# Copyright (C) 2001-2010, Parrot Foundation.
# $Id: pdd09_gc.pod 49269 2010-09-23 08:34:38Z bacek $

=head1 PDD 9: Garbage Collection Subsystem

=head2 Abstract

This PDD specifies Parrot's garbage collection and memory management
subsystems.

=head2 Version

$Revision: 49269 $

=head2 Definitions

=head3 Garbage collection (GC)

Garbage collection is a process of freeing up memory that is no longer used by
the interpreter, by determining which objects will not be referenced again and
can be reclaimed.

=head3 Mark and sweep (MS)

Starting from a known root set, the GC traces all reachable memory objects by
following pointers. Objects reached in this way, and therefore visible for
use by the program, are alive. Objects which are not reached in the trace are
marked dead. In the second stage, sweep, all dead objects are destroyed and
reclaimed.

=head3 Tri-color mark and sweep

Instead of a simple separation of marked (as live) and unmarked (dead), the
object set is divided into three parts: white, gray, and black. The white
objects are presumed dead. The gray objects have been marked as live by some
other object, but haven't yet marked the objects they refer to. The black
objects are live, and have marked all objects they directly refer to.

In the initial run, all objects start as white and the root set is marked
gray.  The marking process changes white objects to gray (marking them from
another gray object), and gray objects to black (when all objects they refer
to are marked). When the gray set is empty, all live objects have been marked
and the white set can be collected. After a collection run, all black objects
are reset to white, the root set to gray, and the process begins again.

The advantage of a tri-color mark over a simple mark is that it can be broken
into smaller stages.

=head3 Copying collection

A copying GC copies objects from one memory region to another during the mark
phase. At the end of the mark, all memory in the old region is dead and the
whole region can be reclaimed at once.

=head3 Compacting collection

The compacting GC moves live objects close together in a single region in
memory. This helps to elimianate fragmented free space and allows the
allocation of large live objects. Compacting and copying collectors are often
similar or even identical in implementation.

=head3 Uncooperative

An uncooperative GC is implemented as a separate module, often without
affecting the remainder of the program. The programmer can write software
without needing to be aware of the operations or implementation of the GC.
The alternative is a cooperative GC, which is often implemented as a reference
counting scheme and requires GC-related logic to be dispersed throughout the
entire program.

=head3 Stop-the-world

A common disadvantage of a simple mark implementation is that the entire
system (including all threads that use the same memory pools) must be
suspended while the whole memory set is examined during marking and
collection.  Normal operation continues only after the whole GC cycle is
performed. This can lead to arbitrarily long pauses during program execution.

=head3 Incremental

In order to alleviate the arbitrarily long pauses in a stop-the-world GC, the
incremental GC breaks the mark and sweep process up into smaller, shorter
phases. Each GC phase may still require the entire program to pause, but the
pauses are shorter and more frequent.

=head3 Real-time

The pauses caused by GC don't exceed a certain limit.

=head3 Generational

The object space is divided between a young generation (short-lived
temporaries) and one or more old generations. Only young generations are reset
to white (presumed dead). The older generations are scanned less often because
it is assumed that long-lived objects tend to live longer.

=head3 Concurrent

GC marking and collection runs as a separate thread, sometimes with multiple
threads participating in GC. On a multi-processor machine, concurrent GC may
be truly parallel.

=head3 Conservative

A conservative GC traces through memory looking for pointers to living
objects. The GC does not necessarily have information about the layout of
memory, so it cannot differentiate between an actual pointer and an integral
value which has the characteristics of a pointer. The Conservative GC follows
a policy of "no false negatives" and traces any value which appears to be a
pointer.

=head3 Precise

A precise GC has intimate knowledge of the memory layout of the system and
knows where to find pointers. In this way the precise collector never has
any false positives.

=head2 Synopsis

Not applicable.

=head2 Description

No GC algorithm is ideal for all workloads. To support multiple workloads,
Parrot provides support for pluggable uncooperative GC cores. Parrot will
attempt to provide a default core which has reasonable performance for most
programs. Parrot provides no built-in support for cooperative GCs.

Parrot uses two separate memory allocation mechanisms: a fixed-size system for
small objects of fixed size (PMC and STRING headers, etc), and a buffer
allocator for arbitrary-sized objects, such as string contents. The default
fixed-size memory allocator uses a SLAB-like algorithm to allocate objects
from large pre-allocated pools. The default buffer allocator uses a compacting
algorithm.

