In Orthanc 1.11.3, we have introduced a Housekeeper thread that 
tries to give back unused memory back to the system.  This is implemented 
by calling malloc_trim every 100ms.

Here is how we validated the effect of this new feature:
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We compared the behaviour of 2 osimis/orthanc Docker images from the mainline
on Feb 1st 2023.  One image without the call to malloc_trim and the other with
this call.

1st test: unconstrained Docker containers
.........................................

5 large studies are uploaded to each instance of Orthanc (around 1GB in total).
A script triggers anonymization of these studies as quick as possible.
We compare the memory used by the containers after 2 minutes of execution 
(using `docker stats`):
- without malloc_trim:                   1500 MB
- with malloc_trim:                       410 MB

2nd test: memory constrained Docker containers
..............................................

Each Orthanc container is limited to 400MB (through the docker-compose configuration
`mem_limit: 400m`)
5 large studies are uploaded to each instance of Orthanc (around 1GB in total).
Each study is anonymized manually, one by one and then, we repeat the operation.
We compare the memory used by the containers after 2 minutes of execution 
(using `docker stats`):
            
# study            without malloc_trim         with_malloc_trim
0                       ~   50 MB                     ~   50 MB
1                       ~  140 MB                     ~  140 MB
2                       ~  390 MB                     ~  340 MB
3                       ~  398 MB                     ~  345 MB
4                  out-of-memory crash                ~  345 MB
5..20                                                 ~  380 MB (stable)

Note: the use of malloc_trim does not guarantee that Orthanc will never reach a
out-of-memory error, especially on very constrained systems.  
Depending on the allocation pattern, the Orthanc memory can get
very fragmented and increase since malloc_trim only releases memory
at the end of each of malloc arena.  However, note that, even long before the 
introduction of malloc_trim, we have observed Orthanc instances running for years
without ever reaching out-of-memory errors and Orthanc is usually considered as
very stable.






malloc_trim documentation
-------------------------

from (https://stackoverflow.com/questions/40513716/malloc-trim0-releases-fastbins-of-thread-arenas)

    If possible, gives memory back to the system (via negative
    arguments to sbrk) if there is unused memory at the `high' end of
    the malloc pool. You can call this after freeing large blocks of
    memory to potentially reduce the system-level memory requirements
    of a program. However, it cannot guarantee to reduce memory. Under
    some allocation patterns, some large free blocks of memory will be
    locked between two used chunks, so they cannot be given back to
    the system.

    The `pad' argument to malloc_trim represents the amount of free
    trailing space to leave untrimmed. If this argument is zero,
    only the minimum amount of memory to maintain internal data
    structures will be left (one page or less). Non-zero arguments
    can be supplied to maintain enough trailing space to service
    future expected allocations without having to re-obtain memory
    from the system.

    Malloc_trim returns 1 if it actually released any memory, else 0.
    On systems that do not support "negative sbrks", it will always
    return 0.