[Vm-dev] faster snapshot support in VM?

[David T. Lewis]

On Fri, Jul 17, 2009 at 01:36:45AM +0200, Nicolas Cellier wrote:
>  
> In recent pharo thread
> [Pharo-project] Save and quit vs. Save - a clue to performance???
> it is said that saveAndQuit is amazingly fast, while save is damned slow...
> ... because it goes through all image startup.
> Cloning the vm/image could be more efficient:
> 
> primFork
>   "return true if we are in the forked image, false in the original"
> 
>   <primitive: XXX>
>    self error: 'fork failed'
> 
> snapshot
>   self primFork ifTrue: [
>       self aboutToSnapshot.
>       self primSaveAndQuit.
>       self returnFromSnapshot].
> 

The primFork that you describe has been part of OSProcess for a long
time. The #primitiveForkSqueakprimitiveForkSqueak primitive is in the
unix flavor of OSProcessPlugin, so it's probably already in whatever
unix VM you are running today.  For what you describe, you would probably
want to invoke it with UnixProcess>>forkHeadlessSqueakAndDoThenQuit:

So you can just try it and see if it gives the performance you want.

> Apart being dependent on another feature of the OS, do you know any
> reason why this would not work?

It definitely does work, but it is very OS dependent, and that's an
extremely good reason for not doing it as a general solution.

I don't understand why save should be "amazingly slow".  What's going
on in the image startup that would cause this be become an issue? It
sounds to me like somebody is not checking the resuming flag in a
#startup: method, hence running a bunch of initialization code that
is not required after an image save.

Dave