Memory Management in SQL Server Analysis Services

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Memory Management in SQL Server Analysis Services

• Steve Wright
• Director of Product Support
• SQL Sentry, Inc.

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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

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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.

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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.

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Measuring SSAS Memory

• MSOLAPSQLMemory Cleaner Memory KB
• MSOLAPSQLMemory gtCleaner Memory (non)shrinkable
  KB

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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

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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

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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.

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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

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Behavior Low Limit Reached

• Cleaner threads start moving data out of memory
  (non-aggresively)
• Perfmon MSOLAPSQLMemory gt Cleaner Memory
  Shrunk KB/sec

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Behavior Total Limit Reached

• Cleaner goes into crisis mode
• More aggressive cleanup
• More threads spawned
• Dramatic performance impact

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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.

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SSAS Memory Management

• Self governing no consideration for low
  physical memory conditions
• Windows File Cache
• Unlike SQL Server, SSAS DBs 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

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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

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Monitoring Example
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The Big Picture
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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

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Common Issues Graphical Anomaly?
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Graphical Anomaly The Investigation
Memory Limit Setting Configured Value Perf Counter Value (KB)
Memory\LowMemoryLimit 2097152 8192
Memory\TotalMemoryLimit 13531488 13312
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Graphical Anomaly The Investigation
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Graphical Anomaly A Comparison
Before
After
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The Investigation Continues
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The Investigation Continues
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The Investigation Continues

• Almost all time spent on non-cached Storage
  Engine operations (SE non-cached)
• Every time query needs data its not found in the
  FE Cache
• FE requests from SE
• SE unable to find in its cache
• Must go to file system

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Caught Red Handed!
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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.

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Confirmation
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Could have been worse!
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Scenario 2

• Preallocation

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Memory Preallocation

• Introduced with SQL 2005 SP2 due to limitations
  with memory allocations in Windows Server 2003
• Edit in msmdsrv.ini
• ltPreAllocategt lt/PreAllocategt
• Preallocates of physical memory on SSAS startup

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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/2008/07/1
  6/running-microsoft-sql-server-2008-analysis-servi
  ces-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

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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

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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)

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PreAllocate Pitfall?

• ltPreAllocategt 80 lt/PreAllocategt
• ltTotalMemoryLimitgt 80 lt/TotalMemoryLimitgt
• ltLowMemoryLimitgt 65 lt/LowMemoryLimitgt

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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

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Resources

• http//sqlcat.com/tags/AnalysisServices/default.a
  spx
• Books Online
• http//cwebbbi.spaces.live.com/default.aspx
• WWW.SQLSENTRY.NET

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Thank you!
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Monitoring

• Perfmon counters
• Another option!