=======================
Nim's Memory Management
=======================
:Author: Andreas Rumpf
:Version: |nimversion|
..
"The road to hell is paved with good intentions."
Introduction
============
This document describes how the multi-paradigm memory management strategies work.
How to tune the garbage collectors for your needs, like (soft) `realtime systems`:idx:,
and how the memory management strategies that are not garbage collectors work.
Multi-paradigm Memory Management Strategies
===========================================
You can choose the memory management strategy to use when compiling source code,
you can pass ``--gc:`` on the compile command with the selected memory management strategy.
- ``--gc:refc`` Deferred `reference counting <https://en.wikipedia.org/wiki/Reference_counting>`_ based garbage collector
with `cycle detection <https://en.wikipedia.org/wiki/Reference_counting#Dealing_with_reference_cycles>`_
by a simple Mark&Sweep that has to scan the full heap,
is only triggered in a memory allocation operation and
it is not triggered by some timer and does not run in a background thread,
`thread local heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_,
references on the stack are not counted for better performance (and easier C code generation), default.
- ``--gc:markAndSweep`` `Mark-And-Sweep <https://en.wikipedia.org/wiki/Tracing_garbage_collection#Copying_vs._mark-and-sweep_vs._mark-and-don't-sweep>`_ based garbage collector,
`thread local heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_.
- ``--gc:boehm`` `Boehm <https://en.wikipedia.org/wiki/Boehm_garbage_collector>`_ based garbage collector,
`stop-the-world <https://en.wikipedia.org/wiki/Tracing_garbage_collection#Stop-the-world_vs._incremental_vs._concurrent>`_,
`shared heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_.
- ``--gc:go`` Go lang like garbage collector,
`stop-the-world <https://en.wikipedia.org/wiki/Tracing_garbage_collection#Stop-the-world_vs._incremental_vs._concurrent>`_,
`shared heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_.
- ``--gc:regions`` `Stack <https://en.wikipedia.org/wiki/Memory_management#Stack_allocation>`_ based garbage collector.
- ``--gc:arc`` Not a garbage collector. Plain `reference counting <https://en.wikipedia.org/wiki/Reference_counting>`_ with
`move semantic optimizations <destructors.html#move-semantics>`_,
`shared heap <https://en.wikipedia.org/wiki/Heap_(programming)>`_,
can be optimized with `sink <destructors.html#sink-parameters>`_ and `lent <destructors.html#lent-type>`_ annotations,
designed to work well with `WebAssembly <https://webassembly.org>`_, `Emscripten <https://emscripten.org>`_,
`hot code reloading <hcr.html>`_ and `address sanitizers <https://en.wikipedia.org/wiki/AddressSanitizer>`_,
basically it is like a shared heap with subgraphs with a single owner,
this is not the same as Swift and ObjectiveC lang ARC because those can not handle cycles,
can use `GOTO based Exception handling <https://nim-lang.org/araq/gotobased_exceptions.html>`_,
may become default in future releases.
- ``--gc:orc`` Not a garbage collector. Similar to ``--gc:arc`` but with improved
`cycle detection <https://en.wikipedia.org/wiki/Reference_counting#Dealing_with_reference_cycles>`_.
`Cycle detection <https://en.wikipedia.org/wiki/Reference_counting#Dealing_with_reference_cycles>`_
will not be the default, because by definition it conflicts with
`deterministic memory management <https://en.wikipedia.org/wiki/Deterministic_memory>`_.
- ``--gc:none`` No memory management strategy nor garbage collector.
You should use `Manual memory management <https://en.wikipedia.org/wiki/Manual_memory_management>`_ with it.
The same Nim code can be compiled to use any of the memory management strategies;
the Nim syntax generally will not change from one memory management strategy to another.
No garbage collector nor memory management is used for `JavaScript and NodeJS
<backends.html#backends-the-javascript-target>`_ compilation targets.
`NimScript <nims.html>`_ target uses Nim VM memory management strategy.
All memory management strategies are supported equally on Nim when possible and aplicable,
even if there is a default one, all others should also work as documented.
If you are new to Nim and just starting, the default memory management strategy is balanced to fit most common use cases.
Cycle collector for garbage collectors
======================================
The cycle collector can be en-/disabled independently from the other parts of
the garbage collector with ``GC_enableMarkAndSweep`` and ``GC_disableMarkAndSweep``.
Realtime support for garbage collectors
=======================================
To enable realtime support, the symbol `useRealtimeGC`:idx: needs to be
defined via ``--define:useRealtimeGC`` (you can put this into your config
file as well).
