Some changes were made to a web application to reduce the amount of bandwidth used. To determine the impact of the changes, a previous benchmark test was compared with a new test. Both tests measured Bytes Received/sec on IIS, Bytes Sent/sec on IIS and Throughput (in Bytes/sec) from the perspective of the load testing tool.

Comparing the tests showed that Bytes Received/sec had been reduced by 1/3 but the Total Throughput had been reduced by 2/3 (Bytes Received/sec was a trivially small value). Where were the missing bytes?

Throughput: Displays the amount of throughput (in bytes) on the Web server during the load test. Throughput represents the amount of data that the Vusers received from the server at any given second. This graph helps you to evaluate the amount of load Vusers generate, in terms of server throughput. (taken from LoadRunner Analysis graph description)
Sent Bytes/sec: Bytes Sent/sec is the rate that data bytes are sent by the Web service. (taken from Perfmon explaination text)

In theory, the values from these two counters should match. But there is one very big gotcha. The Web Service counters that Perfmon uses (which are the same as the Web Server Resources > MS IIS counters in LoadRunner) measure values before IIS hands off its data to any filters.

Perfmon dialog - add counters. Web Service

The web server uses a GZip compression filter. One counter was measuring the data when it was compressed. One was not.

Next time I have to do this, it might be a good idea to also measure the throughput at the NIC level, even though it will include bits from the ethernet packets as well as the higher level encapsulating protocols such as TCP and IP. In Perfmon, this is Network Interface > Bytes Sent/sec. Note that throughput is harder to measure at the TCP or IP level as Perfmon does not give statistics in Bytes, only segments and datagrams respectively.


Published On: February 7, 2004Tags: ,

One Comment

  1. onsoft Technologies April 15, 2009 at 6:57 pm


    Onsoft Technologies is conducting a challenging competition to assist the global recession especially for US and UK based testers.

    We are exploring options to conduct the competition for BEST TEST PLAN .can you please provide your inputs on the same

    here is the rough idea we have brought
    1.participants should be any where from the world
    2.One Director, two post graduate candidates in software testing with more than 1o+years and at least 5 years in writing test plans
    3.The test plan is for SYSTEM TESTING of Decided Webapplication(this will be finalized later)
    4.each participant should pay $20 for registration
    5.Registration will be allowed only for one month.
    6.In June first week the results will be announced

    If you or your friends who want to be the director or reviewer please contact us

    if you want to participate , please write to us along with your profile

    Soon we are launching a website for this then we will update the official information,till then please do not make any payments to anybody

    Let me know your comments and views on this

    ONSOFT Technologies

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