dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

On Saturday 01 November 2008 17:10:09 Martin Sandve Alnæs wrote:
> 2008/11/1 Garth N. Wells <gnw20@xxxxxxxxx>:
> > Martin Sandve Alnæs wrote:
> >> Are there any plans for multicore support in DOLFIN, that is
> >> automatic utilization of multiple CPU cores without significant
> >> user intervention, in the near future? (For example using OpenMP).
> >
> > No immediate plans (message passing is the immediate priority), but
> > obviously this would be very nice to have and we don't want our design to
> > preclude it.
>
> Ok.
>
> >> If yes, am I the only one who thinks it would be a good idea
> >> to consider this in the new Function design?
> >
> > You may be the only that knows enough about multi-threaded programming to
> > be in a position to think properly about it.
>
> Ok, but I don't really know that much about it.
>
> >> I just want to point this out since I think this issue has been too
> >> downplayed in the design discussions. And I don't think anyone
> >> wants yet another redesign of Function within the next months...
> >>
> >> The main issue is that iterating over cells in a mesh and calling a
> >> Function currently involves side effects, modifying its mutable members
> >> cell and facet. Thus the user would have to construct multiple Function
> >> objects for a Function to be used in basically any multicore situation.
> >> Any solution to this must involve sending all kinds of cell data needed
> >> as arguments to eval. Exactly how this should look is a design
> >> discussion.
> >
> > So if we remove
> >
> >  mutable Cell* _cell
> >  mutable int _facet
> >
> > from the private member data of Function, does this resolve the biggest
> > design issue for running multi-threaded?
>
> Yes. Otherwise the threads will overwrite each others current cell all the
> time.

Would it be possible to store the present Cell* and facet in two vectors of 
length #threads. Then we either have to call the cell(), facet() and normal() 
with the present thread number or if it is possible to implement a static 
function that returns the present thread number in these functions. If the 
last option is not possible we need to call the eval function with the 
present thread number.

> User functions need to be threadsafe too though, but we can't do
> anything about that.

Passing the thread number to eval would make it possible to write thread safe 
functions.

This is probably a quite common problem which has been solved in many previous 
applications. 


Johan