Best Practice No 4:- Improve bandwidth performance of ASP.NET sites using IIS compression

My other .NET best practices article

.NET best practice 1:- In this article we discuss about how we can find high memory consumption areas in .NET. You can read about the same at http://www.c-sharpcorner.com/UploadFile/shivprasadk/452069230108152009163244PM/4520692301.aspx?ArticleID=50bdd822-23d0-4baa-ab0a-21314b94d9e5

.NET best practice 2:- In this article we discuss how we can improve performance using finalize / dispose pattern.
http://www.c-sharpcorner.com/UploadFile/shivprasadk/657567608232009132704PM/6575676.aspx?ArticleID=594e945a-ba48-4fee-9ecd-7b182efd076b

.NET best practice 3:- How can we use performance counters to gather performance data from .NET applications http://www.c-sharpcorner.com/UploadFile/shivprasadk/3253453409022009023755AM/32534534.aspx?ArticleID=9dbdcda3-53e9-46c3-9d09-2b46b9d5f5a2

Thanks, Thanks and Thanks

Every bit of inspiration for this article has come from Scott Forsyth's article on IIS compression. You can say I have just created a new version with more details.

http://weblogs.asp.net/owscott/archive/2009/02/22/iis-7-compression-good-bad-how-much.aspx

Thanks to Jimmie to suggest the performance counters and other details of IIS compression.

I also did picked up some bits from this link of Microsoft http://technet.microsoft.com/hi-in/library/bb742379(en-us).aspx .

How does IIS compression work?

Note :- All examples shown in this article is using IIS 6.0. The only reason we have used IIS 6 is because 7.0 is still not that common.

Before we move ahead and talk about how IIS compression works, let's try to understand how normally IIS will work. Let's say the user requests for a 'Home.html' page which is 100 KB size. IIS serves this request by passing the 100 KB HTML page over the wire to the end user browser.



When compression is enabled on IIS the sequence of events changes as follows:-

User requests for a page from the IIS server. While requesting for page the browser also sends what kind of compression types it supports. Below is a simple request sent to the browser which says its supports 'gzip' and 'deflate'. We had used fiddler (http://www.fiddler2.com/fiddler2/version.asp ) to get the request data.

Browser then decompresses the data and displays the same on the browser.

Compression fundamentals: - Gzip and deflate

IIS supports to kind of compressions Gzip and deflate. Both are more or less same where Gzip is an extension over deflate. Deflate is a compression algorithm which combines LZ77 and Huffman coding. In case you are interested to read more about LZ77 and Huffman you can read at http://www.zlib.net/feldspar.html .



Gzip is based on deflate algorithm with extra headers added to the deflate payload.



Below are the headers details which is added to the deflate payload data. It starts with a 10 byte header which has version number and time stamp followed by optional headers for file name. At the end it has the actual deflate compressed payload and 8 byte check sum to ensure data is not lost in transmission.



Google, Yahoo and Amazon use gzip, so we can safely assume that it's supported by most of the browsers.

Enabling IIS compression

Till now we have done enough of theory to understand IIS compression. Let's get our hands dirty to see how we can actually enable IIS compression.

Step 1:- Enable compression

The first step is to enable compression on IIS. So right click on websites if properties and click on the service tab. To enable compression we need to check the below two text boxes from the service tab of IIS website properties. Below figure shows the location of both the checkboxes.



Step 2:- Enable metabase.xml edit

Metadata for IIS comes from 'Metabase.xml' which is located at "%windir%\system32\inetsrv\". In order that compression works properly we need to make some changes to this XML file. In order to make changes to this XML file we need to direct IIS to gives us edit rights. So right click on your IIS server root if go to properties and check 'enable direct metabase edit' check box as shown in the below figure.



Step 4:- Set the compression level and extension types

Next step is to set the compression levels and extension types. Compression level can be defined between 0 to 10, where 0 specifies a mild compression and 10 specifies the highest level of compression. This value is specified using 'HcDynamicCompressionLevel' property. There are two types of compression algorithms 'deflate' and 'gzip'. This property needs to be specified in both the algorithm as shown in the below figures.





We need to also specify which file types need to be compressed. 'HcScriptFileExtensions' help us to specify the same. For the current scenario we specified that we need to compress ASPX outputs before they are sent to the end browser.

Step 5:- Does it really work?

Once you are done with the above 4 steps, it's time to see if the compression really works. So we will create a simple C# asp.net page which will loop "10000" times and send some kind of output to the browser.

protected void Page_Load(object sender, EventArgs e)
{
for (int i; i < 10000; i++)
{
Response.Write("Sending huge data" + "<br>");
}
In order to see the difference before compression and after compression we will run the fiddler tool as we run our ASP.NET loop page. You can download fiddler from http://www.fiddler2.com/fiddler2/version.asp .

Below screen shows data captured by fiddler without compression and with compression. Without compression data is "80501 bytes" and with compression it comes to "629 bytes". I am sure that's a great performance increase from bandwidth point of view.

0, 1,2,3,4...10 IIS compression levels

In our previous section we have set 'HcDynamicCompressionLevel' to value '4'. More the compression level value, more the data size will be less. As we increase the compression level the downside is more CPU utilization. One of the big challenges is to figure out what is the optimum compression level. This depends on lot of things type of data, load etc. In our further coming section we will try to derive which is the best compression level for different scenarios.

3 point consideration for IIS compression optimization

Many developers just enable IIS compression with below default values. But default values do not hold good for every environment. It depends on many other factors like what content type is your site serving. If you site has only static HTML pages then compression levels will be different as compared to site who are serving mostly dynamic pages.

Static data compression

Let's start with the easiest one static content type like HTML and HTM. If a user requests for static page from IIS who has compression enabled, IIS compresses the file and puts the same in
'%windir%\IIS Temporary Compressed Files' directory.

Below is a simple screen which shows the compressed folder snapshot. Compression happens only for the first time. On subsequent calls for the same compressed data is picked from the compressed files directory.



Below are some sample readings we have taken for HTML files of size range from 100 KB to 2048 KB. We have set the compression level to '0'.

As the compression happens only for the first time, we can happily set the compression level to '10'. The first time we will see a huge CPU utilization but on subsequent calls the CPU usage will be small as compared to the compression gains.

Dynamic data compression

Dynamic data compression is bit different than static compression. Dynamic compression happens every time a page is requested. We need to balance between CPU utilization and compression levels.

In order find the optimized compression level, we did a small experiment as shown below. We took 5 files in a range of 100 KB to 2 MB. We then changed compression levels from 0 to 10 for every file size to check how much was the data was compressed. Below are compressed data readings in Bytes.

Compressed file and compression

Compressed files are file which are already compressed. For example files like JPEG and PDF are already compressed. So we did a small test by taking JPEG compressed files and below are our readings. The compressed files after applying IIS compression did not change much in size.

So the conclusion we can draw for compressed files is that we can disable compression for already compressed file types like JPEG and PDF.

CPU usage, dynamic compression and load testing

One of the important points to remember for dynamic data is to optimize between CPU utilization, compression levels and load on the server.