Home > Analysis Services, Configuration, Memory, Windows Server > SSAS <PreAllocate>: What you should know

SSAS <PreAllocate>: What you should know

Preallocating memory for SSAS running on Windows 2003 is a good thing but as with all good things it is essential to know the behavioural changes you will experience, some of which may not be so obvious.

My observations relate to SSAS 2008 SP1 CU8 running on Windows 2003 SP2.

Why PreAllocate?

In my opinion there are 2 reasons which i detail below.

  • The first is the widely stated performance reason surrounding Windows 2003 memory performance. In a nut shell, Windows 2003 did not scale well with many small memory allocations due to fragmentation etc so allocate it up front. Life gets better in Windows 2008 as detailed by the SQLCAT team.
  • The second reason is to ensure SSAS is going to get a slice of the memory and this is very important if your not running on a dedicated SSAS box.

So, what i should i know?

  • When the service starts (don’t forget server startup), if you have assigned "Lock Pages in Memory" to your service account, expect your server to be totally unresponsive for a period of time. Do not panic, the duration of the freeze depends on the amount of memory preallocated but once its done the server becomes responsive again. Make sure the people working with the server know this……
  • Never ever set PreAllocate equal to or greater than <LowMemoryLimit> because if you do the memory cleaner thread is going to spin up and remove data pretty much as soon as it gets into memory. This will seriously hurt performance as your effectively disabling any caching.
  • The shrinkable and nonshrinkable perfmon memory counters are no longer accurate. The counters have “value” when troubleshooting but you must factor in the fact that at least their starting points are wrong.
  • When a full memory dump occurs that dump will be at least the size of the preallocated memory. So, if you preallocate 40gb but SSAS has only written to 2GB of memory its still going to be a 40GB dump so make sure you have the disk space! Hopefully though this is not a situation you should find yourself in very often.

I hope you find this information useful!

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