Thanks mariano. This is what I call science in action. You convinced me
back then and Fuel is a success.
On 21/10/14 03:55, Mariano Martinez Peck wrote:
Just a quick note I would like to share....
For my PhD, I did investigate ImageSegment very very deeply:
http://dl.acm.org/citation.cfm?id=2076323
http://www.slideshare.net/MarianoMartinezPeck/2010-smalltalkspeckobject-swapping
I didn't want to write Fuel just because. I took quite a lot of time
to understand how ImageSegment primitives worked. From that effort, I
remember a few conclusions:
1) I found only few users of ImageSegment
2) The few users I found, were NOT using the real purpose of
ImageSegment, that is, object swapping. It was used instead as an
object serializer. For that, they use #writeForExportOn: which ended
up using SmartRefStream for the rest of the objects.
3) I noticed I could achieve the same performance or even better with
an OO serializer built at the language side, with all the benefits
this means. Of course, having Cog helped here....
In the Fuel paper:
http://rmod.lille.inria.fr/archives/papers/Dias12a-SPE-Fuel.pdf
you can find some benchmark comparison agains IS. Also in my PhD:
http://rmod.lille.inria.fr/archives/phd/PhD-2012-Martinez-Peck.pdf
Cheers,
On Mon, Oct 20, 2014 at 9:56 PM, <[email protected]
<mailto:[email protected]>> wrote:
Hi Eliot,
> Hi All,
>
> I want to check my understanding of reference semantics for
image
> segments as I'm close to completing the Spur implementation.
Specifically
> the question is whether objects reachable only through weak pointers
> should
> be included in an image segment or not.
>
> Remember that an image segment is created from the transitive
closure of
> an
> Array of root objects, the *segment roots*. i.e. we can think of
an image
> segment as a set of objects created by tracing the object graph from the
> segment roots.
>
> The segment always includes the segment roots. Except for the roots,
> objects are excluded from the segment that are also reachable
form the
> roots of the system (the *system roots*, effectively the root
environment,
> Smalltalk, and the stack of the current process).
>
> Consider a weak array in the transitive closure that is not
reachable from
> the system roots, and hence should be included in the segment.
Objects
> referenced from that weak array may be in one of three categories
>
> - reachable from the system roots (and hence not to be included
in the
> segment)
> - *not* reachable form the system roots, but reachable from the
segment
> roots via strong pointers (and hence to be included in the segment)
> - *not* reachable form the system roots, *not* reachable from
the segment
> roots via strong pointers
>
> Should this last category be included or excluded from the
segment? I
> think that it makes no difference, and excluding them is only an
> optimization. The argument is as follows. Imagine that
immediately after
> loading the image segment there is a garbage collection. That
garbage
> collection will collect all the objects in the last category as
they are
> only reachable from the weak arrays in the segment. Hence we are
free to
> follow weak references as if they are strong when we create the
image
> segment, leaving it to subsequent events to reclaim those objects.
>
> An analogous argument accounts for objects reachable from
ephemerons. Is
> my reasoning sound?
> --
> best,
> Eliot
>
>
I think you are right. But there is a risk of somehow, someone,
gaining a
strong reference to the object after the image segment was created,
breaking our invariants when the segment is loaded again.
An object might be (not reachable / strongly reachable / weakely
reachable) from system roots and / or segment roots. This gives us 9
possibilities.
Six of them are easy (and I'll not go into them). The other three are
tricky:
a- Not reachable from system roots. Weakely reachable from segment
roots.
Do not include them. It is best to run a GC before building the image
segment, to get rid of them (run termination, etc). This is to
avoid the
risk of the object gaining somehow a strong reference after the
segment is
built, making the segment miss the weak ref to it. Doing this way
would
also mean that any objects affected by termination would be
consistent,
both in the image and in the segment.
b- Weakely reachable from system roots. Weakely reachable from segment
roots.
Do not include them. If the object manages to survive by gaining a
strong
ref from the system roots, the weak ref will be repaired on
segment load
(Am I right on this?) If the original object was included in the
segment,
then on segment load it would point to a duplicate object that is
about to
be collected (and maybe terminated?) In any case, doing this way
would also
mean that any objects affected by termination would be consistent,
both in
the image and in the segment.
c- Weakely reachable from system roots. Strongly reachable from
segment
roots.
Do include them. It seems reasonable to run a GC and get rid of
them after
unloading the segment, to avoid the risk of the object gaining
somehow a
strong ref in the image, and being duplicated on segment load. But
doing as
I say means that we would be loading into the image an object that was
already terminated, although in the state it had before running
termination. Not really sure if this is ok. There could be some
risk of
objects in the segment being in some pre-termination state, with some
objects in the image being in some after-termination state. In any
case,
this would suggest bad design... So perhaps it makes sense to throw an
exception in these cases?
I hope this rant is of use.
Cheers,
Juan Vuletich
--
Mariano
http://marianopeck.wordpress.com
