VMworld 2014/Extreme Performance Series: Understanding Virtualized Memory Management Performance

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Extreme Performance Series: Understanding Virtualized Memory Management Performance (INF1420)

Motivations

configured memory size:

  • too small impacts performance
  • too large impacts overhead

number of VMs per host:

  • too many impacts performance
  • too few wastes host memory

memory reclamation method:

  • properly implemented, will minimally impact performance

Layers

Memory Layers

  • App (ex. Java)
  • VM (Guest OS)
  • Host (ESX)

Each layer does memory management differently

Terminology

  • Memory Size
    • Free Memory
    • Allocated Memory
      • Active Memory
      • Idle Memory

Memory Undercommit

  • Sum of all VM memory is less than host memory
  • No reclamation of guest memory, even if VM has free/idle memory

Memory Overcommit

  • Sum of all VM memory is more than host memory
  • ESX may map only a subset of VM memory

Active, Idle, Cache, Free, Ballooned

entitlement

Memory Entitlement and Reclamation

  • compute memory entitlement for each vm, then reclaim guest memory if entitlement is less than consumed
  • computed on Reservation, Limit, Shares, and Memory demand
  • esx can only classify memory broadly as active and idle
  • sampling baed demand estimation is reasonably accurate

Parameters:

  • configured memory size
  • memory reservation
  • memory limit
  • memory shares
  • idle memory ("idle memory tax")

Memory Reclamation Techniques

Memory Reclamation Techniques

  • Transparent page sharing - de-dup identical memory pages (low cpu cost technique, uses content hasing)
  • Ballooning
  • Compression
  • Swapping

Performance Implications

Designed to handle burst memory pressure well

Constant memory pressure should be handled by DRS/vMotion etc.

Monitoring Tools

VM Sizing

  • guard against "active memory" reclamation
  • sized larger than highest demand during peak loads

Page Sharing & Large Page

  • Savings can be significant for homogeneous VMs (like VDI)
  • "Preventers":
    • ASLR (Address space Layout Randomizaiton) - for security
    • Super fetching (proactive caching)
    • Host large pages (ESXi does not share large pages)
      • Due to fewer TLB misses and faster page table look up time

Best Practices

  • Install VMware Tools and enable ballooning - better performance than host swapping
  • Provide sufficient swap space in guest
  • Place guest swap file/partition on separate vdisk
  • Don't disable memory compression
  • Host cache is "nice to have"

Optimize Host Swapping

  • Sharing before swap
  • Compressing before swap
  • Swap to host cache (SSD)
    • Target 20% for SSD
    • Too big host cache is potentially a waste

Transient memory Pressure

Transient memory pressure is not a problem

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