Mitosis: high performance, scalable virtual memory

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Mitosis high performance, scalable virtual memory

• Juan Navarro Alan Cox
• Rice University

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Motivation

• Current VM systems are based on two-decade-old
  data structures
• Inappropriate design for todays workloads
• More memory, more and larger processes
• Shared libraries
• Processes use VM (no longer transparent)
• Garbage collection
• Distributed shared memory

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Problem areas

• Studied large applications that stress VM
• Found problems in
• Address space management
• Copy-on-write management
• TLB coverage

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Address space management

• Problem example
• Finding unused address space does not scale
• Hurts performance if many mappings
• Solution
• Augmented tree
• keep track of mappings and available space
• 40 improvement in Flash web server

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Copy-on-write management

• Reference counting issue
• Mach-like systems object chains
• there are at most N references to this page
• leads to swap memory leak and other problems

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Copy-on-write management (cont)

• SVR4-like systems reference arrays
• there are exactly N references to this page
• unnecessary space and time overhead
• Solution versioned trees
• Is this the only reference to this page?

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COW management versioned trees
P2
P1
2
A
B
C
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TLB coverage

• TLB covers a small fraction of memory
• Up to 75 slowdown due to TLB misses
• Solution multiple page sizes
• Transparent support from the OS?
• Requires better study of
• Promotion, demotion and replacement policies
• Multiprocess workloads

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• More info
• www.cs.rice.edu/jnavarro/mitosis
• Acks
• Work supported by a USENIX student research grant