# Copyright (C) 2001-2009, Parrot Foundation.

=head1 NAME

docs/memory_internals.pod - Memory Internals

=head1 ABSTRACT

This document tries to explain the internals of the Parrot memory subsystem,
and the data structures related to memory management and garbage collection.

=head1 OVERVIEW

All allocated items are managed in memory pools. Memory pools hold collections
of similar items, items of the same data type and usually of the same size.
These can be roughly divided into two kinds: pools holding fixed sized items
and pools holding variable sized items.

A pool is organized as a linked list of big chunks, called I<Arenas>. Each
Arena holds and manages a set number of items of a single size and shape.

=head2 Abbreviations used here

=over 4

=item * GC

Garbage collector. A garbage collector is a system that finds memory objects
that are not being used by the system, and frees them automatically. Garbage
collection is broken into two distinct phases: the I<mark phase> that searches
for unreferenced objects, and the I<sweep phase> that frees all dead objects.

=item * DOD

Dead object detection, a deprecated term that refers to the I<mark phase>
of the garbage collector.

=back

By scanning through all the interpreter's registers, stacks and the processor
stack, all objects that are detected by the garbage collector are marked as
alive. Objects in all pools not alive at the end of the mark phase are
considered dead and are collected.

See F<docs/pdds/pdd09_gc.pod> for details about the garbage collector system.

=head1 Top down: the interpreter

A overall layout of the interpreter's memory management looks like so:

    typedef struct Interp {
        ...
        struct Memory_Pools *mem_pools;
        ...
    } Interp;

All object-like things that get allocated during the execution of parrot
bytecode are managed from the C<mem_pools> member of the interpreter
structure.

=head1 Memory Pools

C<struct Memory_Pools> holds pointers to a variety of different kinds of managed
memory. A simplification looks similar to this:

    typedef struct Memory_Pools {
        struct Var_Size_Pool *memory_pool;
        ...
        struct Fixed_Size_Pool * header_pool;
        struct Fixed_Size_Pool ** sized_pools;
    } Memory_Pools;

C<memory_pool> and C<header_pool> are variable and fixed sized pool pointers
respectively. These are just two examples, there are other C<Var_Size_Pool>s
and C<Fixed_Size_Pool>s in the Parrot system. Pools of type
C<struct Var_Size_Pool> are for variable-size objects, such as constant string
buffers. Pools of type C<struct Fixed_Size_Pool> are for fixed-size objects
such as headers or PMCs.

=head1 Fixed sized items

Fixed-size items are either objects by themselves, like a C<PMC>, or are
header structures of variable-sized objects like C<STRING>.  The general
object of this kind is a buffer-like object, which consists of a
C<Buffer> or a C<PObj> header at the beginning connected to a variable-sized
memory block for data.

Buffer-like objects of the same size are maintained in the list of
C<sized_pools>, which manage objects of the same size in one slot.

Every garbage collector must supply 4 function pointers: an initialization
routine, a de-initialization routine, a function to initialize a new memory
pool, and a function to initiate a garbage collection run. Every pool also
requires the garbage collector to supply 4 additional function pointers: a
function to allocate a new arena for the pool, a function to find more items
(possibly through running the garbage collector, or allocating objects
otherwise), a function to add an unused item to the free list, and a function
to retrieve a new free item from the pool.

Every pool maintains a list of unused items within the pool, called the
C<free_list> When there is need for a new object it is taken off the free
list, and when it stops being used, it will be put back on the free list
again. GC mark determines which objects are not being used, and GC sweep
adds those unused items back to the free list.

If the free list is empty then a GC run could be initiated. This may be
able to find some dead objects and free them up for immediate reuse.
However, if there are no objects that can be immediately recycled, a new
arena will be allocated.

Arenas, which contain the fixed-sized items used by Parrot, are never returned
to the operating system, even if they are empty. In the future, this behavior
may change, however. Variable-sized buffers may be returned when they are no
longer being used.

=head2 General structure of a buffer-like item

    struct parrot_object_t {
        unsigned flags;
        ...
    } PObj;

The flags field may contain a series of flags which indicate the type, status,
configuration, and special requirements of each item. C<Buffer>s, C<PMC>s, and
C<PObj>s all have this basic field in common.

C<PMC>s and C<Buffer>s each have an additional field which contain a pointer
to a block of data.

=head2 GC-related PObj flags

Only three flags need to be checked during a mark run: C<PObj_live_FLAG>,
C<PObj_on_free_list_FLAG>, and C<PObj_is_special_PMC_FLAG>.  Normally these
flags are stored in C<PObj-E<gt>flags>, meaning that each PMC must be accessed
during the mark run.

F<pobj.h> provides macros to facilitate referencing individual object flags:
C<gc_flag_SET>, C<gc_flag_CLEAR> and C<gc_flag_TEST>. They make up a portable
way of manipulating the GC-relevant object flags.

