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HOW TO : Increasing number of processes that can be run by a user in Linux

By default, most of the Linux distros limit the number of processes that a user can spawn. This is put in place to limit (un)intended cases when a process might just fork off processes without a limit and bring down a server.

For RHEL (and CentOS), the default is 1024 processes per user. In some cases, you do need to increase the number of processes that a particular user can spawn. For example if you are running a database or an application server, you definitely want to tweak this number because these apps tend to create a lot of threads.

As a side note, if you run into this limitation on a machine running jboss, you typically see an error with the following string in your server logs [code]java.lang.OutOfMemoryError: unable to create new native thread.[/code]

. Looking at the error, one would think it is related to memory issues :).

OK.. back to the subject at hand. Here is the process for identifying your limits and then tweaking them as required in RHEL or CentOS.

  • Check the current limits on the number of processes a user can run by executing [code]ulimit -u[/code]
  • Edit the /etc/security/limits.conf file and add the required limits. You can get all the possible options by running man limits.conf. For example, if I wanted all the users to have a soft limit of 2000 and a hard limit of 4000, my limits.conf file wold look like this [code]# Increase the number of threads per process
    *       soft    nproc   200
    *       hard    nproc   4000 [/code]
  • Edit the /etc/security/limits.d/90-nproc.conf file and update it to have the same soft limits. By default, it has 1024 as the limit. So an updated file with my new limits as in the example above would look like this [code]
    # Default limit for number of user’s processes to prevent
    # accidental fork bombs.
    # See rhbz #432903 for reasoning.

    *          soft    nproc     2000[/code]

  • Restart the server. The updated settings won’t take affect until this is done
  • Check if you have the new limits by running [code]ulimit -u[/code]

You can also check the limits of a particular user by finding a process ID being executed by that user and running [code]sudo cat /proc/PROCESS_ID/limits [/code]

March 14, 2012 by HOWTO Linux Uncategorized 2

HOW TO : Configure Jboss for writing web access logs

One of the capabilities of Jboss is that it can serve HTTP traffic. By default Jboss does not log any of the HTTP traffic in it’s log files. Here is a quick howto on enabling this logging. This post is specific to Jboss 4.x (ancient!!) and I will post another one soon on how do it in version 5.x and newer.

Edit the server.xml file located in $JBOSS_HOME/servers/$PROFILE/deploy/jboss-web.deployer and replace the commented out access logger section as such

FROM

[code]<!–
<Valve className="org.apache.catalina.valves.AccessLogValve"
prefix="localhost_access_log." suffix=".log"
pattern="common" directory="${jboss.server.log.dir}"
resolveHosts="false" />
–> [/code]

TO

[code]<Valve className="org.apache.catalina.valves.AccessLogValve"
prefix="localhost_access_log." suffix=".log"
pattern="common" directory="${jboss.server.log.dir}"
resolveHosts="false" /> [/code]

This will start creating a file with the format localhost_access_log.CURRENT_DATE.log in the $JBOSS_HOME/server/$PROFILE/log folder

But it isn’t fun if you just leave the default logging right :). The pattern formats of common and combined are similar to the standard apache logging options. But if you wanted to have certain content and format in the log files, you have a lot of options. Jboss community has documented all the data that is exposed through this valve at http://docs.jboss.org/jbossweb/latest/api/org/apache/catalina/valves/AccessLogValve.html

So say, I want to log the referrer header, user agent and the value of a cookie called JSESSONID and log all this data into a file called jboss_web_access_log, I setup the options as such

[code]<Valve className="org.apache.catalina.valves.AccessLogValve"
prefix="jboss_web_access_log." suffix=".log"
pattern="%h %p %l %u %t %r %s %b ‘%{Referer}i’ ‘%{User-Agent}i’ ‘%{JSESSIONID}c’"
directory="${jboss.server.log.dir}"
resolveHosts="false" /> [/code]

March 13, 2012 by HOWTO Technology Uncategorized Web 0

HOW TO : Clear unused swap memory in Linux

Inspired by a G+  post by Thomas Weeks .

swap memory is something used by the OS to essentially swap data to and forth if the main memory is not available. It is several times slower than RAM, since it uses hard disk to store the memory. And if you are constantly swapping, your system performance is going to be impacted quite a lot. You should always ensure that  your system is not swapping by adding the required RAM and/or stopping your application(s) from using so much memory. At times, because of spike in utilization, the OS might briefly use swap. And when it does, it doesn’t release the memory from swap. So from an analysis prospective, it makes it difficult to check (quickly) if your system is using swap or not. This is similar to errors on an interface in a router. Unless you clear them and monitor, you don’t know when the errors happened.

I was not aware that you could turn off swap devices while the OS is running and then enable them again. So here are the commands to do that in Linux

[code]swapoff -a[/code]

This essentially disables swap on all devices configured for swap in /etc/fstab

[code]swapon -a[/code]

This does the opposite of the first command. Enabled swap on all devices that have swap configured.

Tom put this into a nice alias by doing the following

[code]alias unswap=’sudo swapoff -a && sudo swapon -a'[/code]

Thx Tom…

March 9, 2012 by HOWTO Linux Technology Uncategorized 2

HOW TO : Clear screen based on OS in python scripts

I like shiny new toys :). Even though perl is pretty powerful and more than enough for the simple tasks I get to automate from time to time, I want to start learning python and find out first hand, why the whole geek community is raving about it.

As I start to write new scripts in python, I wanted to document how I used to do some things in perl and how I implemented them in python.

One of the standard features of any script I write is to “clear” the screen before starting to send output to the console. Here is the comparison between perl and python

perl

[code]system $^O eq ‘MSWin32’ ? ‘cls’ : ‘clear’; [/code]

python

[code]

# Clear screen, based on the OS
if (os.name == ‘nt’):
os.system("cls")
else:
os.system("clear")

[/code]

March 8, 2012 by HOWTO Programming Technology Uncategorized 0

Project Uptime : Progress Report – 1

Here is the first update on Project Uptime. I spun up a new server (doesn’t that sound so odd.. spun up a new server!! :)) with 512MB of RAM running Ubuntu 11.1o (Oneiric Ocelot). First order of business after spinning up the server?

  • Update to the latest and greatest patches

[code] sudo apt-get update [/code]

  • Update to the latest kernel.
    • First check the version of kernel you are running

[code] uname -r [/code]

    • Check the repository for latest version

[code] apt-cache search linux-image [/code]

    • Install latest version

[code] sudo apt-get install linux-image-LATEST-VERSION [/code]

    • Restart server

[code] sudo init 6 [/code]

March 7, 2012 by HOWTO Linux Uncategorized 2