"HowTo" Posts


Understanding Linux CPU stats

By Derek Bullet_white Posted in HowTo Bullet_white Comments Comments

Your Linux server is running slow, so you follow standard procedure and run top. You see the CPU metrics:



But what do all of those 2-letter abbreviations mean?

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Restricting process CPU usage using nice, cpulimit, and cgroups

By Derek Bullet_white Posted in HowTo Bullet_white Comments Comments

juggle

The Linux kernel is an incredible circus performer, carefully juggling many processes and their resource needs to keep your server humming along. The kernel is also all about equity: when there is competition for resources, the kernel tries to distribute those resources fairly.

However, what if you've got an important process that needs priority? What about a low-priority process? Or what about limiting resources for a group of a processes?

The kernel can't determine what CPU processes are important without your help.

Most processes are started at the same priority level and the Linux kernel schedules time for each task evenly on the processor. Have a CPU intensive process that can be run at a lower priority? Then you need to tell the scheduler about it!

There are at least three ways in which you can control how much CPU time a process gets:

  • Use the nice command to manually lower the task's priority.
  • Use the cpulimit command to repeatedly pause the process so that it doesn’t exceed a certain limit.
  • Use Linux’s built-in control groups, a mechanism which tells the scheduler to limit the amount of resources available to the process.

Let's look at how these work and the pros and cons of each.

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Understanding disk inodes

By Derek Bullet_white Posted in HowTo

You try creating a file on a server and see this error message:

No space left on device

...but you've got plenty of space:

df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/xvda1            10321208   3159012   6637908  33% /

Who is the invisible monster chewing up all of your space?

Why, the inode monster of course!

What are inodes?

An index node (or inode) contains metadata information (file size, file type, etc.) for a file system object (like a file or a directory). There is one inode per file system object.

An inode doesn't store the file contents or the name: it simply points to a specific file or directory.

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Slow Server? This is the Flow Chart You're Looking For

By Andre Bullet_white Posted in HowTo Bullet_white Comments Comments

Your high-powered server is suddenly running dog slow, and you need to remember the troubleshooting steps again. Bookmark this page for a ready reminder the next time you need to diagnose a slow server.

Get on "top" of it

Linux's top command provides a wealth of troubleshooting information, but you have to know what you're looking for. Reference this diagram as you go through the steps below:

top output

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Scout is Hiring! Come Build the Future of Monitoring as Developer #2

By Andre Bullet_white Posted in HowTo Bullet_white Comments Comments

We take pride in building a server monitoring product our customers love with a lean, flat team. We're looking to add the fourth human to our close-knit group.

So, what's special about being a Ruby dev @ Scout?

First, great people! Second, great tech: come build beautiful realtime monitoring visualizations in d3 and Ruby. There won't be an LDAP integration in sight, we promise. Third, you'll have a tremendous impact as developer #2.

About you

Please be comfortable with Ruby and a framework like Rails or Sinatra. You should have some Javascript chops. Experience with high-throughput, data-heavy apps will give you a leg up.

Beyond the technical chops, the single most important thing is your initiative. Will you dive into a problem unprompted? Point out problems and give suggestions on fixing it? Given a high-level goal, can you break it actionable chunks, ask for help when you need it, and see everything through to completion? We're a flat organization, and we won't micro-manage your work.

Benefits

Competitive salary, health care reimbursement, and unlimited vacation time.

Location

We'll consider great remote candidates, but we'd love for you to join us in Fort Collins, Colorado.

A few things about Fort Collins: best place to live (Money Magazine), ranked 3rd on the Best Bicycle Cities list, one of the Ten Best Vacation Cities for Beer Lovers, and 300 days of sunshine! Our office is located minutes from Old Town, the heart of Fort Collins.

How to apply

Email us at punch.the.clock@scoutapp.com. Resumes are fine, but a more personal email is better.

 

Understanding CPU Steal Time - when should you be worried?

By Derek Bullet_white Posted in HowTo Bullet_white Comments Comments

A big thanks to Eric Lindvall of Papertrail for adding steal time to Scout's CPU Usage Plugin and helping out on this blog post!

Netflix tracks CPU Steal Time closely. In fact, if steal time exceeds their chosen threshold, they shut down the virtual machine and restart on a different physical server.

If you deploy to a virtualized environment (for example, Amazon EC2), steal time is a metric you'll want to watch. If this number is high, performance can suffer significantly. What is steal time? What causes high steal time? When should you be worried (and what should you do)?

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