On 03.06.2011 10:38, Norbert Hartl wrote:

Am 03.06.2011 um 10:20 schrieb Mariano Martinez Peck:

If I look at the serializers I know the mechanics are simple:

1. take an object and look it up in a table.
2. if the object is in the table it has already been serialized. We take a reference from the table and write it down
3. if it isn't in the table it means the object has not been serialized and the contents need to be written. After that the object with a reference representation is written to the table
4. while writing the content of an object (inst vars) I encounter simple objects (direct string representation) or complex objects (composite). In the latter case we start over from 1.

If the object graph is well interconnected than the odds are high that a stack will grow high until the first reference can be written. With a small pier kernel I already matched a stack size of 1000.

In Fuel we  traverse the graph but instead of using a recursion we use our own simple stack implementation where we push and pop objects.
But Martin may explaint better than me.

I'm interested in details how it works and if it avoids the deep stack. Is there additional information about that?

In 4. you said "while writing the content of an object (inst vars) I encounter simple objects (direct string representation) or complex objects (composite). In the latter case we start over from 1.".
So... Imagine object X with two instVars Y and Z. Imagine the method of the traverse is called #analye:.  So you do #analize: X.  Then, in such method you check whether X has regular pointers to regular objects, and if true, you analize them. So in this case you would send #analyze: Y and #analize: Z, generating the recursion. Right ?

yes, the time between analyse: Y and analyse: Z is dependent on the graph that is attached to Y. Y can have objects that have further objects etc. So leaving X aside, Y can be the first object to serialize and Z the last.

Ok, instead of sending #analyze: Y and #analize: Z  what we do is just to do a push on a stack:   #push: Y  and #push: Z. And then we are done with X, so we pop from the stack and we continue with the next object (at some point in the future we will pop Y and Z) When there are no more objects in the stack it means we are done.

How do you write X? I can see that you turn a recursive trace into depth-by-depth trace but the difficult part is to write X. In X you need a reference representation of any sort. How do you derive that from the stack? 

For performance (I think) we did FLSimpleStack. You can check also FLSimpleStackTest. Both are in Fuel repo.

Would something like that help for the stack in GemStone?

Yes. With this approach the resulting stack size is much smaller. To exceed the stack setting you need a graph containing an object which has a shortest route that is larger than the stack limit. And I think this is very unlikely :) 

I would need to change Sixx in order to have it work like this. I need to think about it.

Thanks,

Norbert

Yes, using a specific stack in the way Fuel does relies on there being a separate analyze-phase before the actual serialization of  the objects.
(Due to wanting to group objects of the same type together when serializing)

You could do the same in a "raw" serializer like SIXX:
1) pop first object from stack
2) if any of the instvars need to be serialized, and their ids can not be found in cache, push self + missing objects
3) if not, serialize self, then goto 1

The key difference is you can not considering an object "done" during an analysis phase, but rather push it back on the stack for reevaluation after its dependencies have been handled in the same way.

So the resulting stack will be much larger than what you see in Fuel, but still kept outside the call stack.

Cheers,
Henry