Hacker News Reader

Comment by sillysaurus3 | Hacker News Reader

This is going to sound amusing at best, but would you clarify what it means to boot?

To justify the question a bit: booting traditionally meant physically turning a system on. The boot time included BIOS initialization, a concept now blurred by the advent of virtualization.

150ms is such an absurdly short amount of time that I'm left wondering what booting is in this context.

View on HN

VMs usually do have BIOS (sometimes you can see it flicker on the screen) but like NeutronBoy said, the hypervisor just creates the virtual hardware devices in a pre-initialized state so the BIOS has to do almost no work and it completes in a fraction of a second. Clear Containers boots even faster by not using BIOS; the hypervisor directly loads the kernel and initrd into RAM. So in this case "booting" means starting the kernel, mounting the root filesystem (accelerated using DAX), running init, starting dockerd, etc.

Actually Clear Containers does have a BIOS, because Linux requires one eg to read E820 data and to set up the virtual video. However the one Clear Containers uses (from kvmtool) is extremely minimal -- it's literally enough of a BIOS just to answer the int calls that modern Linux makes at boot and that's all. IIRC it's hundreds of lines of code only.

> 150ms is such an absurdly short amount of time that I'm left wondering what booting is in this context.

Clear Linux was announced about a year ago, and it does boot absurdly quickly

https://clearlinux.org

https://lwn.net/Articles/644675/

It does this thanks to a technology called DAX and the fact that systemd boots really fast.

https://www.kernel.org/doc/Documentation/filesystems/dax.txt

DAX is a small part of it, but Intel made many changes throughout the stack, mostly to the Linux kernel.

> This is going to sound amusing at best, but would you mind defining what it means to boot?

I imagine that most of the time would be in mocking some/all of the hardware interfaces to present to the VM, and running your init processes (and all that entails for whatever OS you're running).

For perspective, I just timed (not very well) how long it takes windows to run the c-program "exit": "int main() { return 0; }", compiled with gcc 4.8.1, -O3 -std=c11 -Wall, stripped[1]. From a warm disc cache it takes ~3ms. From cold(er) it takes ~19s.

Taking a 50x hit to run "exit" from a container doesn't sound bad, but it doesn't sound all that far fetched either.

[1] time util from pstools, as installed by scoop.sh - similar to why gcc (not eg msvc - it's all in my path atm, no work needed :)

~19ms, obviously, not 19 seconds.

Reply on news.ycombinator.com