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Impact of TCP offload and ‘Received Side Scaling’ on traffic handling

March 9th, 2010 Leave a comment Go to comments
 

While doing a performance test on one of our customer environments we observed the impact of TCP offload and “Receive Side Scaling” (RSS) settings on the interface card on Windows web servers in combination with traffic handling.

Setup:

1. 2x Mercury Load Runner generators hitting public URL of customer

2. Served by 3x Windows2003 SP2 servers, running IIS6

3. Load being balanced by Cisco CSS11503 to web farm.

 

The CPU performance graph of the web servers with TCP offload and RSS enabled on the internet facing (FRONT) interface:image1-with-tcp-offload-enabled

 

Similarly but a more outdated graph even more clearly showing that traffic is alternating from one web server to another:

image1-1-with-TCP-offloading-enabled

 

Most interesting right!?

What makes this traffic to alternate if the load balancer has been set up to distribute the load evenly across the farm resp each Load Runner vuser to clear its cookies and session cache after each request?

We then stumbled over this read, knowing that TCP offload to network card is a classic one , but still:
http://blogs.msdn.com/psssql/archive/2010/02/21/tcp-offloading-again.aspx

And found out the characteristic that when TCP offload and RSS were disabled, the load is more evenly spread across the web farm:

 image2-with-tcp-offload-disabled

I find this pretty cool.

Any comments?

 

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