SortMem...

Hi
in the function ExecChooseHashTableSize() 
(~/src/backend/executor/nodeHash.c)
are determined the bytes of the hash table. The correspondent code is: 

/*
* Target in-memory hashtablesize in SortMem kilobytes
*/ hash_table_bytes = SortMem *1024L 

(~/src/include/miscadmin.h)
extern DLLIMPORT int SortMem; 

What SortMem represents? Where is evaluated and what is its value? 

I want to create 2 hash tables. One for the inner and one for the outer rel. 
What should be the hash_table_bytes in that case? 

Thanks in advance!-martha