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Rick Heiges

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PLE - Discussion in the new 64-bit World

On 32-bit systems, generally we did not sound alarm bells if PLE (Page Life Expectancy) was above 300 seconds (5 minutes).  As I have been working more and more with 64-bit systems, I catch myself wondering if there is a different "benchmark" for PLE on 64-bit systems.  Recently, a colleague and I had a discussion.  Although we did not come to any conclusions, I thought that I would bring forward some of the discussion points for your enjoyment.

Addresses:  64-bit systems take half as many addresses to reference the same "real estate" in memory.  So this means it flsuhses twice as many "pages" as compared to 32-bit systems for the same number of addresses.  Does this mean that the if PLE is above 150 on a 64-bit system that we do not need to sound the alarm?

Percentage of VAS:  32-bit systems have a 4GB Virtual Address Space (VAS).  64-bit systems do not have this limitation and may natively address much more memory. If a 32-bit system has a PLE value of 300 with 4GB, that would be less pressure in my view than the same 300 PLE value in a 64-bit system with 64 GB.  This is because when a page gets in memory, I can expect it to be in there for 5 minutes.  BUT many more pages are being "churned" with a PLE of 300 in the 64-bit system than the 32-bit system.  Since the 64-bit system has 16x more memory (in the case above), do I sound the alarm when PLE goes below 4800?

Other factors - I am sure there are many other factors to consider in figuring this out.  In my brief search of the web, I found little on this idea that the PLE value may have a different threshold than 300 in the new 64-bit world.

The SAME:  It could still be the same generally accepted value of 300, but you have to wonder.

My thought is that the truth is based on sevral of these factors.  I just have not had the time to fully test them out, but I hope this has been interesting.  The 300 PLE value that we have known for a long time is probably based on a 4GB VAS number.  The 64-bit world is not limited to that, and the PLE threshold may vary based upon amount of memory or even if AWE is turned on.  AWE on 64-bit? yes - there is a case for that too althogh we won't discuss it here and now. 

If someone reading this on has additional information, I'd love to have it posted as a comment for discussion.

P.S. Be sure to check out for quick "10 Minute Tidbits" about SQL Server topics.

Published Sunday, August 27, 2006 7:26 AM by RickHeiges

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Dis4ea said:

I wouldn't trust the assumption that it automatically needs to be higher now, although it sounds very logical.  But with 64bit the memory above 4GB can be used for other purposes too like for example plan cache.  My personal experience is at least monitor the memory utilisation on 64 bit when you first start using your applications on it; in our environment it acts VERY different from the 32 bit server.  I'll make sure to follow up your posts about this ;-)

I'm curious on your point about AWE on 64-bit.  As far as I know 64 bit doesn't really care about the AWE setting.  There is the 'Lock Pages In Memory' policy that is actually the 'AWE behavior' that is recommended.
September 6, 2006 11:46 PM

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