Project Euler

Signed up recently to Project Euler, seems like a good way to keep your computer science/programming skills sharp. Although I'm surprised on the forums everyone's using C/Assembly to solve the problems. Got a nice badge auto generated for me as well:

The problems seem like they could get a bit addictive, but definitely a lot of fun and probably better than a spending time on facebook.

How to see which package a file belongs to

Previously I wrote a post talking about how to see what files were installed as a part of a package. In this post, I'll talk about going to other way i.e. how to tell which package a file belongs to

On Debian/Ubuntu you can do so using the dpkg search function:

# sudo dpkg -S /etc/init.d/whoopsie
whoopsie: /etc/init.d/whoopsie

On RedHat/CentOS you can use the rpm command:

# rpm -qf /usr/bin/bash

Configuring boot services

One of the common tasks when setting up a server is to configure whether a service is set to start up on boot or not. This is handled differently on different versions of Linux.

To list all services and whether they're set to run on boot:

RHEL/CentOS/Fedora

chkconfig --list

Debian/Ubuntu

rcconf

NOTE: this program doesn't come installed by default

Enable a service to run on boot:

RHEL/CentOS/Fedora

chkconfig [service name] on

Debian/Ubuntu

update-rc.d [service name] enable

Disable a service from running on boot

RHEL/CentOS/Fedora

chkconfig [service name] off

Debian/Ubuntu

update-rc.d [service name] disable

Tomcat and Virtual Hosts

In this guide I'll go through setting up some very simple virtual hosts on an Tomcat server. This guide assumes the steps gone through to setup Tomcat 6 on Ubuntu as per this previous post.

So, the first step is to define the host under /etc/tomcat6/server.xml:

Put the above line in the Catalina "Engine" section. The "name" attribute will be used as the hostname to match and the "appBase" will define where Tomcat will look for the applications to run off of this host. If you'd like to define some aliases for this virtual host, you can do so with a nested "Alias" directive as described in the Tomcat documentation.

Next, if we want to define the Context we simply create the directory for it under Catalina:

mkdir /etc/tomcat6/Catalina/example.org

And then for a simple application we can just copy the ROOT app from the default context:

cp /etc/tomcat6/Catalina/localhost/ROOT.xml /etc/tomcat6/example.org

This will define the Context for our application. Next, we will need to create the application directory to actually hold our applications for this virtual host and copy the relevant application files to this new directory. This is done with:

mkdir /var/lib/tomcat6/example.org
cp -r /var/lib/tomcat6/webapps/ROOT /var/lib/tomcat6/example.org

Then I modified the "index.html" file under "example.org/ROOT/" to display "example.org" instead of the default "It Works!" so that we would know when the Virtual Host was being accessed. Once this is done, we can go ahead and restart Tomcat in order to apply the changes:

sudo service tomcat6 restart

To test out that this configuration is working, I added a line to my hosts file (/etc/hosts under linux) on my desktop machine to point "example.org" to the IP address of the VM that I had installed Tomcat on. This allowed me to type in http://example.org:8080 and have the request go to the Tomcat server.

If everything worked out well, going to the virtual host at http://example.org:8080 should yield the modified page, where as going to http://[Tomcat server IP]:8080 will result in the default page.

So, there you have it, that's the short story on how to setup up virtual hosts on Apache Tomcat.

Ubuntu - The following packages have been kept back

If you've used Ubuntu for long enough, you'll find that eventually you'll run into a problem when upgrading the installed packages. When running apt-get from the command line, the problem manifests itself as the following:

$ sudo apt-get upgrade
[sudo] password for srdan:
Reading package lists... Done
Building dependency tree      
Reading state information... Done
The following packages have been kept back:
  linux-headers-server linux-image-server linux-server
0 upgraded, 0 newly installed, 0 to remove and 3 not upgrade

The short answer is that you should be able to upgrade by running the "apt-get dist-upgrade" command:

$ sudo apt-get dist-upgrade
Reading package lists... Done
Building dependency tree      
Reading state information... Done
Calculating upgrade... Done
The following NEW packages will be installed:
  linux-headers-3.2.0-27 linux-headers-3.2.0-27-generic linux-image-3.2.0-27-generic
The following packages will be upgraded:
  linux-headers-server linux-image-server linux-server
3 upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 51.2 MB of archives.
After this operation, 217 MB of additional disk space will be used.
Do you want to continue [Y/n]?

