What is HTTP Throttling?
A major challenge SharePoint administrators have faced in the past and see again is lack of resources and the odd behaviors it produces.
One scenario is an fully loaded Web Front End server. As a WFE is processing requests, it can reach to a level where it is not immediately responding to a request
due to a lack of resources. It will put this request in a queue , but that capacity for storing requests is also limited . Once the queue fills up, it will start dropping requests
until load decreases. It will not be an issue for a GET request, But what if a user who has just spent an hour or more taking a survey or filling out an application form????
If that PUT request is dropped, time spent in vain and will have no option but to start over.
To avoid this issue, Microsoft has introduced HTTP Throttling to protect a server during peak load. By default, this feature monitors the available memory in
megabytes and the ASP.NET requests in queue. It will monitors these counters, and generates a health score for the server from 0 to 9, where 0 being
the best. By default the monitor checks every five seconds. If the score is 9 for three continuous attempts, then the server will enter a throttled state. In this
state, SharePoint will send a 503 server busy message to all GET requests, including the crawler if you happen to be indexing. All timer jobs will be paused,
which enables the server to concentrate on finishing existing requests and hopefully makes room for anyone doing a PUT request, like that user who just spent an
hour filling out a form. The monitoring continues every five seconds, and throttling is disabled after one occurrence of a score below 9.
This feature can be configured using Central Administration, to be enabled or disabled per web application.
It can be turned on and off through the Central Administration Web Site by selecting a Web Application from the Web Application management page,
Click on the drop-down menu under General Settings on the ribbon, and selecting Resource Throttling. At the bottom of the popup dialog box is a
setting HTTP Request Monitoring and Throttling for managing Request Throttling.
Using Windows PowerShell, you can go a step further and view and edit the thresholds using the following cmdlets:
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Hope this help
Reference taken from one of SharePoint 2010 Admin book.