What is the HAVE_PSI_INTERFACE macro used for?

I've been reading MySQL 5.5 source code, and got confused by the macro HAVE_PSI_INTERFACE, which is appeared in many source file of the whole project. For example, in the source file storage/example/ha_example.cc, there is the following code: ``` #ifdef HAVE_PSI_INTERFACE static PSI_mutex_key ex_key_mutex_example, ex_key_mutex_EXAMPLE_SHARE_mutex; static PSI_mutex_info all_example_mutexes[]= { { &ex_key_mutex_example, "example", PSI_FLAG_GLOBAL}, { &ex_key_mutex_EXAMPLE_SHARE_mutex, "EXAMPLE_SHARE::mutex", 0} }; static void init_example_psi_keys() { const char* category= "example"; int count; if (PSI_server == NULL) return; count= array_elements(all_example_mutexes); PSI_server->register_mutex(category, all_example_mutexes, count); } #endif ``` So what does the HAVE_PSI_INTERFACE mean? Specifically, what does the PSI stand for? And what is the macro HAVE_PSI_INTERFACE used for? Thanks.