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Comment by yazaddaruvala | Hacker News Reader

My understanding about using unikernels is that it helps you optimize. Unikernels allow you to trade built in safety and portability for customization and performance.

If there is only one process, the webserver, why deal with a scheduler?

If you're not using the safety features of an OS why have them?

Why context switch if its not needed?

You're hosting a key value store on an SSD. Why deal with a file system abstraction?

Basically, as I understand it, unikernels are trying to democratize application deployment "on the metal" (or currently on xen) without the legacy fundamentals that drove the designs of OSes like Windows or Linux which were built for shared mainframes in university basements.

For more info here is Xen talking about unikernels, http://wiki.xenproject.org/wiki/Unikernels

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Each different Unikernel approach has its benefits and trade-offs. Rumprun lets you 'unikernelize' legacy software using well-established OS components (eg out of NetBSD) -- but you're still exposed to classes of bugs and potential run-time errors. MirageOS gives you safety, compile-time checks and produces really small binaries -- but everything is written in OCaml.

So it's not quite about trading safety and portability, as you can retain those depending on the approach you take (eg, MirageOS currently lets you target Unix, Xen-on-x86 and Xen-on-ARM from exactly the same code-base).

You can also try out static sites in MirageOS [1] and I gave a talk at polyconf last weekend with slides at [2].

[1] http://amirchaudhry.com/from-jekyll-to-unikernel-in-fifty-li...

[2] http://amirchaudhry.com/unikernels-polyconf-2015/

I think another aspect I recall reading somewhere is security: if you can do the job with a far simpler system, potentially you end up with less attack surface (assuming the fancy new simpler system is actually well built and tested, etc).

E.g.

  > Why use Mirage OS?
  >
  > The cloud has so far mostly been used to
  > consolidate existing operating systems and manage
  > them more conveniently. The interface between
  > guest kernels, applications and VMs are all
  > managed very differently, leading to serious
  > inefficiencies, unreliability and insecurity.

http://www.xenproject.org/developers/teams/mirage-os.html

https://mirage.io

Further, you have less to lose if that surface is breached; If you penetrate a unikernel static content server, you can't really do much on that system; It won't have a shell, compilers, or any of the tooling needed to expand that hole. Because your OS is so much simpler, the cleanup is simpler too; Nuking the machine and starting over is much simpler when you've designed for lightweight unikernels.

In the very beginning, there was no OS and applications ran directly from boot. Then people started realising that applications had lots of duplicate code to do the same tasks, and abstracted that into a library linked into every application, which eventually grew into a full OS. It looks like with all the popularity of VMs/hypervisors, which are sort of like OSs themselves, multiplexing the virtual hardware to real, and the growing complexity of OSs, we're now going back to applications without an OS. It reminds me of these:

https://en.wikipedia.org/wiki/PC_booter

One thing that helped to steer into that direction is that developers started to realize that programming languages with rich runtimes, can have those runtimes play the OS role anyway.

For example, I only care about POSIX when not using "comes with batteries/repository manager" languages.

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