Independts of chains?

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
Post Reply
shadab alam
Posts: 14
Joined: November 06 2013
Affiliation: CMU

Independts of chains?

Post by shadab alam » September 10 2014

Hello All,

Is it technically/scientifically right to run cosmomc several time with one chain and combine the chains while using getdit to get the marginalized likelihood?
I can run hundreds of single processor job but I can't run parallel job on many processor due to some technical difficulty.

Sorry if its a repetition, I couldn't find any discussion on the topic in cosmocoffee arxives.

Thanks,
Shadab

Jason Dossett
Posts: 97
Joined: March 19 2010
Affiliation: The University of Texas at Dallas
Contact:

Independts of chains?

Post by Jason Dossett » September 14 2014

You can actually run numbered single chain jobs using for example

Code: Select all

./cosmomc test.ini 1
./cosmomc test.ini 2
This will produce numbered chains just like if you had run in MPI. Of course while the jobs are running they wont crosstalk. You will simply have a purely MCMC sampling of the parameter space.

You can then run getdist on these chains in the usual way, set the number of chains you ran by setting the chain_num parameter in your getdist ini file.

shadab alam
Posts: 14
Joined: November 06 2013
Affiliation: CMU

Independts of chains?

Post by shadab alam » September 15 2014

Thanks Jason,

That helps a lot.
Could you clarify another thing related to proposal update.
Since these chains will be completely independent including the proposal update. Running chains independently is going to bias the results or make things very inefficient for convergence?

Thanks,
Shadab

Jason Dossett
Posts: 97
Joined: March 19 2010
Affiliation: The University of Texas at Dallas
Contact:

Independts of chains?

Post by Jason Dossett » September 15 2014

Running the chains independently should not bias your results, rather, as you said, it will just be very inefficient in achieving convergence.

shadab alam
Posts: 14
Joined: November 06 2013
Affiliation: CMU

Independts of chains?

Post by shadab alam » September 15 2014

Thanks Jason
I appreciate your help :)

Shadab

Post Reply