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From Phil Steitz <phil.ste...@gmail.com>
Subject Re: [math] Re: Sparse matrices not supported anymore?
Date Fri, 08 Nov 2013 20:20:14 GMT
On 11/8/13 11:59 AM, Ted Dunning wrote:
> On Fri, Nov 8, 2013 at 11:47 AM, Luc Maisonobe <luc@spaceroots.org> wrote:
>
>>> is there still consensus that we are going to remove the sparse
>>> implementations with 4.0?
>> Well, I really think it is a pity, we should support this. But lets face
>> it: up to now we have been unable to do it properly. S├ębastien who tried
>> to do something in this direction has left the project and nobody
>> replaced him.
>>
> I have done a fair bit of noodling and was unable to come up with a
> solution that is performant.
>
> The issue is that you essentially have to maintain a additional bitmask of
> exceptional values in addition to the implicit bitmask of non-zero
> elements.  I don't see any way of determining that exceptional value
> bitmask short of a full scan.  Moreover, the cost of propagating the
> exceptional value bitmask significantly changes the cost of various
> operations because exceptions require an OR while multiplication allows use
> of an AND.  Furthermore, even after the operation itself and the operation
> on the exception bitmask are done, there needs to be another scan of the
> results to find new exceptional values.
>
>
> So the upshot is that dealing with this will cost at least a significant
> integer degradation in performance at no benefit relative to the normal
> user's expectations with regard to sparse vector operations.  I say no
> benefit because no other package handles this sort of issue so users are
> very used to imprecise handling of exceptional values.
>
So why not just "doc and punt" - document the deficiencies that we
know about and the fact that we are not going to try to "fix" them
(which, IIUC is what other packages do)?

Phil

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