on my MS SQL SERVER 2008R2 run daily OLAP processing tasks in SSAS. During the processing the SSAS takes lots of CPU and RAM. As a result, users of Reporting Services ( which mostly access relational database based Reports experience a very poor performance while opening the reports.
is there an option to limit the SSAS resources? or priorize the reporting server higher that SSAS, so the report generation will not be affected by SSAS processing?
You can limit the memory and the number of threads that Analysis Services uses by changing configuration settings. The easiest way to do that is by right-clicking on the server node in Object Explorer in management Studio, and selecting "Properties".
The properties are documented at http://msdn.microsoft.com/en-us/library/ms174556(v=sql.105).aspx. For your purposes, memory and thread pool properties are probably where you want to change settings, depending what is your bottleneck.
You can reduce the parallelism of processing as well in the processing request changing server settings, using (or not using) the Parallel element, possibly including the MaxParallel attribute. To see the XMLA for the different settings, you can just right click on the object that you want to process in Management Studio and select "Process". Then click "Change Settings", change the settings as required, and click OK. Do not click on OK in the main dialog, but on "Script" in the top toolbar, and have a look at the generated XMLA.