Is the memory management model documented for MySQL? I want to know
what APIs exist to allocate memory in the context of a user
connection. Are there heaps for global, connection duration and
statement duration allocation? And if there are, how are they
accessed?
I am curious because of a bug in MySQL 5.0 --
http://bugs.mysql.com/bug.php?id=40097. A per-THD variable references
stack-allocated memory used by a String instance.
I found this, but it doesn't describe what/where MEM_ROOT instances
exist in the context of a connection (THD).
http://forge.mysql.com/wiki/MySQL_memory_handling_and_memory_handling_in_Falcon
This states there is a connection duration memory heap -- use sql_alloc()
http://forge.mysql.com/wiki/MysysLibraryAndAlgorithms
There appears to be a lot of allocation that does not use the
per-connection MEM_ROOT. String (from sql/sql_string.{cc,h}) uses
my_malloc and my_realloc to get memory. my_malloc and my_realloc don't
use per-THD heaps like sql_alloc() and get memory directly from
malloc(). Is there a goal to make all allocation done in the context
of a user query allocated via the per-connection MEM_ROOT? It seems
like a bad idea to allow memory references between objects from
different allocation arenas -- things allocated from malloc()
referencing things allocated from sql_alloc() and vice-versa.
Finally, String provides 'operator new' that uses MEM_ROOT to allocate
the String object from that MEM_ROOT. But if that String instance ever
needs to reallocate memory for the contained string, it will get it
from malloc() and not use the MEM_ROOT. Is there an unstated
assumption that all uses of this won't need to allocate more memory?
--
Mark Callaghan
mdcallag@stripped
