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There is not much here, but this site serves as a reminder of past things we had to dig up on google or another search engine that we thought were note-worthy and that we thought we might need again.

This site is not ment to be a one stop shop for everyone, nor is it ment to be complete, well thought out, etc; but only a place to store our 'notes'.

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Last entry: 2008-12-12 08:56
86 entries written

Friday, March 14. 2008

Posted by Matthew Isam in Linux at 15:48

Memory Usage - Climbing to 98% is NORMAL

Linux
Worried that your memory usage seems high?
Don't be - Linux is quite smart in the way it handles memory - it will use all that it can in order to speed up basic operation. If you add more memory, the system will use it up too, same with taking out memory.

So - If high memory usage doesn't indicate a problem what does?

High or climbing swap usage are the "memory problem" in Linux. Swap space is a section of the hard drive used as memory when the machine has run out of physical RAM, Hard drrives are however much slower than RAM, so this needs to be looked at.

Climbing SWAP usage - If your SWAP file is monotonically increasing (i.e. it NEVER falls) then this might indicate a problem. - when this situation has been ongoing for a few months (and you've tried increasing the SWAP space, but it's been filled again then come and ask questions...

Details: For speed and efficiency, any available physical memory (RAM) is used to speed up file throughput and access times. In other words, you will usually see your physical memory usage at 90% or higher.

Explanation: RAM is 100x faster than any harddrive in existence with avg read/write speeds measured in nanoseconds, compared to harddrive read/write speeds measured in microseconds. It makes much more sense to keep more of the operating system in the ultra highspeed RAM space, rather than swapping it to the harddrive and back. Thus, linux will attempt to completely fill the RAM, leaving a tiny bit available as a buffer. When all available RAM is used, it will then write to the swap space on the HD. This increases the overall system performance.

Thursday, February 28. 2008

Posted by Matthew Isam in Linux at 13:54

Time in cron

Linux

The times in cron are local times. This means, that if you configure cron to start some program at 7 in the morning, it is 7 in the morning local time as determined by the system Timezone.

This causes headaches, if the system Timezone is not the same you have set for yourself using the TZ enviroment variable, or you have two host in different Timezones that need to start someting at the same time on both hosts.

Thursday, February 28. 2008

Posted by Matthew Isam in Linux at 13:53

Linux Job Management, Process Management, and Help

Linux

Linux Help Commands

apropos apropos keyword - Show all commands with the keyword in their description. The same as the "man -k" command.
help Bash shell help for the bash builtin command list. The help command gets help for a particular command.
man Get help from the manual for a command.
man man -k keyword - Show all commands with the keyword in their description
"man 2 kill" - Display page 2 of the kill command
manpath Determine user's searchpath for manpages.
info Documentation on Linux commands and programs similar to the man pages but navigation is organized different.

Linux Job Management

at Similar to cron but run only once.
atq Lists the user's pending jobs. If the user is the superuser, everybody's jobs are listed.
atrm Deletes at jobs.
atrun Run jobs queued for later execution
batch Executes commands when system load levels drop below 0.8 or value specified in atrun invocation.
cron A deamon used to set commands to be run at specific times. Starts the commands in the crontab file. Used to clean up temporary files periodically in the /var/tmp and /tmp directories.
nice Run a program with modified scheduling priority.
nohup Run a command immune to hangups, with output to a non-tty.
watch Execute a program periodically showing output full screen.

Linux Process management

bg Starts a suspended process in the background
fg  Starts a suspended process in the foreground
gitps A graphical process viewer and killer program.
jobs Lists the jobs running
kill Ex: "kill 34" - Effect: Kill or stop the process with the process ID number 34.
killall Kill processes by name. Can check for and restart processes.
pidof Find the process ID of a running program
ps Get the status of one or more processes. Options:
  • u (more info)
  • a (see all)
  • -l (technical info)
Meanings:
  • PPID-parent process ID
  • PID-process ID
ps ax |more to see all processes including daemons
pstree Display the tree of running processes.
sa Generates a summary of information about users' processes that are stored in the /var/log/pacct file.
skill Report process status.
snice Report process status.
top Display the processes that are using the most CPU resources.
CTRL-C Kills the current job.
&  At the end of the command makes it run in the background.

Thursday, February 28. 2008

Posted by Matthew Isam in Linux at 13:49

Linux/UNIX Crontab Basics

Linux

cron is a unix, solaris utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon. These tasks are often termed as cron jobs in unix , solaris.
Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times.

Continue reading "Linux/UNIX Crontab Basics"
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