[Networkit] LAMG not converning for small tolerance

Simon Adrian simon.adrian at tum.de
Sun Nov 6 16:19:03 CET 2016


Dear Moritz,

thank you for your reply.

The runtime for the entire program is less than 10 seconds, so I would rule 
this out as the source of the problem.

Nevertheless, I have increased the value by a factor of ten, but as expected, 
I could not observe any difference.

Best regards,
Simon


Am Freitag, 4. November 2016, 19:27:50 CET schrieb Moritz von Looz:
> Dear Simon,
> 
> thanks for your email and example!
> 
> In both the original unit test and your example code, the maximum
> convergence time is set to ten minutes with "status.maxConvergenceTime = 10
> * 60 * 1000". It is to be expected that with increasing precision and input
> size, the time until convergence grows until it no longer fits into the set
> maximum.
> 
> Do you mean it took unusually long and failed to converge even when
> increasing the maximum time?
> 
> All the best,
> Moritz
> 
> Am 02.11.2016 um 19:04 schrieb Simon Adrian:
> > I have played around with the test example cpp/numerics/test/LAMGGTest.cpp
> >  of the LAMG implementation.
> > 
> > It works well when the I use the default tolerance (i.e.,
> > status.desiredResidualReduction = 1e-6* b.length() /
> > (hierarchy.at(0).getLaplacian() * x - b).length();)
> > 
> > When I decrease the tolerance, it stops working at around 1e-10 for the
> > "wing" graph, that is, it does not converge and the output for the final
> > residual is "-nan".
> > 
> > I'm not an expert in the LAMG algorithm, but it seems to me that this is
> > not the correct behavior (given that there is some distance to machine
> > precision from the tolerance).
> > 
> > I compiled a slightly modified version without the googletest environment
> > (see attachment); I presume that this should not effect the observation.
> > 
> > Thank you very much for your help!
> > 
> > Best regards,
> > Simon
> > 
> > 
> > 
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