Help, my CPU slot no longer folds !

By , the in Folding@Home Project - No comment
Help, my CPU slot no longer folds !
Recently, your CPU slot might have stopped folding with the following messages in the log :

Connecting to
Failed to get assignment from '': Empty work server assignment
Connecting to
Failed to get assignment from '': Empty work server assignment
Exception: Could not get an assignment

In its default behaviour, the client always configure automatically the CPU slot to use fewer threads of the CPU than the total available :

  • if you have a GPU slot, the client reserves 1 thread to feed it
  • if you use little aggressive settings (Folding Power set to Light or Medium), the client will not use all threads

This behaviour usually creates a non even number of threads to be used for the CPU slot (if you have an 8 threads CPU, the client will attempt to use at most 7) which implies compatibility issues with a lot of simulated systems when this odd number is a prime number. This limitation comes from algorithms used by Gromacs (the science code) to decompose the system in parts computed by each thread. To determine the supported combinations, the researchers relied on returns from testers for each project and set up the assignment servers accordingly. Gromacs used to work well from 1 to 8 threads, then for every value that didn't contain a prime greater than 7 when factored (this means that all prime numbers greater than 7 were excluded).

Lately, the Folding@Home project started to follow official Gromacs recommendations more strictly, which exclude odd prime numbers greater than or equal to 7 from supported decompositions.

If you are in this situation with you CPU slot, the workaround is quite simple. You just have to change the automatic behaviour of the client (-1 value) to a forced setup (6 threads instead of 7 if you use an 8 threads CPU for instance) with the following method :
In FAHControl, click on Configure.
In Slots tab, select the CPU slot and click on Edit.
Near the top of the window, change the number of CPUs.
Click on OK then on Save.

If you need further assistance to configure your client, do not hesitate to post on our forum.