The long answer comes from the man page of the "apt-get" command. In particular, if you look at the description of the "upgrade" argument, two sentences stick out:

"under no circumstances are currently installed packages removed, or packages not already installed retrieved and installed."

"New versions of currently installed packages that cannot be upgraded without changing the install status of another package will be left at their current version."

Because you can't upgrade the "linux-image-server" (a.k.a. the kernel) without upgrading the headers as well (technically you can, but it can lead to serious problems) it won't let you upgrade them using the "upgrade" command. Either that, or the "new" linux image package requires that a "new" linux headers package be installed, violating the requirement that packages not already installed not be installed.

The reason that the "dis-upgrade" command works, where the "upgrade" command does not is that "dist-upgrade" takes into account dependencies between packages. Also from the man page:

"dist-upgrade ... intelligently handles changing dependencies with new versions of packages; apt-get has a 'smart' conflict resolution system, and it will attempt to upgrade the most important packages at the expense of less important ones if necessary."

This does beg the question though as to why not just use "dist-upgrade" all the time or incorporate the conflict resolution system into the "upgrade" command?

I suspect the answer has something to do with how each "edition" of a distribution is defined. i.e. Ubuntu 12.04 will ship with version 3.1.13 of the "at" package and all other packages that are a part of this "edition" should work with that version. Having many different packages depend on specific versions of other packages could end up in a situation where it becomes very difficult to upgrade any individual package.

svn:ignore property, STS and Subversive

Working with a combination of Springsource Tool Suite, Subversion and the Subversive plugin, one of the things that really bugs me is the issues I keep on getting as a result of messed up source control of the "target" directory.

This directory typically holds all of the compiled classes as well as a stacktrace log and a few other things. In general it never really needs to be put under version control.

One of the things I keep forgetting to do, that frustrates the hell out of me is to set the svn:ignore property as soon as the repository is created. You see, without this property set, the target directory is submitted (added) to version control and once it's in there, there's no (easy) way of getting it out, as the Subversion documentation says:

"...Once an object is under Subversion's control, the ignore pattern mechanisms no longer apply to it..."

And even if you set the "svn:ignore" property on the directory, it still probably won't work properly. Instead committing all of the class files and so forth that you couldn't give two craps about.

All this is well and good, but could be written off as a minor problem, if it weren't for the fact that when you try to revert your changes, Subversion could very well pick up "conflicts" between the class files, and in fact can put you in a situation where you're struggling to get a functional checkout of the repository, simply because of the "target" directory.

The problem seems to be with the nature of the files being stored in the directory being constantly deleted/recreated etc... which seems to screw up Subversion. At the moment there doesn't seem to be a fix for the issue i.e. every time I see the problem, I just manually manage the conflicting files.

Linux Kernel Swappiness

There is a tendency of the Linux kernel to use memory as file system cache. This generally improves performance and is considered to be a "good thing". However, one thing that the kernel also occasionally does is take the memory allocated to running processes and swap them to disk, in order to use that memory for file systems cache. Now, this can and does result in processes becoming slower, especially if they've been running (sitting in memory) for a while, but haven't been actively used.

Luckily, there is a way in which you can define this behaviour and it's called the kernel "swappiness" value. The value has a range of 0 to 100, with zero roughly meaning that process memory will never get swapped out for the sake of disk caching and a value of 100 means that process memory is very aggressively swapped out, in favour of disk caching. A more in depth explanation of how the kernel manages swappiness, can be found here.

By default, this value is set to 60, which is configured more for server throughput, rather than desktop responsiveness.On my desktops I usually set the value to 10, which seems to be a good fit for desktop responsiveness.

The way to set it on Ubuntu is:

$ sudo sysctl vm.swappiness=10

and to make the changes permanent, just add the following line to /etc/sysctl.conf:

vm.swappiness=10