Janus Dam Nielsen wrote:


On 21/10/2009, at 20.28, Marcel Keller wrote:
Martin Geisler wrote:
Janus Dam Nielsen <janus.niel...@alexandra.dk <mailto:janus.niel...@alexandra.dk>> writes:
Hi Marcel,

I am not opposed to your suggestion. However I would like to point out
that in VIFF you compute on shares and not field elements!
Well, we've actually made the outer runtime interfaces in such a way
that add, mul, xor, etc... accept both integers, FieldElements and
Shares. The methods then wrap their input as needed -- or they *dont*
wrap it if that leads to a short cut (e.g., constant multiplication)

I agree (see also my answer).
I would still like to stress that Shares are the basic values in VIFF. The interface is then designed in such a way, so that we can do various optimizations, but computing on field elements (a particular representation of a share) is an optimization, which I am very happy that the interface allows us to do. But it is still an optimization.

I agree that the design maybe is not optimal semantically, IMHO because Shares are Deferreds. This leads to the situation that whenever a callback returns a Share, the Deferred code converts it into the result carried by the Share. However, a list of Shares returned by a callback is not converted.

Computing directly on the field elements is hacking the abstractions
of VIFF. Computation on field elements or rather the representation of
a Share can be useful as an optimization, however this optimization
should be confined within applications or runtimes, and should not
progress over interface boundaries as I fear you are suggesting.
I think we are in agreement: public methods on the runtimes will keep
returning Shares. Methods used internally in runtimes can return other
things as needed. To me it sounds like a better API to require
preprocessing functions to return a list of Deferreds:
 [D(?), D(?), ...],
instead of a Deferred list of tuples containing Deferreds :-)
I think it will simplify the interface nicely, at least for consumers.
Using simpler types also leads to less memory usage which has a positive
effect on performance, as Marcel notes. So let's go for it.

So this makes 2 votes in favour of it and 1 against it. Maybe we should have a meeting to discuss it. What do you think?
I can agree on this as well, as long as we don't make field elements canonical.

You mean that the preprocessing infrastructure should not be restricted to FieldElements but can handle other items such as the tuples used by the Orlandi runtime. Is that correct? It was never my plan to introduce this restriction. I just want to get rid of the Deferreds in the preprocessing pool because I think that they are unnecessary there.
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