=head2 Implementation

Parrot supports pluggable garbage collection cores, so ultimately any
uncooperative garbage collection model devised can run on it.

Parrot really has two independent GC models, one used for objects (PMCs) and
the other used for buffers (including strings). The core difference is that
buffers cannot contain other buffers, so incremental marking is unnecessary.

=head3 Terminology

A GC run is composed of two distinct operations: Finding objects which are
dead (the "trace" or "mark" phase) and freeing dead objects for later reuse
(the "sweep" phase). The sweep phase is also known as the collection phase.
The trace phase is less frequently known as the "dead object detection" phase.

=head3 Marking

Each PMC and STRING has a C<flags> member which is a bitfield of various
flags. Three flags in particular are important for GC operation.
C<PObj_live_FLAG> is set if the object is currently alive and active.
C<PObj_on_free_list_FLAG> is set if the object is currently on the free list
and is available for reallocation. A third flag, C<PObj_grey_FLAG> can be used
to support tricolor mark. Despite the given names of these flags, they can be
used by the active GC core for almost any purpose, or they can be ignored
entirely if the GC provides another mechanism for marking the various life
stages of the object. These flags are typically not used outside the GC
subsystem.

=head4 Special PMCs

=head4 Root Set

The root set for the GC mark is the interpreter object and, if necessary,
the C system stack. If the C system stack is traced, the GC is conservative.

=head4 Initiating a mark and sweep

Depending on the core in use, the mark and sweep phases may be initiated in
different ways. A concurrent core would always be running in the background.
The most common mechanism for a non-concurrent core is to initiate a run of
the GC system when an attempt is made to allocate

=head4 Object marking

To mark a PMC, the C<Parrot_gc_mark_pmc_alive> function is called. To mark a
STRING, the C<Parrot_gc_mark_string_alive> function is called. These functions
mark the object alive, typically by setting the C<PObj_live_FLAG> flag.

If the PMC contains references to other PMCs and STRINGS, it must have the
C<PObj_custom_mark_FLAG> flag set. If this flag is set, the C<mark> VTABLE
for that PMC is called to mark the pointers in that PMC. The custom_mark flag
is ignored in STRINGs.

=head4 Buffer Marking

Buffers are always attached to a fixed-size header, or several headers. During
the mark phase of the fixed-size objects, owned buffers are flagged as alive.
At somet time after the fixed-size objects are marked, the buffer pool is
compacted by moving all alive buffers to a new pool and then freeing the old
pool back to the operating system.

=head3 Collection

When all objects have been marked, the collection phase begins.

=head4 Collecting objects

During the sweep phase, objects which had previously been alive but were not
traced in the most recent mark phase are dead and are collected. If the
C<PObj_custom_destroy_FLAG> is set on a PMC, the GC will call the C<destroy>
VTABLE on that PMC to do custom cleanup. This flag is ignored in STRINGs.

The GC does not collect dead PMCs in any particular order and does not
guarantee any ordering of collection between dependant PMCs. Some GC cores may
enforce some ordering or dependency recognition, but this is not guaranteed.

=head3 Finalization

When the interpreter object is destroyed, the GC system is finalized. During
finalization, all living PMCs in the system are destroyed unconditionally and
all memory owned by the interpreter is freed back to the operating system.

=head3 Internal Structures

A GC core is defined in memory by a structure of function pointers to various
routines that perform the primitive operations of the GC. A GC core must
define most of the pointers in the C<< interp->gc_sys >> structure, which is
a C<GC_Subsystem> structure.

C<GC_Subsystem> has the following fields:

=over 4

=item C<void (*finalize_gc_system) (PARROT_INTERP)>

Function to finalize the GC system, by freeing all PMCs and returning all
allocated memory to the operating system.

=item C<void (*destroy_child_interp)(Interp *dest_interp,
Interp *child_interp)>

=item C<void (*do_gc_mark)(PARROT_INTERP, UINTVAL flags)>

Perform a GC mark and sweep run, or at least run a single increment of it.

=item C<void (*compact_string_pool)(PARROT_INTERP)>

Compact the string pool and destroy all unused buffers.

