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 sqlblog.com has additional information, I'd love to have it posted as a comment for discussion.

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