Re: [igraph] releasing the GIL in the python extension

[Tamas Nepusz]

> Tamas, in the python extension, why do you not release the GIL when  
> performing calculations on graphs? Was there a specific reason not  
> to do so?
Yes, there was. The igraph C core is not thread-safe:

http://igraph.sourceforge.net/doc/html/igraph-errorhandlingthreads.html

In a nutshell: whenever igraph allocates memory for some purpose, it  
pushes the address of the allocated memory block to a global stack  
along with a destructor function that should be called should an error  
occur. If the calculation that required the memory block runs to  
completion without errors, it cleans up the allocated memory itself  
and then unrolls the stack. If an error happens, the calculation calls  
an error handler routine which unrolls the stack (calling the  
destructors associated to the memory addresses). Since there is only a  
single global stack, igraph functions that are run in parallel in  
different threads might end up freeing each other's memory should an  
error occur in one of them. Therefore there's no use in running  
multiple igraph functions in separate threads at the same time. That's  
why I keep the GIL locked -- even if you create multiple threads in  
Python and start calling igraph functions from them, the GIL will  
ensure that igraph's internal stack remains in a valid state.

--
Tamas