With this switch the garbage collector supports the following operations:
.. code-block:: nim
proc GC_setMaxPause*(maxPauseInUs: int)
proc GC_step*(us: int, strongAdvice = false, stackSize = -1)
The unit of the parameters ``maxPauseInUs`` and ``us`` is microseconds.
These two procs are the two modus operandi of the realtime garbage collector:
(1) GC_SetMaxPause Mode
You can call ``GC_SetMaxPause`` at program startup and then each triggered
garbage collector run tries to not take longer than ``maxPause`` time. However, it is
possible (and common) that the work is nevertheless not evenly distributed
as each call to ``new`` can trigger the garbage collector and thus take ``maxPause``
time.
(2) GC_step Mode
This allows the garbage collector to perform some work for up to ``us`` time.
This is useful to call in a main loop to ensure the garbage collector can do its work.
To bind all garbage collector activity to a ``GC_step`` call,
deactivate the garbage collector with ``GC_disable`` at program startup.
If ``strongAdvice`` is set to ``true``,
then the garbage collector will be forced to perform collection cycle.
Otherwise, the garbage collector may decide not to do anything,
if there is not much garbage to collect.
You may also specify the current stack size via ``stackSize`` parameter.
It can improve performance, when you know that there are no unique Nim
references below certain point on the stack. Make sure the size you specify
is greater than the potential worst case size.
These procs provide a "best effort" realtime guarantee; in particular the
cycle collector is not aware of deadlines yet. Deactivate it to get more
predictable realtime behaviour. Tests show that a 2ms max pause
time will be met in almost all cases on modern CPUs (with the cycle collector
disabled).
Time measurement with garbage collectors
----------------------------------------
The garbage collectors's way of measuring time uses
(see ``lib/system/timers.nim`` for the implementation):
1) ``QueryPerformanceCounter`` and ``QueryPerformanceFrequency`` on Windows.
2) ``mach_absolute_time`` on Mac OS X.
3) ``gettimeofday`` on Posix systems.
As such it supports a resolution of nanoseconds internally; however the API
uses microseconds for convenience.
Define the symbol ``reportMissedDeadlines`` to make the
garbage collector output whenever it missed a deadline.
The reporting will be enhanced and supported by the API in later versions of the collector.
Tweaking the garbage collector
------------------------------
The collector checks whether there is still time left for its work after
every ``workPackage``'th iteration. This is currently set to 100 which means
that up to 100 objects are traversed and freed before it checks again. Thus
``workPackage`` affects the timing granularity and may need to be tweaked in
highly specialized environments or for older hardware.
Keeping track of memory with garbage collectors
-----------------------------------------------
If you need to pass around memory allocated by Nim to C, you can use the
procs ``GC_ref`` and ``GC_unref`` to mark objects as referenced to avoid them
being freed by the garbage collector.
Other useful procs from `system <system.html>`_ you can use to keep track of memory are:
* ``getTotalMem()`` Returns the amount of total memory managed by the garbage collector.
* ``getOccupiedMem()`` Bytes reserved by the garbage collector and used by objects.
* ``getFreeMem()`` Bytes reserved by the garbage collector and not in use.
* ``GC_getStatistics()`` Garbage collector statistics as a human-readable string.
These numbers are usually only for the running thread, not for the whole heap,
with the exception of ``--gc:boehm`` and ``--gc:go``.
In addition to ``GC_ref`` and ``GC_unref`` you can avoid the garbage collector by manually
allocating memory with procs like ``alloc``, ``alloc0``, ``allocShared``, ``allocShared0`` or ``allocCStringArray``.
The garbage collector won't try to free them, you need to call their respective *dealloc* pairs
(``dealloc``, ``deallocShared``, ``deallocCStringArray``, etc)
when you are done with them or they will leak.
Heap dump
=========
The heap dump feature is still in its infancy, but it already proved
useful for us, so it might be useful for you. To get a heap dump, compile
with ``-d:nimTypeNames`` and call ``dumpNumberOfInstances`` at a strategic place in your program.
This produces a list of used types in your program and for every type
the total amount of object instances for this type as well as the total
amount of bytes these instances take up. This list is currently unsorted!
You need to use external shell script hacking to sort it.
The numbers count the number of objects in all garbage collector heaps, they refer to
all running threads, not only to the current thread. (The current thread
would be the thread that calls ``dumpNumberOfInstances``.) This might
change in later versions.