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We will discuss methods used to manage memory in SQL Server Analysis Services ... SQL CAT Tech Notes. Running Microsoft SQL Server 2008 Analysis Services on Windows Server ...

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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
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
  • 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
  • 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 continues24
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 handed26
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


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
  • 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 pitfall35
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
  • Books Online
  • Perfmon counters
  • Another option!