==========================
Nim's Garbage Collector
==========================

:Author: Andreas Rumpf
:Version: |nimversion|

..


  "The road to hell is paved with good intentions."


Introduction
============

This document describes how the GC works and how to tune it for
(soft) `realtime systems`:idx:.

The basic algorithm is *Deferred Reference Counting* with cycle detection.
References on the stack are not counted for better performance (and easier C
code generation). The GC **never** scans the whole heap but it may scan the
delta-subgraph of the heap that changed since its last run.


The GC is only triggered in a memory allocation operation. It is not triggered
by some timer and does not run in a background thread.

To force a full collection call ``GC_fullCollect``. Note that it is generally
better to let the GC do its work and not enforce a full collection.


Cycle collector
===============

The cycle collector can be en-/disabled independently from the other parts of
the GC with ``GC_enableMarkAndSweep`` and ``GC_disableMarkAndSweep``. The
compiler analyses the types for their possibility to build cycles, but often
it is necessary to help this analysis with the ``acyclic`` pragma (see
`acyclic <manual.html#acyclic-pragma>`_ for further information).

You can also use the ``acyclic`` pragma for data that is cyclic in reality and
then break up the cycles explicitly with ``GC_addCycleRoot``. This can be a
very valuable optimization; the Nim compiler itself relies on this
optimization trick to improve performance. Note that ``GC_addCycleRoot`` is
a quick operation; the root is only registered for the next run of the
cycle collector.


Realtime support
================

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 GC supports the following operations:

.. code-block:: nim
  proc GC_setMaxPause*(MaxPauseInUs: int)
  proc GC_step*(us: int, strongAdvice = false)

The unit of the parameters ``MaxPauseInUs`` and ``us`` is microseconds.

These two procs are the two modus operandi of the realtime GC:

(1) GC_SetMaxPause Mode

    You can call ``GC_SetMaxPause`` at program startup and then each triggered
    GC 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 GC and thus take  ``MaxPause``
    time.

(2) GC_step Mode

    This allows the GC to perform some work for up to ``us`` time. This is
    useful to call in a main loop to ensure the GC can do its work. To
    bind all GC activity to a ``GC_step`` call, deactivate the GC with
    ``GC_disable`` at program startup.

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 unless the cycle collector
is triggered.


Time measurement
----------------

The GC'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 GC output whenever it
missed a deadline. The reporting will be enhanced and supported by the API in
later versions of the collector.


Tweaking the GC
---------------

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
-----------------------

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 GC. 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 GC.
* getOccupiedMem(): bytes reserved by the GC and used by objects.
* getFreeMem(): bytes reserved by the GC and not in use.

In addition to ``GC_ref`` and ``GC_unref`` you can avoid the GC by manually
allocating memory with procs like ``alloc``, ``allocShared``, or
``allocCStringArray``. The GC won't try to free them, you need to call their
respective *dealloc* pairs when you are done with them or they will leak.
