[Paraview] distributed stream tracer scalability issue

[John Biddiscombe]

Burlen

I have had performance issues with the Distributed Stream tracer, but in 
fact I found that in general, the problem of it not being very well 
optimized for parallel operation was not the main trouble. If you are 
using Unstructured Grids, and they are large (in my case 20million cells 
in a block), then the main time was taken by the building of cell links 
which are used to FindCEll inwhich an integration point lies. I modified 
the stream tracer interpolation to use a BSP tree (or CellLocator) and 
found a huge improvement in execution time. (minutes instead of hours).

Secondly. the parallelization of the stream tracer is an inherent 
problem. One cannot integrate the streamline in block 2, until it has 
reached a boundary in block 1 - one must wait until the streamling 
traverses one block before passing it to the next. In actuality, the 
implementation could be improved with more intelligent seeding and 
rending/receiving of streamline seeds etc between iterations.

The Particle tracer code could be modifed to produce streamlines in a 
serial or distributed manner and ought to give a 'reasonably' optimal 
solution to the problem - but in fact the chaps at kitware are at the 
moment (they tell me) in the process of revamping the streamline code to 
make use of CellLocators - and for this reason I recently committed my 
BSP tree code.

Here's how to check your bottleneck.
Find a large StructuredGrid dataset which is loaded in parallel. 
Generate streamlines. Time it. Convert the grdi to UnstructuredGrid and 
do the same. If test 1 takes 1 minute and test 2 1 hour, then it isn't 
the parallization that's the real issue, but the grid being used.

JB




> We've been using the distributed stream tracer to generate 100s-1000s 
> of stream lines per time step. It's very slow, and it doesn't scale at 
> all.  The class comments say as much. I'm sure there is a reason why 
> this implementation was chosen. Is there something that generally 
> prevents real parallel implementation? Is there a better 
> implementation available out there?
>
> There is this post a while back
> http://www.paraview.org/pipermail/paraview/2009-July/012959.html
>
> What's the status?
>
> Thanks
> Burlen
>
>
>
>
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-- 
John Biddiscombe,                            email:biddisco @ cscs.ch
http://www.cscs.ch/
CSCS, Swiss National Supercomputing Centre  | Tel:  +41 (91) 610.82.07
Via Cantonale, 6928 Manno, Switzerland      | Fax:  +41 (91) 610.82.82