=item C<void (*mark_special)(PARROT_INTERP, PMC *)>

Mark a special PMC. A PMC is special if it has the C<PObj_is_special_FLAG>
flag set.

=item C<void (*pmc_needs_early_collection)(PARROT_INTERP, PMC *)>

Flag a PMC as needing early collection.

=item C<void (*init_pool)(PARROT_INTERP, struct Fixed_Size_Pool *)>

Initialize a new memory pool.

=item C<PMC* (*allocate_pmc_header)(PARROT_INTERP, UINTVAL flags)>

Allocate a new PMC object from the system.

=item C<void (*free_pmc_header)(PARROT_INTERP, PMC *)>

Free a PMC object back to the system.

=item C<STRING* (*allocate_string_header)(PARROT_INTERP, UINTVAL flags)>

Allocate a new STRING header from the system.

=item C<void (*free_string_header)(PARROT_INTERP, STRING*)>

Free a STRING object back to the system.

=item C<Buffer* (*allocate_bufferlike_header)(PARROT_INTERP, size_t size)>

=item C<void (*free_bufferlike_header)(PARROT_INTERP, Buffer*, size_t size)>

=item C<int  (*is_pmc_ptr)(PARROT_INTERP, void*)>

Determine if the given pointer is or resembles a valid PMC pointer.

=item C<int  (*is_string_ptr)(PARROT_INTERP, void*)>

Determine if the given pointer is or resembles a valid STRING pointer.

=item C<void (*mark_pobj_header)(PARROT_INTERP, PObj*)>

=item C<void (*mark_pmc_header)(PARROT_INTERP, PMC *)>

Mark a PMC alive.

=item C<void* (*allocate_pmc_attributes)(PARROT_INTERP, PMC *)>

Allocate attribute storage for a PMC. The size of the attributes structure is
determined from the PMCs VTABLE.

=item C<void (*free_pmc_attributes)(PARROT_INTERP, PMC *)>

Free an attribute structure back to the system.

=item C<void (*allocate_string_storage)
(PARROT_INTERP, STRING *str, size_t size)>

Allocate buffer storage for a string.

=item C<void (*reallocate_string_storage)
(PARROT_INTERP, STRING *str, size_t size)>

Resize existing string storage to fit data of the new size.

=item C<void (*allocate_buffer_storage)
(PARROT_INTERP, ARGMOD(Buffer *buffer), size_t nsize)>

Allocate buffer storage for any purpose.

=item C<void (*reallocate_buffer_storage)
(PARROT_INTERP, ARGMOD(Buffer *buffer), size_t newsize)>

Reallocate or resize existing buffer storage.

=item C<void* (*allocate_fixed_size_storage)(PARROT_INTERP, size_t size)>

Allocate storage for a fixed-size header which is not a PMC or a STRING. The
contents of this structure are not marked automatically by GC.

=item C<void (*free_fixed_size_storage)(PARROT_INTERP, size_t size, void *)>

Free a fixed-size structure back to the system.

=item C<void* (*allocate_memory_chunk)(PARROT_INTERP, size_t size)>

=item C<void* (*reallocate_memory_chunk)(PARROT_INTERP, void *data,
size_t newsize)>

=item C<void* (*allocate_memory_chunk_with_interior_pointers)(PARROT_INTERP,
size_t size)>

=item C<void* (*reallocate_memory_chunk_with_interior_pointers)(PARROT_INTERP,
void *data, size_t oldsize, size_t newsize)>

=item C<void (*free_memory_chunk)(PARROT_INTERP, void *data)>

=item C<void (*block_mark)(PARROT_INTERP)>

Block the GC mark from occuring.

=item C<void (*unblock_mark)(PARROT_INTERP)>

Unblock the GC mark.

=item C<unsigned int (*is_blocked_mark)(PARROT_INTERP)>

Query the blocked state of the GC mark.

=item C<void (*block_sweep)(PARROT_INTERP)>

Block the GC sweep phase.

=item C<void (*unblock_sweep)(PARROT_INTERP)>

Unblock the GC sweep phase.

=item C<unsigned int (*is_blocked_sweep)(PARROT_INTERP)>

Query the blocked state of the GC sweep.

=item C<size_t (*get_gc_info)(PARROT_INTERP, Interpinfo_enum)>

Query information about the GC core.

