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Page life expectancy diving to sub-10 on a 128 GB server


Page life expectancy diving to sub-10 on a 128 GB server

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Kevin Dahl
Kevin Dahl
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Just wondering if either of the two posters experiencing this issue got any further with MS support on it?

I'm having what appears to be a similar issue - PLE crashes randomly during business hours when memory available for SQL Server to use is abundant. I've looked for corresponding workloads to match the times this occurs, but there's generally nothing of note. I've checked and it's not a VMWare ballooning issue, and none of the other metrics from the VMWare console look suspicious. Stats from SQL Sentry show plenty of free memory not allocated to SQL Server available (generally 4-6GB), and regularly show 30-35GB of the memory allocated to SQL Server available as well. The buffer will build up to about 35GB a couple of times a day, but then regularly crashes down as low as 1GB.

We're on SQL 2012 SP1/CU2.

Just trying to get some more information before I determine whether to contact MS, or bring in a more experienced DBA resource. Thanks for any updates you can provide!
TheSQLGuru
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Wasn't SQL 2012 SP1 CU4 supposed to be the build that had the fix?

In my experience you usually have to be able to tell Microsoft that all BIOS/FIRMWARE/DRIVER/VMWARE/WIN/SQL/ETC patches have been applied. That done? firmware/drivers have been most common cause of memory flushes in my past...

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Kevin G. Boles
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sql_handle
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Kevin Dahl - Some relevant details will be whether NUMA pass through is enabled in VMWare. This is enabled by default with more than 8 vCPUs, but for 8 vCPUs and below it is disabled unless configured manually.

Here's why that is relevant: when SQL Server is aware of the NUMA configuration, it attempts to allocate a buffer pool of equal size on each NUMA node, the sum of which is the total buffer pool target size. However, especially in a VM, each NUMA node may not have the same number of physical cores, and may not have the same amount of physical RAM. Sizable amounts of "foreign/away pages" can result. In some cases, especially during buffer pool ramp up, a large amount of memory can end up orphaned as memory allocation requests are first granted with foreign memory and almost immediately afterward with local memory. In these cases, the local memory is used (to optimize memory latency), but the foreign memory is not released... in order to prevent repetitive cycles of allocation and release while the buffer pool is growing. While that may be a reasonable strategy, there are scenarios where the result is a large amount of orphaned, unused memory.

Bob Dorr describes this a bit in this post.
http://blogs.msdn.com/b/psssql/archive/2012/12/13/how-it-works-sql-server-numa-local-foreign-and-away-memory-blocks.aspx

SQL Server 2012 SP1 CU3 contains this fix which addresses some of the scenarios which can lead to orphaned memory and memory stalls.
http://support.microsoft.com/kb/2819662


However, in some cases, using trace flag 8048 (to remove spinlock contention among serialized threads during memory allocation) and trace flag 8015 (to ignore NUMA configuration and manage SQL Server CPU resources and memory resources in single large pools instead of NUMA node-aligned pools) produce benefits far beyond what the SQL Server 2012 SP1 CU3 fix alone provide.

All that said, that type of issue has been around since SQL Server 2005. It seems that the other posters in this thread have found a few similar memory management behaviors that may have been newly introduced in SQL Server 2012.
Tommy Bollhofer
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In our case, this was remediated in CU4 for SQL Server 2012 SP1.

An update is available for SQL Server 2012 Memory Management
http://support.microsoft.com/kb/2845380

CU4 for SQL Server 2012 SP1
http://support.microsoft.com/kb/2833645/en-us

http://www.sqlservercentral.com/Forums/Topic1415833-2799-3.aspx#bm1458905

Tommy

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Don Halloran
Don Halloran
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Yep, the link Tommy has provided is the fix for the issue he and I were having.

We have not yet updated to CU4, as we're still seeing issues with performance on RTM. We did an ERP upgrade recently that took 20 hours to run on the production box, but only 8 on a significantly less powerful UAT box. The workload is identical in each case, basically a bunch of .sql files being run against a database, configurations are identical except that the UAT box is not NUMA, is not clustered, and has far less CPU and RAM. Both instances are 2012 RTM, so this issue is not related to the fix provided in CU4.

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Kevin Dahl
Kevin Dahl
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Thanks for all the updates. Some very useful information!
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