memory decommit does not work with large pages

[romange]

Seems that mimalloc does not decommit memory that was comitted using large pages. See

mimalloc/src/arena.c

Line 540 in 03020fb

if (arena->memid.is_pinned || arena->blocks_purge == NULL) return false;

and

mimalloc/src/arena.c

Line 832 in 03020fb

memid.is_pinned = is_large;

what's the motivation behind this? why large pages can not be decomitted?

[daanx]

Ah, on Windows, large OS pages (2Mib) (called huge pages on Linux), or huge OS pages (1GiB) are "pinned" in memory and cannot be de-committed. In particular huge OS pages are always mapped to physical memory and cannot be shared between processes for example (which I why you generally need to give permission on Windows for processes to "lock" memory).

Maybe this is too restrictive though since on Linux these restrictions may not hold (in particular since we "decommit" on Linux by using mprotect with PROT_NONE).

[romange]

Yes, for long running processes like dragonfly we would like to have some kind of control over the RSS usage. We have even implemented a special administrative command that calls mi_heap_collect for all the heaps in the system. It gives us confidence that Dragonfly itself does not leak memory and all the memory is accountable.

Another issue that we see when using large pages (2MB) is that the RSS usage for a long-running process can constantly grow for some usage patterns: the gap between RSS and sum(heap){mi_heap_visit_blocks(heap)} widens with time. We took a decision to stop using large 2MB pages recently, and we see that the memory usage of our servers became more stable now.