=back

=head4 The Memory_Pools structure

The C<Memory_Pools> structure contains pointers to a variety of memory pools,
each used for a specific purpose. Two are Var_Size_Pool pointers (memory_pool,
constant_string_pool), and six are Fixed_Size_Pool structures (pmc_pool,
constant_pmc_pool, constant_string_header_pool).

The C<Memory_Pools> structure holds function pointers for the core defined
interface of the currently active GC subsystem: C<init_pool>, C<do_gc_mark>,
C<finalize_gc_system>. It holds various accounting information for the GC
subsystem, including how many GC runs have been completed, amount of memory
allocated since the last run, and total memory allocated. This accounting
information is updated by the GC system. The current block level for GC mark
and sweep phases is stored in the C<Memory_Pools> structure.
(See L<Blocking GC>.)

The pointer C<void *gc_private> is reserved for use by the currently active GC
subsystem (with freedom for variation between GC implementations).

=head4 The Var_Size_Pool structure

The C<Var_Size_Pool> structure is a simple memory pool. It contains a pointer
to the top block of the allocated pool, the total allocated size of the pool,
the block size, and some details on the reclamation characteristics of the
pool.

=head4 The Fixed_Size_Pool structure

The C<Fixed_Size_Pool> structure is a richer memory pool for object
allocation. It tracks details like the number of allocated and free objects
in the pool, a list of free objects, and for the generational GC
implementation maintains linked lists of white, black, and gray PMCs. It
contains a pointer to a simple C<Var_Size_Pool> (the base storage of the
pool). It holds function pointers for adding and retrieving free objects in
the pool, and for allocating objects.

=head3 Internal API

Each GC core provides a standard interface for interaction with the core.

=head4 Initialization

Each GC core declares an initialization routine as a function pointer,
which is installed in F<src/memory.c:mem_setup_allocator()> after
creating C<mem_pools> in the interpreter struct.

=over 4

=item C<void Parrot_gc_XXX_init(Interp *)>

A routine to initialize the GC system named C<XXX>.

The initialization code is responsible for the creation of the header pools
and fills the function pointer slots in the interpreter's C<mem_pools>
member.

=back

=head4 Memory_Pools structure function pointers

Each GC system declares 3 function pointers, stored in the Memory_Pools
structure.

=over 4

=item C<void (*init_gc_system) (Interp *)>

Initialize the GC system. Install the additional function pointers into
the Memory_Pools structure, and prepare any private storage to be used by
the GC in the Memory_Pools->gc_private field.

=item C<void (*do_gc_mark) (Interp *, int flags)>

Trigger or perform a GC run. With an incremental GC core, this may only
start/continue a partial mark phase or sweep phase, rather than performing an
entire run from start to finish. It may take several calls to C<do_gc_mark> in
order to complete an entire run of an incremental collector.

For a concurrent collector, calls to this function may activate a concurrent
collection thread or, if such a thread is already running, do nothing at all.

The C<do_gc_mark> function is called from the C<Parrot_gc_mark_and_sweep>
function, and should not usually be called directly.

C<flags> is one of:

=over 4

=item C<0>

Run the GC normally, including the trace and the sweep phases, if applicable.
Incremental GCs will likely only run one portion of the complete GC run, and
repeated calls would be required for a complete run. A complete trace of all
system areas is not required.

=item GC_trace_normal | GC_trace_stack_FLAG

Run a normal GC trace cycle, at least. This is typically called when there
is a resource shortage in the buffer memory pools before the sweep phase is
run. The processor registers and any other system areas have to be traced too.

Behavior is determined by the GC implementation, and might or might not
actually run a full GC cycle. If the system is an incremental GC, it might
do nothing depending on the current state of the GC. In an incremental GC, if
the GC is already past the trace phase it may opt to do nothing and return
immediately. A copying collector may choose to run a mark phase if it hasn't
yet, to prevent the unnecessary copying of dead objects later on.

=item GC_lazy_FLAG

Do a timely destruction run. The goal is either to detect all objects that
need timely destruction or to do a full collection. This is called from the
Parrot run-loop, typically when a lexical scope is exited and the local
variables in that scope need to be cleaned up. Many types of PMC objects, such
as line-buffered IO PMCs rely on this behavior for proper operation.

No system areas have to be traced.