=head1 Variable sized items

Variable-sized items do not exist by themselves, they are always wrapped by
a buffer structure that contains a pointer to the data information about them.
The variable-sized data items are managed in two different pools: the
C<memory_pool>, which contains a general mish-mash of data types, and the
C<constant_string_pool> which contains immutable string buffers used by
programs running on Parrot.

Here, different memory allocation schemes jump in:

=head2 Copying GC

A C<memory_pool> gets allocated in big blocks, namely a C<Memory_Block>.
When some part is needed, e.g. to store a string, this part is carved out of
the memory block, until this block is used up. If no more space is available in
this memory block, a special copying garbage collection (GC) is started.
This copies all living items of all used memory blocks into one new block,
which holds thereafter only used items tightly packed together.

The old memory blocks, containing sparse unused parts and used parts already
copied to the new place, are then unused altogether and get C<free()>ed
thereafter. When GC doesn't provide enough free space needed for a new item,
a new block is added to the memory pool.

This also implies that buffers are moved around during their life. Users of
these buffers are therefore not allowed to hold pointers to buffers over pieces
of code that might trigger a GC run, like C<Parrot_allocate()> or
C<Parrot_str_compare()>.

=head2 Defragmenting allocator

An alternative to the above is to use a memory allocator, which is as fast as
the above and does reuse C<free()>ed items in a memory-conserving way.
Actually, the malloc implementations in some systems' F<libc> efficiently
provide this, such as the glibc malloc based on Doug Lea's allocator.

Using this allocator, all variable sized items are just allocated via a plain
C<malloc()> call, or resized with C<realloc()>, and after they lose their
existence (ie when the mark phase detects that the managing buffer header is no
longer in use) they are C<free()>ed. That's all. The underlying allocator
collects these pieces, coalesces them if possible to form bigger pieces, and
then puts them on free lists, sorted by size.  Eventually, when a new
allocation request arrives, it may give them back to Parrot.

So here, the variable length C<memory_pool> is unused. You can consider this
pool to live inside the allocator. Buffers allocated this way don't move
around, except when reallocated, of course.

The F<Configure.pl> option C<--gc> allows one to use either method.

=head2 Buffer_Tail and COW

Both implementations have the same problem to solve: C<STRING>s that get
copied, or parts of strings as the result of a substr() call, do not allocate
new space in the memory pool to hold this string or part of string. They just
use a C<new_string_header()> and set up a pointer (C<strstart>) pointing
somewhere into the original string and remember the used length there in
C<bufused>.

This is all OK, as long as the original and the lazy copy of the string are not
modified. So that's well-known and called COW (copy on write). We don't have
to make a copy of the buffer until one of the copies tries to modify it. In
practice, strings often get copied from place to place but do not often get
modified, so this optimization saves us a lot of time.

Now, during GC (or freeing buffers) the problem arises: to whom does this
string belong? You shouldn't copy the same string to different places, thus
rendering COW to a noop, nor are you allowed to free this same string more then
once. Both allocation schemes therefore use a part of the allocated string to
do this bookkeeping.

The C<malloc()>/C<free()> approach stores a refcount at C<bufstart>. During the
mark phase all dead users increment the refcount, living users set it to an
huge value.  When freeing the buffer, the string is only freed if the refcount
reaches zero.

=head1 Simplified Figure

                         +--------------+
      +--------------<---| Memory Pools |<---------+
      |                  +--------------+---+      |
      |                                     |      |
      |         +------+    +-----------+   |  +=============+
      |         | S0   |<---| Registers |<--)--| Interpreter |
      |         +------+    +-----------+   |  +=============+
      |     +---| S1   |                    |
      |     |   +------+                    +----------+
 +-------+  |                                          |
 | Blk 1 |--)-->+----------+    +--------------+   +---------+
 +-------+  |   | Buffer 1 |    | OnestringAno |   | Block 1 |
 | Blk 2 |  |   +----------+    | therstring.. |   +---------+
 +-------+  |   | Buffer 2 |    | ..String...  |<--| Block 2 |
 | .     |  |   +----------+    +--------------+   +---------+
 +-------+  |   | ...      |        ^    ^         | ...     |
Fixed Size  |   +----------+        |    |         +---------+
   Pool     +-->| Buffer N |--------+----+        Var Size Pool
                +----------+
                 Buffer          Memory Block

Now consider that the I<Interpreter> shall be a C<PMC> living in C<Buffer X>
of the underlying interpreter and is currently running F<perldoc
docs/memory_internals.pod>, and then redo this figure, with all the blanks
filled in.

=head1 FILES

src/gc/api.c, src/gc/gc_private.h, pobj.h, mark_sweep.[ch],
alloc_resources.[ch], string.[ch]. Other garbage collector
implementations may use separate files as well.

=head1 BUGS

To minimize this bugs section - please patch this document and keep it up to
date - thanks.

=head1 AUTHOR

Leopold Tötsch C<lt@toetsch.at>

=head1 VERSION

0.1.1 June 2008
