03 April 2013

Advanced IIS compression configuration

Enhancing the standard static and dynamic content compression can improve your site's performance.  The standard "out of the box" configuration works great for most situations but every site is slightly different and as such there is always room for fine tuning.

The Basics

Install the compression components
The enable compression on your IIS site you need to have the IIS performance features installed.  To add these open the Server Manager then:

  • Add roles and Features 
  • Select Role Based 
  • Server Roles 
  • Web Server IIS 
  • Web Server
  • Performance 
  • Static content Compression and Dynamic Content Compression


Configure the default settings
There are a few global settings you can change quite easily.

  • Open the IIS management Console
  • Select The Server level
  • Open Compression


Here you have a few options that pertain to whether to enable static and or dynamic content compression.

You can also specify a minimum file size to compress / compressed cache location and a size limit for the amount of content to keep.

Compressing tiny files often does not yield any benefit so just ignoring those is recommended.
The amount of compressed content to keep is 100MB by default.  If your application generates loads of data you would probably want to increase this.

Advanced Configuration
If you are going to be making changes to these setting you need to know what is happening.  I strongly recommend using HTTP Watch to monitor your progress as I found it the best tool when tweaking these settings.

If you want to change additional setting you will have to open the IIS management console.  Select the server level, open Configuration editor.  From the Section drop down select System.Webserver/httpCompression.


There are 17 setting here that can be changed.  Most of them typically would not need to be changed at all.  The fields I am normally interested in are


  • dynamicTypes
  • staticTypes


dynamicTypes and staticTypes refer to the MIME types that will and won't be compressed.  Something to keep in mind is that IIS will only serve MIME types that are approved.

Adding an additional MIME type for compression
As an example we will add bmp files as a content type to compress


  • Open the staticTypes field
  • Click Add
  • enabled True
  • Mime type image/bmp
  • Close the windows
  • Open the dynamicTypes field
  • Click Add
  • enabled True
  • Mime type image/bmp
  • From the Actions Pane click Apply
  • Restart IIS with iisrest


If we look at a .bmp download now we can see that the content is being compressed

In this example you can see that we saved 93.8 % of the required bandwidth.

Choosing the MIME type are not always straight forward.  Some files are already compressed an example of this is .jpg images. Here you can see how compressing the .jpg actually makes it bigger!

If you would like to compress all other images and exclude .jpg add the following to the dynamicTypes and staticTypes field


  • Enabled = False | MIME Type = image/jpeg
  • Enabled = True | MIME Type = image/*



CPU concerns
All the compressing is great for saving bandwidth but it adds to CPU load.  In my experience the additional load negligible given the massively powerful CPUs available nowadays, You can however set thresholds to guard against CPU congestion due to compression overhead


  • dynamicCompressionDisableCpuUsage
  • dynamicCompressionEnableCpuUsage


  • staticCompressionDisableCpuUsage
  • statricCompressionEnableCpuUsage


These set the upper threshold to disable compression and the resume threshold.  Literally the other side of the story is the client device,  the client would incur a similar decompression overhead.  Since the client is only concearned with it one conversation it is not normally an issue, but it is a good thing to keen in mind.

Conclusion
Compressing HTTP content can be a huge bandwidth saver, if you get to know your web application properly you can tweak out some additional performance gains.  Be cautious though, because being over optimistic can actually take you backwards.

If you would like to know more about the other setting available in the configuration editor check out http://www.iis.net/configreference/system.webserver/httpcompression

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