Memory is sometimes not released by free() in libc

[Dimitar Misev]

It appears that in some cases memory is not really released and returned by free(). It's pretty complex and not very clear why. Especially with GB of allocated memory for tiles, this is a huge issue.

So far I've come across one solution of replacing the system malloc with tcmalloc or jemalloc; but this still doesn't seem to work in all cases, e.g. it works on Ubuntu 16.04, but not on Ubuntu 14.04.

Another solution I stumbled upon is to use ​malloc_trim() into trying to force release all free memory from the top of the heap. I've placed this in ObjectBroker::clearBroker() (called on transaction commit/abort), and this appears to work consistently.