Re: [Xen-devel] Current LibXL Status

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Thu, 2016-02-18 at 17:26 +0000, George Dunlap wrote:

> According to them, the ocaml garbage collector never frees memory.ÂÂIt
> grows its own internal heap as necessary, but it never reduces the
> size of its heap.

Assume that's true: Wow!

Although, I presume this is a factor of the current/particular
implementation, rather than a fundamental property of an ocaml runtime, so
in theory it could change (and we have here a good real world argument why
it perhaps should do!)

> So no -ENOMEM call or OOM callback can make a failed malloc succeed.
> 
> One might then ask if libxl could simply allocate memory *from the
> ocaml heap* itself.ÂÂIt turns out that is also not tenable: Data in
> the ocaml heap is stored in a heavily coded format.ÂÂ(For example
> integers are stored as (n*2+1), so that pointers can all be even and
> non-pointers can all be odd.)
> 
> The only thing they said might be improved is:
> 
> 1.ÂÂTo know that libxl would never call exit() for any other reason
> (which it seems is true)
> 
> 2. ToÂÂhave a callback in OOM conditions.ÂÂIt's unlikely the process
> as a whole could do anything but exit, but the ocaml runtime itself
> might still have internal heap available, which would allow it to exit
> more gracefully.
> 
> I think if Haskell or any library *is* capable of integrating with the
> garbage collector, we'll have to wait until someone who understands
> the language actually writes bindings and can ask for something they
> know to be useful for that language.

Agreed.

Hopefully it is possible to make the callback without needing to malloc
anything!

If I were adding an API for #2 to libxl (which must therefore be stable) I
think I might be inclined to allow for the possibility of the callback
returning "please retry" since it would be inconvenient in the usual API
stability ways to retrofit it, I would guess, but really until a language
even exposes that possibility we don't really know if it is worthwhile
doing so.

Ian.

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