duktape/doc/low-memory.rst

=======================
Low memory environments
=======================

Overview
========

This document describes suggested feature options for reducing Duktape
memory usage for memory-constrained environments, which are one important
portability target for Duktape.

The default Duktape options are quite memory conservative, and significant
Ecmascript programs can be executed with e.g. 1MB of memory.  Currently
realistic memory targets are roughly:

* 256-384kB flash memory (code) and 256kB system RAM

  - Duktape compiled with default options is feasible

  - Duktape compiles to around 200-210kB of code (x86), so 256kB is
    technically feasible but leaves little space for user bindings,
    hardware initialization, communications, etc; 384kB is a more
    realistic flash target

* 256-384kB flash memory (code) and 128kB system RAM

  - Duktape feature options are needed to reduce memory usage

  - A custom pool-based memory allocation with manually tuned pools
    may be required

  - Aggressive measures like lightweight functions, 16-bit fields for
    various internal structures (strings, buffers, objects), pointer
    compression, external strings, etc may need to be used

* 256kB flash memory (code) and 96kB system RAM

  - Requires a bare metal system, possibly a custom C library, etc.

  - http://pt.slideshare.net/seoyounghwang77/js-onmicrocontrollers

There are four basic goals for low memory optimization:

1. Reduce Duktape code (flash) footprint.  This is currently a low priority
   item because flash size doesn't seem to be a bottleneck for most users.

2. Reduce initial memory usage of a Duktape heap.  This provides a baseline
   for memory usage which won't be available for user code (technically some
   memory can be reclaimed by deleting some built-ins after heap creation).

3. Minimize the growth of the Duktape heap relative to the scope and
   complexity of user code, so that as large programs as possible can be
   compiled and executed in a given space.  Important contributing factors
   include the footprint of user-defined Ecmascript and Duktape/C functions,
   the size of compiled bytecode, etc.

4. Make remaining memory allocations as friendly as possible for the memory
   allocator, especially a pool-based memory allocator.  Concretely, prefer
   small chunks over large contiguous allocations.

Suggested feature options
=========================

* Use the default memory management settings: although reference counting
  increases heap header size, it also reduces memory usage fluctuation
  which is often more important than absolute footprint.

* Reduce error handling footprint with one or more of:

  - ``DUK_OPT_NO_AUGMENT_ERRORS``

  - ``DUK_OPT_NO_TRACEBACKS``

  - ``DUK_OPT_NO_VERBOSE_ERRORS``

  - ``DUK_OPT_NO_PC2LINE``

* If you don't need the Duktape-specific additional JX/JC formats, use:

  - ``DUK_OPT_NO_JX``

  - ``DUK_OPT_NO_JC``

* Features borrowed from Ecmascript E6 can usually be disabled:

  - ``DUK_OPT_NO_ES6_OBJECT_SETPROTOTYPEOF``

  - ``DUK_OPT_NO_ES6_OBJECT_PROTO_PROPERTY``

  - ``DUK_OPT_NO_ES6_PROXY``

* If you don't need regexp support, use:

  - ``DUK_OPT_NO_REGEXP_SUPPORT``

* Duktape debug code uses a large, static temporary buffer for formatting
  debug log lines.  If you're running with debugging enabled, use e.g.
  the following to reduce this overhead:

  - ``-DDUK_OPT_DEBUG_BUFSIZE=2048``

More aggressive options
=======================

The following may be needed for very low memory environments (e.g. 128kB
system RAM):

* Consider using lightweight functions for your Duktape/C bindings and to
  force Duktape built-ins to be lightweight functions:

  - ``DUK_OPT_LIGHTFUNC_BUILTINS``

* Enable other 16-bit fields to reduce header size; these are typically
  used together (all or none):

  - ``DUK_OPT_REFCOUNT16``

  - ``DUK_OPT_STRHASH16``

  - ``DUK_OPT_STRLEN16``

  - ``DUK_OPT_BUFLEN16``

  - ``DUK_OPT_OBJSIZES16``

* Enable heap pointer compression, assuming pointers provided by your allocator
  can be packed into 16 bits:

  - ``DUK_OPT_HEAPPTR16``

  - ``DUK_OPT_HEAPPTR_ENC16(p)``

  - ``DUK_OPT_HEAPPTR_DEC16(x)``

* Enable data pointer compression if possible.  Note that these pointers can
  point to arbitrary memory locations (outside Duktape heap) so this may not
  be possible even if Duktape heap pointers can be compressed:

