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virtualizationhyper-vwindows-subsystem-for-linux

What is meant by a "lightweight vm" as discussed in the technology stack for WSL2?

My understanding is that Docker on Windows currently uses a "regular VM" under the hood. WSL2 (and Docker) will switch to using a lightweight VM. But what does this actually mean; is it just using a smaller initial memory foot print with some memory passthrough technnique, or is there more to it?

Solution

  • TL;DR

    The big change is in the move from a virtualized Linux system call interpreter for the Windows kernel in WSL to a full-on Linux kernel provided in WSL2. This move dramatically cuts down on virtualization overhead.

    Juicy Details

    Directly from the DevBlogs Post on the announcement of WSL2:

    Microsoft will be shipping a Linux kernel with Windows ... This kernel has been specially tuned for WSL 2. It has been optimized for size and performance to give an amazing Linux experience on Windows.

    This is a departure from the ways of the current (as of writing) WSL which doesn't make use of a proper Linux kernel, demonstrated in the original WSL overview from 2016.

    WSL executes unmodified Linux ELF64 binaries by virtualizing a Linux kernel interface on top of the Windows NT kernel.

    The WSL LXCore service runs an interpreter of sorts for native Linux system calls as well as running its own VolFs and DriveFs operations to provide file access between WSL and Windows 10, which essentially performs the role of a traditional VM's translation layer the likes of VirtualBox.

    WSL components chart

    Citation: MSDN Blog

    Little is known as of yet about the exact system employed for WSL2, what we do know is from the Build2019 WSL2 talk. To help answer the question regarding file system changes and the light VM:

    Timestamp: 29:58

    Here, we see that the Linux kernel runs alongside the NT Kernel instead of as a virtualized environment on top of it. (as a Windows service). The lightweight VM likely comes into play for facilitating the necessary interactions between the two kernels.

    Timestamp: 41:11

    This gives a peek into the inner workings of that interoperability layer. Discussed verbally in the Build2019 talk, the two kernels serve each other files via natively hosted file servers (inaccessible to the Windows userspace via means other than WSL2).

    Again, much is still up in the air from our perspective as users due to the limited details currently available to us at the time of writing.