Cells: A Virtual Mobile Smartphone Architecture (SOSP '11)

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Columbia University, papers, slides, video

1. Problem

The popularity of smartphones results in the need that a user may purchase several smartphones for different uses. Virtual machine (VM) mechanisms may be the solution, which enables separate and isolated instances of a smartphone software stack to run on the same ARM hardware.

2. Challenges

Traditional VMs for desktops and servers can not be applied to smartphones for

  1. limited resources can not fully support running an another entire OS
  2. existing design cannot leverage new devices, such as GPS, camera, GPUs, …

3. Solution

Key observation: one app at a time

  • Cells, a new, lightweight virtualization architecture for enabling multiple virtual phones (VPs) to run simultaneously on the same smartphone hardware with high performance.
    • not entire OS virtualization but multiple isolated virtual phones (VP) on a single OS.
    • a novel file system layout based on unioning which minimizes memory consumption
  • Architecture
    • one foreground VP for displaying and multiple background VPs
    • remap OS resource id to virtual ones and then each VP has its own private virtual namespaces
    • foreground VP has direct access to hardware. If the foreground VP does not acquire exclusive access, the hardware can be shared by background VPs.
      • Kernel-Level Device Virtualization to support transparency and performance by introducing device namespace
      • User-Level Device Virtualization to support portability and transparency by proxy
  • Graphics
    • Linux framebuffer (FB) provides an abstraction to a physical display, so a new FB device driver mux_fb is introduced between virtual addresses and physical addresses, providing an identical device interface to all VPs.
    • The foreground VP uses the GPU directly, but background VPs use GPU render into their respective backing buffers.
  • Power Management
    • early suspend interface: it is blocked by Cells for background VPs
    • fbearlysuspend interface: Cells makes it namespace-aware. Block & unblock.
    • weak locks interface: The same weak lock can be locked and unlocked independently by multiple device namespaces.
  • Phone calls: combination use of VoIP and cellular network
    • VoIP provides telephone numbers for VPs
    • Proxy libraries

Implementation & evaluation

  • Up to 5 VPs on Nexus 1, Nexus S …
  • Modest memory overhead (less than 2%)
  • Imperceptible switch time among VPs

4. Conclusion

  • First OS virtualization fully supports smartphone hardware with native performance
  • One app at a time -> foreground and background usage model
  • No need of modifications for applications