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417b90a7e5
- add additional parameters parsing (other implementations will just
ignore them). E.g. if in RST we have:
.. code:: nim
:test: "nim c $1"
...
then in Markdown that will be:
```nim test="nim c $1"
...
```
- implement Markdown interpretation of additional indentation which is
less than 4 spaces (>=4 spaces is a code block but it's not
implemented yet). RST interpretes it as quoted block, for Markdown it's
just normal paragraphs.
- add separate `md2html` and `md2tex` commands. This is to separate
Markdown behavior in cases when it diverges w.r.t. RST significantly —
most conspicously like in the case of additional indentation above, and
also currently the contradicting inline rule of Markdown is also turned
on only in `md2html` and `md2tex`. **Rationale:** mixing Markdown and
RST arbitrarily is a way to nowhere, we need to provide a way to fix the
particular behavior. Note that still all commands have **both** Markdown
and RST features **enabled**. In this PR `*.nim` files can be processed
only in Markdown mode, while `md2html` is for `*.md` files and
`rst2html` for `*.rst` files.
- rename `*.rst` files to `.*md` as our current default behavior is
already Markdown-ish
- convert code blocks in `docgen.rst` to Markdown style as an example.
Other code blocks will be converted in the follow-up PRs
- fix indentation inside Markdown code blocks — additional indentation
is preserved there
- allow more than 3 backticks open/close blocks (tildas \~ are still not
allowed to avoid conflict with RST adornment headings) see also
https://github.com/nim-lang/RFCs/issues/355
- better error messages
- (other) fix a bug that admonitions cannot be used in sandbox mode; fix
annoying warning on line 2711
96 lines
4.0 KiB
Markdown
96 lines
4.0 KiB
Markdown
=======================
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Nim's Memory Management
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=======================
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.. default-role:: code
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.. include:: rstcommon.rst
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:Author: Andreas Rumpf
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:Version: |nimversion|
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..
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> "The road to hell is paved with good intentions."
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Multi-paradigm Memory Management Strategies
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===========================================
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.. default-role:: option
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Nim offers multiple different memory management strategies.
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To choose the memory management strategy use the `--mm:` switch.
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**The recommended switch for newly written Nim code is `--mm:orc`.**
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ARC/ORC
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-------
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`--mm:orc` is a memory management mode primarily based on reference counting. Cycles
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in the object graph are handled by a "cycle collector" which is based on "trial deletion".
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Since algorithms based on "tracing" are not used, the runtime behavior is oblivious to
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the involved heap sizes.
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The reference counting operations (= "RC ops") do not use atomic instructions and do not have to --
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instead entire subgraphs are *moved* between threads. The Nim compiler also aggressively
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optimizes away RC ops and exploits `move semantics <destructors.html#move-semantics>`_.
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Nim performs a fair share of optimizations for ARC/ORC; you can inspect what it did
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to your time critical function via `--expandArc:functionName`.
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`--mm:arc` uses the same mechanism as `--mm:orc`, but it leaves out the cycle collector.
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Both ARC and ORC offer deterministic performance for `hard realtime`:idx: systems, but
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ARC can be easier to reason about for people coming from Ada/C++/C -- roughly speaking
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the memory for a variable is freed when it goes "out of scope".
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We generally advise you to use the `acyclic` annotation in order to optimize away the
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cycle collector's overhead
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but `--mm:orc` also produces more machine code than `--mm:arc`, so if you're on a target
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where code size matters and you know that your code does not produce cycles, you can
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use `--mm:arc`. Notice that the default `async`:idx: implementation produces cycles
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and leaks memory with `--mm:arc`, in other words, for `async` you need to use `--mm:orc`.
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Other MM modes
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--------------
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.. note:: The default `refc` GC is incremental, thread-local and not "stop-the-world".
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--mm:refc This is the default memory management strategy. It's a
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deferred reference counting based garbage collector
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with a simple Mark&Sweep backup GC in order to collect cycles. Heaps are thread-local.
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`This document <refc.html>`_ contains further information.
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--mm:markAndSweep Simple Mark-And-Sweep based garbage collector.
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Heaps are thread-local.
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--mm:boehm Boehm based garbage collector, it offers a shared heap.
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--mm:go Go's garbage collector, useful for interoperability with Go.
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Offers a shared heap.
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--mm:none No memory management strategy nor a garbage collector. Allocated memory is
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simply never freed. You should use `--mm:arc` instead.
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Here is a comparison of the different memory management modes:
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================== ======== ================= ============== ===================
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Memory Management Heap Reference Cycles Stop-The-World Command line switch
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================== ======== ================= ============== ===================
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ORC Shared Cycle Collector No `--mm:orc`
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ARC Shared Leak No `--mm:arc`
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RefC Local Cycle Collector No `--mm:refc`
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Mark & Sweep Local Cycle Collector No `--mm:markAndSweep`
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Boehm Shared Cycle Collector Yes `--mm:boehm`
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Go Shared Cycle Collector Yes `--mm:go`
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None Manual Manual Manual `--mm:none`
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================== ======== ================= ============== ===================
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.. default-role:: code
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.. include:: rstcommon.rst
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JavaScript's garbage collector is used for the `JavaScript and NodeJS
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<backends.html#backends-the-javascript-target>`_ compilation targets.
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The `NimScript <nims.html>`_ target uses the memory management strategy built into
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the Nim compiler.
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