Extremely slow to converge

[ferrigno]

Hi Johannes,

I am trying to use BXA to compare two models on an XMM-Newton data set and I have issues of convergence, probably due to some parameter degeneracy, even if models are not very complex:

constant * TBabs * pow (This one converged in an hour or so) constant * TBabs * pcfabs * pow

This last one has been running for more than one day and it does not look to be converging. The warning is: /pyenv/versions/3.8.2/lib/python3.8/site-packages/ultranest/integrator.py:1632: UserWarning: Sampling from region seems inefficient (0/40 accepted in iteration 2500). To improve efficiency, modify the transformation so that the current live points (stored for you in BXA-pcfabs/extra/sampling-stuck-it%d.csv) are ellipsoidal, or use a stepsampler, or set frac_remain to a lower number (e.g., 0.5) to terminate earlier. warnings.warn(warning_message)

I have already put frac_reami=0.5 in my parameters, but I do not understand well this thing of ellipsoids...

Do you have some suggestions to increase efficiency ?

You find attached some files from the run

debug.log sampling-stuck-it%d.csv

The WIP notebook is in https://gitlab.astro.unige.ch/xmm-newton-workflows/0862410301 which uses the call to BXA in https://gitlab.astro.unige.ch/ferrigno/pysas/-/blob/master/pyxmmsas/init.py#L420

[JohannesBuchner]

Making a corner plot of the CSV file, gives me this:

Most of the plots look ellipsoidal. However, in the first panel of the second row, it seems there is a inverted L-shape. This can be inefficient to sample with the MLFriends algorithm. Your choices are to:

• try to reparameterize so that it is easier to sample
• to use a different algorithm (for example, a step sampler), to continue.
• If you know that some regions of the parameter space are unreasonable, update the prior.

Looking at the two arms of this L:

The left half tightly constrains the first parameter to a narrow range, while the right allows it to be wider.

I don't know what your parameters mean, but I want to give you an example of a beneficial reparametrization. Lets say, the first and third parameter are each a normalisation of components, each with a log-uniform prior. The data constrain to first order the sum of these components. Therefore, a L shape appears, where at least one of the two component normalisations has to exceed a threshold. To remove the L shape, one can use a total normalisation parameter, and a relative normalisation parameter (e.g., the ratio between the two components, or similar).