Line On KDE 4.2

Here is a quick way to create desktop shortcuts for kde 4.2.
/usr/share/applications/ and /usr/share/applications/kde4 contains all the shortcut files (*.desktop).

Just copy what ever you want to ~/Desktop. Thats it!

fsck.ext3 Unable to resolve UUID error

I installed Kubuntu 9.04 recently and within a day i ran into a peculiar problem. When i boot my system an error message "fsck.ext3 Unable to resolve UUID=". This error generally happens when there is some information mismatch between what is specified in /etc/fstab and what information the OS gives to fsck utility. In this case, its quite easy to solve the problem.

Enter the root password to enter the Maintenance mode when asked during boot.

(Control - D to continue):

List the partition table of your system.

#fdisk -l

Device Boot Start End Blocks Id System
/dev/sda1 * 1 1913 15361888+ 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2 1914 3186 10225372+ 83 Linux
/dev/sda3 3187 12161 72091687+ 5 Extended
/dev/sda5 3187 9265 48829536 83 Linux
/dev/sda6 9266 11697 19535008+ 83 Linux
/dev/sda7 11698 12161 3727048+ 82 Linux swap / Solaris

#vim /etc/fstab

Verify that each device entry specified in the file matches the correct file-system.This error can occur if FS type is wrongly specified. For example, /dev/sda7 is shown as ext3 FS when it clearly a swap in the above case. Correct such errors and reboot. This should solve the problem.

If the entries are correct then, possible problem lies in the UUID specified in fstab. To solve this problem

Determine the UUID of each and every device mentioned in fstab. This exmaple shows for sda1. Repeat this for all devices.

#vol_id -u /dev/sda1
b056f084-eb83-4374-9635-0b5904ff520a

A line taken from my fstab
# /virt was on /dev/sda1 during installation
UUID=b056f084-eb83-4374-9635-0b5904ff520a /virt ext3 relatime 02

Note that UUID matches the entry in fstab. If there is a mismatch, then the entry shown by fstab must be replaced by vol_id. Thats it! save and reboot. This will solve the problem.

REASON: The problem was solved but i was not sure what caused this problem. On further analysis i was able to narrow down on what caused the issue. Apparently, i did a format of a partition (/dev/sda1) which is NOT the / partition using gpartd. Gpart does not allow formats on mounted partitions, so the tools unmounts the partition and formats it. During this step, udev recognizes the new formated device as a new device and old one being removed(formated). Hence it assigned a new UUID which is now not same the one specified in /etc/fstab. Thats why on reboot i ran into this issue.

Change/Resassign Interface Name In Linux

Depending on the linux distro, the interface name given to network cards differ.
For example, on one distro Intel Giga bit Ethernet card might be detected as eth0 and Broadcom card as eth1 while with another distro this might be reversed. This causes a lot of problem, when one uses automated scripts which hardcode "eth0" and "eth1". In older distros it was matter of changing few network scripts to get this working. One can also use "ip" command to make this change temporarily. But to make it permanent it needs to be changed in udev. Here is how simple it is to change the ethernet names.
NOTE: This is only tested on F-10 and Ubuntu 9.04. Its expected to be the same for other distros. Only the name of the udev script might change.

Edit the rule script:

#vim /etc /udev/rules.d/70-persistent-net.rules

Here the list of n/w adapters and their names are specified!
Change the field NAME="eth0" to NAME="eth1" or vice versa
Remember to rename the other adapter whose name you have borrowed.
Restart the system and you will find the new names interchanged!