# Glas :

## [glas] (sparse) operations and temporary storage

From: Gunter Winkler (guwi17_at_[hidden])
Date: 2006-12-19 11:41:02

Hallo,

will the glas-concepts consider the need for temporary storage or is this only
interesting for the backends?

Lets for example consider a linear combination of two sparse vectors:

z = alpha x + beta y

in the case x and y being conformant (the pairs (index,value) are sorted by
index) the result can be computed in order by a parallel run through both
vectors. The index sequence of z is just a merge of two sorted sequences.
However, if x or y are not conformant one could either use
z = alpha x; z += beta y;
or utilize a temporary dense vector "temp" known to be zero everywhere:
temp = alpha x; // sparse assign because temp is zero ("scatter")
temp += beta y; // sparse update of a dense vector
for each index i in x do {
z(i) = temp(i); temp(i) = 0;
} // "gather and clear"
for each index i in y do {
if temp(i)!=0 {
z(i) = temp(i);
temp(i) = 0;
}
} // "gather if nonzero and clear"

One important point is, that the vector "temp" has to be reused many times in
order to amortize is allocation cost. Thus it has to be a parameter of the
assign operation.

(This algorithm is expected to be even faster than the conformant merge,
especially when more vectors are combined - but I never tried it.)

The question is now, should this be considered in the general concepts, or
could this be hidden inside backend functions, or should these optimizations
left to the enduser by only providing some building blocks?

Another question is, whether to have concepts (and implementations) which are
independent of the actual implementation and provide refinements for dense
and sparse types (which might be quite different) or to have concepts (and
models) mainly for dense types and refine these for sparse types.

What do you think?

mfg
Gunter

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