  - ``DUK_OPT_DATAPTR16``

  - ``DUK_OPT_DATAPTR_ENC16(p)``

  - ``DUK_OPT_DATAPTR_DEC16(x)``

  - **UNIMPLEMENTED AT THE MOMENT**

* Enable C function pointer compression if possible.  Duktape compiles to
  around 200kB of code, so assuming an alignment of 4 this may only be
  possible if there is less than 56kB of user code:

  - ``DUK_OPT_FUNCPTR16``

  - ``DUK_OPT_FUNCPTR_ENC16(p)``

  - ``DUK_OPT_FUNCPTR_DEC16(x)``

  - **UNIMPLEMENTED AT THE MOMENT**

* Enable a low memory optimized string table variant which uses a fixed size
  top level hash table and array chaining to resolve collisions.  This makes
  memory behavior more predictable and avoids a large continuous allocation
  used by the default string table:

  - ``DUK_OPT_STRTAB_CHAIN``

  - ``DUK_OPT_STRTAB_CHAIN_SIZE=128`` (other values possible also)

* Use "external" strings to allocate most strings from flash (there are
  multiple strategies for this, see separate section):

  - ``DUK_OPT_EXTERNAL_STRINGS``

  - ``DUK_OPT_EXTSTR_INTERN_CHECK(ptr,len)``

  - ``DUK_OPT_EXTSTR_FREE(ptr)``

* Enable struct packing in compiler options if your platform doesn't have
  strict alignment requirements, e.g. on gcc/x86 you can:

  - ``-fpack-struct=1`` or ``-fpack-struct=2``

Notes on low memory measures
============================

Pointer compression
-------------------

Can be applied throughout (where it matters) for three pointer types:

* Compressed 16-bit Duktape heap pointers, assuming Duktape heap pointers
  can fit into 16 bits, e.g. max 256kB memory pool with 4-byte alignment

* Compressed 16-bit function pointers, assuming C function pointers can
  fit into 16 bits

* Compressed 16-bit non-Duktape-heap data pointers, assuming C data
  pointers can fit into 16 bits

Pointer compression can be quite slow because often memory mappings are not
linear, so the required operations are not trivial.  NULL also needs specific
handling.

External string strategies (DUK_OPT_EXTSTR_INTERN_CHECK)
--------------------------------------------------------

The feature can be used in two basic ways:

* You can anticipate a set of common strings, perhaps extracted by parsing
  source code, and build them statically into your program.  The strings will
  then be available in the "text" section of your program.  This works well
  if the set of common strings can be estimated well, e.g. if the program
  code you will run is mostly known in advance.

* You can write strings to memory mapped flash when the hook is called.
  This is less portable but can be effective when the program you will run
  is not known in advance.

Note that:

* Using an external string pointer for short strings (e.g. 3 chars or less)
  is counterproductive because the external pointer takes more room than the
  character data.

The Duktape built-in strings are available from build metadata:

* ``dist/duk_build_meta.json``, the ``builtin_strings_base64`` contains
  the byte exact strings used, encoded with base-64.

Strings used by application C and Ecmascript code can be extracted with
various methods.  The Duktape main repo contains an example script for
scraping strings from C and Ecmascript code using regexps:

* ``util/scan_strings.py``

There are concrete examples for some external string strategies in:

* ``dist/examples/cmdline/duk_cmdline_ajduk.c``

Summary of potential measures
=============================

Heap headers
------------

* Compressed 16-bit heap pointers

* 16-bit field for refcount

* Move one struct specific field (e.g. 16-bit string length) into the unused
  bits of the ``duk_heaphdr`` 32-bit flags field

Objects
-------

* Tweak growth factors to keep objects always or nearly always compact

* 16-bit field for property count, array size, etc.

* Drop hash part entirely: it's rarely needed in low memory environments
  and hash part size won't need to be tracked

* Compressed pointers

Strings
-------

* Use an indirect string type which stores string data behind a pointer
  (same as dynamic buffer); allow user code to indicate which C strings
  are immutable and can be used in this way

* Allow user code to move a string to e.g. memory-mapped flash when it
  is interned or when the compiler interns its constants (this is referred
  to as "static strings" or "external strings")

* Memory map built-in strings (about 2kB bit packed) directly from flash

* 16-bit fields for string char and byte length

* 16-bit string hash

* Rework string table to avoid current issues: (1) large reallocations,
  (2) rehashing needs both old and new string table as it's not in-place.
  Multiple options, including:

  - Separate chaining (open hashing, closed addressing) with a fixed or
    bounded top level hash table

