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Memory Management in SQL Server Analysis Services. Steve Wright Director of Product Support SQL Sentry, Inc. About the Presenter. Steve Wright – Director of Product Support for SQL Sentry Inc. Headquartered in Huntersville, NC

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Memory management in sql server analysis services

Memory Management in SQL Server Analysis Services

Steve Wright

Director of Product Support

SQL Sentry, Inc.


About the presenter
About the Presenter

  • Steve Wright – Director of Product Support for SQL Sentry Inc.

  • Headquartered in Huntersville, NC

  • SQL Sentry provides tools to monitor your entire Microsoft BI Platform

  • WWW.SQLSENTRY.NET


About this presentation
About this presentation…

  • We will discuss methods used to manage memory in SQL Server Analysis Services

  • Expected behavior when applying these methods.

  • How to monitor that behavior

  • Examples of Issues when these methods are mis-applied.


Ssas memory categories
SSAS Memory Categories

  • Shrinkable

    • Easily reduced and returned to the OS

    • SE and FE caches

  • Nonshrinkable

    • Used for more essential system-related activites

    • Memory Allocators

    • Metadata Objects

    • Not easily reduced.


Measuring ssas memory
Measuring SSAS Memory

  • MSOLAP$SQL:Memory Cleaner Memory KB

  • MSOLAP$SQL:Memory >Cleaner Memory (non)shrinkable KB


Ssas memory limits
SSAS Memory Limits

  • Used to determine how to allocate and manage internal memory

  • Configurable with SSAS properties with SSMS or edit msmdsrv.ini

  • Memory\LowMemoryLimit

  • Memory\TotalMemoryLimit

  • Refers to physical memory on the server


Memory lowmemorylimit
Memory\LowMemoryLimit

  • Default Value 65 in SSAS2008, 75 in SSAS2005

  • Percentage if between 0 and 100.

  • Bytes if greater than 100.

  • Refers to total physical memory on machine


Memory totalmemorylimit
Memory\TotalMemoryLimit

  • Default Value 80

  • Percentage if between 0 and 100.

  • Bytes if greater than 100.

  • Total amount of memory the SSAS process(msmdsrv.exe) can consume.


Total memory limit cont
Total Memory Limit cont.

  • If set to 0, SSAS will use no memory for caching

  • Total available to SSAS depends on platform

    • 32-bit limited to 3GB with /Gb switch enabled

    • 64-bit provides all memory available


Behavior low limit reached
Behavior – Low Limit Reached

  • Cleaner threads start moving data out of memory (non-aggresively)

  • Perfmon – MSOLAP$SQL:Memory > Cleaner Memory Shrunk KB/sec


Behavior total limit reached
Behavior – Total Limit Reached

  • Cleaner goes into crisis mode

  • More aggressive cleanup

  • More threads spawned

  • Dramatic performance impact


Behavior between low and total
Behavior Between Low and Total

  • Economic memory management

  • Memory Price per KB

  • Depends on amount available based on limits

  • Memory is free and the livin’ is easy under the Low Limit!

  • Memory price begins to increase as total memory increases between Low and Total.


Ssas memory management
SSAS Memory Management

  • Self governing – no consideration for low physical memory conditions

  • Windows File Cache

    • Unlike SQL Server, SSAS DB’s are collection of files on files system and use file system cache

    • SSAS data may be loaded in RAM as file cache

    • Not part of SSAS process memory or limits


Monitoring perfmon
Monitoring - Perfmon

  • Memory Usage KB

  • Memory Limit Low(High) KB

  • Cleaner Memory KB

  • Cleaner Memory (Non)Shrinkable KB

  • Cleaner Memory Shrunk KB

  • Do NOT rely on Task Manager




Graphical anomaly background
Graphical Anomaly - Background

  • SQL Sentry Customer – Financial Services

  • Beta tester of SQL Sentry Performance Advisor for Analysis Services

  • Eager to test as they were suffering from SSAS performance issues

  • Scheduled Microsoft to come on-site to help








The investigation continues2
The Investigation Continues

  • Almost all time spent on non-cached Storage Engine operations (SE non-cached)

  • Every time query needs data it’s not found in the FE Cache

  • FE requests from SE

  • SE unable to find in its cache

  • Must go to file system



Caught red handed1
Caught Red Handed!

  • Total Queries shows how many times storage engine went to the file system to read data from a partition or aggregation.

  • Want this to be as close to 0 as possible.

  • Exceptions are when Agg is first hit and loaded into cache or cache is cold.




Scenario 2

Scenario #2

Preallocation


Memory preallocation
Memory Preallocation

  • Introduced with SQL 2005 SP2 due to limitations with memory allocations in Windows Server 2003

  • Edit in msmdsrv.ini

  • <PreAllocate> </PreAllocate>

  • Preallocates % of physical memory on SSAS startup


Sql cat tech notes
SQL CAT Tech Notes

  • Running Microsoft SQL Server 2008 Analysis Services on Windows Server 2008 vs. Windows Server 2003 and Memory Preallocation: Lessons Learned

  • http://sqlcat.com/technicalnotes/archive/2008/07/16/running-microsoft-sql-server-2008-analysis-services-on-windows-server-2008-vs-windows-server-2003-and-memory-preallocation-lessons-learned.aspx

  • Author: Carl Rabeler Contributors: Eric Jacobsen, Thomas Kejser Technical Reviewers: Brad Daniels, Lindsey Allen, John Desch, Wayne Robertson, Kevin Cox


Sql cat report summary
SQL CAT Report Summary

  • SSAS 2008 runs equally well on Windows 2008 with or without preallocation

  • SSAS 2008 runs substantially better on Windows 2003 WITH preallocation

  • Essentially equal to Windows 2008


Points to consider
Points to Consider

  • Be sure to set low enough so sufficient memory remains for other processes and avoid paging.

  • Be sure to set high enough for it to help SSAS

  • Use peak value for memory counters as guideline

  • Also applies to SSAS 2005 (no change in code)


Preallocate pitfall
PreAllocate Pitfall?

  • <PreAllocate> 80 </PreAllocate>

  • <TotalMemoryLimit> 80 </TotalMemoryLimit>

  • <LowMemoryLimit> 65 </LowMemoryLimit>


Preallocate pitfall1
PreAllocate Pitfall?

  • PreAllocate does not “override” memory limits

  • Memory cleaner acts as if all preallocated memory is in use.

  • In this case PreAllocate = TotalMemoryLimit

  • Result – cleaner goes into crisis mode

  • Performance suffers drastically


Resources
Resources

  • http://sqlcat.com/tags/Analysis+Services/default.aspx

  • Books Online

  • http://cwebbbi.spaces.live.com/default.aspx

  • WWW.SQLSENTRY.NET



Monitoring
Monitoring

  • Perfmon counters

  • Another option!


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