[vbox-dev] Speeding up VM boot/restore

[Nicolay Vizovitin]

Hello,

my question boils down to the following: what ways are there to speed
up either VM boot process or VM restore from a state snapshot? The
latter case seems more interesting as restore generally happens
faster. Further in the mail I share my thoughts on this matter, some
additional data and reasoning.

We are attempting to implement some kind of isolation environment
based on virtual machine for testing "small" programs. Think of
ACM-type programming competitions as a sample use-case. As a
consequence, one of the most pressing concerns is container
initialization time (either boot, or restore from snapshot).

Of course there are ways to decrease this time without touching VM,
such as OS optimization, or hardware upgrade. But those ways proved
limited. Currently the best time of initialization we archived is
about 2 seconds (for restore) for "repackaged" version of Windows 2003
[tested on: Host OS – Ubuntu 10.04, CPU – Intel Core 2 Duo T5750,
Memory – 3 Gb, Guest memory – 256 Mb]. This result is quite good, but
not good enough. So we need some way to speed up the process using VM.

We already tried to place the snapshot and disk image in RAM
beforehand. Unfortunately speedup was only about 0.5 seconds. Also we
tried to optimize VirtualBox VM settings. No promising results there
yet, but we are still working in this direction.

Also there was an idea how to gain (probably) significant speedup. As
simultaneous launch of several VMs from a single snapshot will be
required, one could implement some kind of copy-on-write mechanism for
VM's memory pages. Let's say there is already a paused VM in memory.
To spawn a new container instance we "clone" that machine and use the
memory pages of the original one, but in a copy-on-write fashion. I.e.
until the first write to the page memory from original VM is used.
Hopefully this reduces average VM memory footprint, as well as new VM
spawn time. We hope that in that case startup time reduction would be
considerable.

So, what do you think, would VM "clone" time be better than restoring
machine from a normal snapshot? If so, what gain could be expected? If
not, why (maybe complex processing takes more time while restoring
snapshot than loading it)? Any advice on where to start to implement
the described feature is greatly appreciated.

Maybe there is a simpler way to gain reduced startup time?

If not, what will you suggest to enable launching multiple VM
instances from a single configuration, provided changes to VM are not
required to be saved after VM is shut down?


--Best regards,
  Nicolay