  - Various tree structures like red-black trees

* Compressed pointers

Duktape/C function footprint
----------------------------

* Lightweight functions, converting built-ins into lightweight functions

* Lightweight functions for user Duktape/C binding functions

* Magic value to share native code cheaply for multiple function objects

* Compressed pointers

Ecmascript function footprint
-----------------------------

* Motivation

  - Small lexically nested callbacks are often used in Ecmascript code,
    so it's important to keep their size small

* Reduce property count:

  - _pc2line: can be dropped, lose line numbers in tracebacks

  - _formals: can be dropped for most functions (affects debugging)

  - _varmap: can be dropped for most functions (affects debugging)

* Reduce compile-time maximum alloc size for bytecode: currently each
  instruction takes 8 bytes, 4 bytes for the instruction itself and 4 bytes
  for line number.  Change this into two allocations so that the maximum
  allocation size is not double that of final bytecode, as that is awkward
  for pool allocators.

* Improve property format, e.g. ``_formals`` is now a regular array which
  is quite wasteful; it could be converted to a ``\xFF`` separated string
  for instance.

* Spawn ``.prototype`` on demand to eliminate one unnecessary object per
  function

* Use virtual properties when possible, e.g. if ``nargs`` equals desired
  ``length``, use virtual property for it (either non-writable or create
  concrete property when written)

* Write bytecode and pc2line to flash during compilation

* Compressed pointers

Contiguous allocations
----------------------

Unbounded contiguous allocations are a problem for pool allocators.  There
are at least the following sources for these:

* Large user strings and buffers.  Not much can be done about these without
  a full rework of the Duktape C programming model (which assumes string and
  buffer data is available as plain ``const char *``).

* Bytecode/const buffer for long Ecmascript functions:

  - Bytecode and constants can be placed in separate buffers.

  - Bytecode could be "segmented" so that bytecode would be stored in chunks
    (e.g. 64 opcodes = 256 bytes).  An explicit JUMP to jump from page to page
    could make the executor impact minimal.

  - During compilation Duktape uses a single buffer to track bytecode
    instructions and their line numbers.  This takes 8 bytes per instruction
    while the final bytecode takes 4 bytes per instruction.  This is easy to
    fix by using two separate buffers.

* Value stacks of Duktape threads.  Start from 1kB and grow without
  (practical) bound depending on call nesting.

* Catch and call stacks of Duktape threads.  Also contiguous but since these
  are much smaller, they're unlikely to be a problem before the value stack
  becomes one.

Notes on function memory footprint
==================================

Normal function representation
------------------------------

In Duktape 1.0.0 functions are represented as:

* A ``duk_hcompiledfunction`` (a superset of ``duk_hobject``): represents
  an Ecmascript function which may have a set of properties, and points to
  the function's data area (bytecode, constants, inner function refs).

* A ``duk_hnativefunction`` (a superset of ``duk_hobject``): represents
  a Duktape/C function which may also have a set of properties.  A pointer
  to the C function is inside the ``duk_hnativefunction`` structure.

In Duktape 1.1.0 a lightfunc type is available:

* A lightfunc is an 8-byte ``duk_tval`` with no heap allocations, and
  provides a cheap way to represent many Duktape/C functions.

RAM footprints for each type are discussed below.

Ecmascript functions
--------------------

An ordinary Ecmascript function takes around 300-500 bytes of RAM.  There are
three objects involved:

- a function template
- a function instance (multiple instances can be created from one template)
- automatic prototype object allocated for the function instance

The function template is used to instantiate a function.  The resulting
function is not dependent on the template after creation, so that the
template can be garbage collected.  However, the template often remains
reachable in callback style programming, through the enclosing function's
inner function templates table.

The function instance contains a ``.prototype`` property while the prototype
contains a ``.constructor`` property, so that both functions require a
property table.  This is the case even for the majority of user functions
which will never be used as constructors; built-in functions are oddly exempt
from having an automatic prototype.

Duktape/C functions
-------------------

A Duktape/C function takes about 70-80 bytes of RAM.  Unlike Ecmascript
functions, Duktape/C function are already stripped of unnecessary properties
and don't have an automatic prototype object.

Even so, there are close to 200 built-in functions, so the footprint of
the ``duk_hnativefunction`` objects is around 14-16kB, not taking into account
allocator overhead.

Duktape/C lightfuncs
--------------------

Lightfuncs require only a ``duk_tval``, 8 bytes.  There are no additional heap
allocations.