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Enormous Page Life Expectancy (PLE)?


Enormous Page Life Expectancy (PLE)?

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mmcardle
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I've been running perfmon and analysing the results on a production Windows 2008 R2 server running SQL Server 2008 R2 and I have a consistently high PLE. And when I say high, I mean gigantic.

The server has 24GB of RAM, so a comfortable value is apparently 24 / 4 * 300 = 1800 seconds or 30 minutes

Over the last 12 hours (7pm last night to 2pm today), perfmon has reported this:
Average 3,643,293
Median 3,637,530
Min 3,633,329
Max 3,672,127
Std Deviation 11,952

Just ran this (at 3pm):

[SELECT [object_name], [counter_name], [cntr_value]
FROM sys.dm_os_performance_counters
WHERE [object_name] LIKE '%Manager%'
AND [counter_name] = 'Page life expectancy'[code language="sql"]


And I get this:


Which is (roughly) 3.6 million seconds or 60,000 minutes or 1,000 hours or nearly 30 days!

A month?????

Is this too high? Is there such a thing as PLE being too high?




mmcardle
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Max server memory is set in SQL Server to 16GB.

Available MBytes over the same period was:

Gulfstream Portal Tool
Average 4,559
Median 4,567
Min 4,061
Max 4,628
Std Deviation 49




GilaMonster
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Why is that a problem?


And no, 'a comfortable value is apparently 24 / 4 * 300 = 1800 seconds or 30 minutes', is not a comfortable level, that's a way of calculating the point below which suggests memory pressure, or at least too much churn.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass


mmcardle
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GilaMonster - Thursday, August 17, 2017 10:05 AM
Why is that a problem?


And no, 'a comfortable value is apparently 24 / 4 * 300 = 1800 seconds or 30 minutes', is not a comfortable level, that's a way of calculating the point below which suggests memory pressure, or at least too much churn.

Well, I didn't think so, but when I Googled on "page expectancy level high" (and they said the same as you Gail), the biggest "high" example I found was 70,000 not 3,600,600!

Does this mean the infrastructure team will be saying I should drop the amount of RAM allocated to this server in VMware as it's not being used?




GilaMonster
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mmcardle - Thursday, August 17, 2017 11:10 AM
Does this mean the infrastructure team will be saying I should drop the amount of RAM allocated to this server in VMware as it's not being used?

Tell the Infrastructure team to go to hell. And it's not unused memory, not sure what implied that.


Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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Chris Harshman
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Page Life Expectancy doesn't tell you how much memory is being used, just a rough idea of how long a page in memory stays in memory. To see h ow much memory is being used try queries like these:

-- system level info 2008
SELECT cpu_count/hyperthread_ratio AS physical_cpu_count,
physical_memory_in_bytes / 1048576 AS physical_memory_meg,
bpool_committed / 128 AS bpool_commited_meg,
bpool_commit_target / 128 AS bpool_commit_target_meg
FROM sys.dm_os_sys_info

-- SQL Server process memory
select physical_memory_in_use_kb / 1024 AS physical_memory_in_use_meg,
locked_page_allocations_kb / 1024 as locked_in_memory_meg,
page_fault_count AS sql_page_fault_count, memory_utilization_percentage, process_physical_memory_low, process_virtual_memory_low
from sys.dm_os_process_memory



https://docs.microsoft.com/en-us/sql/relational-databases/system-dynamic-management-views/sys-dm-os-sys-info-transact-sql
https://docs.microsoft.com/en-us/sql/relational-databases/system-dynamic-management-views/sys-dm-os-process-memory-transact-sql


Chris Harshman
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You probably want to check things like memory usage and PLE over time, instead of making knee-jerk reactions to what you see on one particular day. It may be that this particular system has more activity on a specific day of the week or month, so reducing VM memory now would leave the more intense processes short on resources. A PLE that high typically means that it's been high for quite a while, the PLE starts at 0 when you first start the server, and will gradually increase until activity causes pages in memory to be removed so other pages can be read in (in a least recently used way) which will lower the PLE.
mmcardle
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GilaMonster - Thursday, August 17, 2017 11:45 AM
mmcardle - Thursday, August 17, 2017 11:10 AM
Does this mean the infrastructure team will be saying I should drop the amount of RAM allocated to this server in VMware as it's not being used?

Tell the Infrastructure team to go to hell.

Haha...I will.

Cheers Gail




mmcardle
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Thank you for all the replies on this post. All taken on board and will be acted upon.

Thanks again.



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