Optimizations for instances with a lot of hostgroups #3349
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This commit rewrites MySQL_HostGroups_Manager::read_only_action() in way to reduce the calls to SQLite. It relies on two new std::set() : - read_only_set1: if the server is already in a writer hostgroup - read_only_set2: a cache to track the first time that a server has RO=0 Added also an index on MySQL_HostGroups_Manager, ON mysql_servers (hostname,port)
HostGroups are stored in an array of pointers. This commit allows the lookup using an unordered_map if the number of Hostgroups is greater than 100. For less number of hostgroups, a sequential scan seems more efficient. This also introduce a test for micro benchmark.
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This PR speeds up performance in MySQL_HostGroups_Manager if there are a lot of servers/hostgroups.
Specifically:
MySQL_HostGroups_Manager::read_only_action()can scale hundreds or thousands of times. This avoid unnecessary connection pool locksMySQL_HostGroups_Manager::HGC_lookup()uses anunordered_mapin case there are more than 100 hostgroups