C++ performance of global variables

Question

For clarification: I know how evil globals are and when not to use them :)

• Is there any performance penalty when accessing/setting a global variable vs. a loca

Answer 1

It's more of the way how you use data stored in your variables that matters performance wise then how you declare them. I'm not sure about the correct terminology here, but one can define two types of data access. Shared access (where you access same data from different parts of the code) and private data, where each part has its own data. By default global variables imply shared access and local imply private access. But both types of access can be achieved with both types of variables (i.e. local pointers pointing to the same chunk of memory, or global array where each part of code access different part of array).

Shared access has better caching, lower memory footprint, but is harder to optimize, especially in multi threaded environment. It is also bad for scaling especially with NUMA architecture..

Private access is easier to optimise and better for scaling. Problems with private access usually exist in situation where you have multiple copies of same data. The problems usually associated with these scenario's are higher memory footprint, synchronization between copies, worse caching etc.