=item GC_finish_FLAG

Finalize and destroy all living PMCs. This is called during interpreter
destruction. The GC subsystem must clear the live state of all objects
and perform a sweep in the PMC header pool, so that destructors and finalizers
get called. PMCs which have custom destructors rely on this behavior for
proper operation.

=back

=item C<void (*finalize_gc_system) (Interp *)>

Called during interpreter destruction. Free used resources and memory pools.
All PMCs must be swept, and PMCs with custom destroy VTABLE functions must
have those called.

=item C<void (*init_pool) (Interp *, Fixed_Size_Pool *)>

Initialize the given pool. Populates the C<Fixed_Size_Pool> structure with
initial values, and sets a series of function pointers for working with the
pool. The function pointers used with the pool are discussed next.

=back

=head4 Fixed_Size_Pool function pointers

Each GC core defines 4 function pointers stored in the C<Fixed_Size_Pool>
structures. These function pointers are used throughout Parrot to implement
basic behaviors for the pool.

=over 4

=item C<PObj * (*get_free_object) (Interp *, Fixed_Size_Pool*)>

Get a free object from the pool. This function returns one free object from
the given pool and removes that object from the pool's free list. PObject
flags are returned clear, except flags that are used by the garbage collector
itself, if any. If the pool is a buffer header pool all other object memory
is zeroed.

=item C<void (*add_free_object) (Interp *, Fixed_Size_Pool *, PObj *);>

Add a freed object to the pool's free list. This function is most often called
internally to the GC itself to add items to the free list after a sweep, or
when a new arena is created to add the new items to the free list. It does
not need to be used in this way, however.

=item C<void (*alloc_objects) (Interp *, Fixed_Size_Pool *);>

Allocate a new arena of objects for the pool. Initialize the new arena and add
all new objects to the pool's free list. Some collectors implement a growth
factor which increases the size of each new allocated arena.

=item C<void (*more_objects) (Interp *, Fixed_Size_Pool *);>

Reallocation for additional objects. It has the same signature as
C<alloc_objects>, and in some GC cores the same function pointer is used for
both. In some GC cores, C<more_objects> may do a GC run in an attempt to free
existing objects without having to allocate new ones. This function may also
call C<pool->alloc_objects> internally, to allocate objects if a GC run fails
to free any old objects.

=back

=head4 Write Barrier

Each GC core has to provide the following macros. All of these might be
defined empty, for GC cores which do not use them.

=over 4

=item C<GC_WRITE_BARRIER(Interp *, PMC *agg, PMC *old, PMC *new)>

This macro is invoked when in aggregate C<agg> the element C<old> is getting
overwritten by C<new>. Either C<old>, C<new>, or both may be NULL.

=item C<GC_WRITE_BARRIER_KEY(Interp *, PMC *agg, PMC *old, PObj
*old_key, PMC *new, PObj *new_key)>

Similar to C<GC_WRITE_BARRIER>. Invoked when a hash key C<new_key> is
inserted into hash C<agg> with value C<new>, possibly replacing a key/value
pair C<old_key> and C<old>, respectively. Any of C<old>, C<old_key>, C<new>
or C<new_key> might be C<NULL>.

=back

=head3 Blocking GC

Being able to block GC is important, so newly allocated Buffers or PMCs won't
be collected before they're attached to the live tree. Parrot provides locking
mechanisms to prevent the GC from taking certain actions, such as marking
or sweeping. GC block functions are nesting, and multiple calls to a lock
function requires the same number of calls to the corresponding unlock
function in order to operate the GC normally again. The following functions
are used to block the GC from performing certain actions:

=over 4

=item Parrot_block_GC_mark(Interp *interpreter)

Block the GC mark phase for the passed interpreter, but do not block the sweep
phase. In a stop-the-world collector, this will prevent the entire collection
run, but in an incremental collector this will only block if the GC is in the
trace state.

=item Parrot_block_GC_sweep(Interp *interpreter)

Block the GC sweep phase for the passed interpreter, but do not block the
trace phase.

=item Parrot_unblock_GC_mark(Interp *interpreter)

Unblock the GC mark phase for the passed interpreter, but do not unblock a
blocked sweep phase, if it is blocked using C<Parrot_block_GC_sweep>.

=item Parrot_unblock_GC_sweep(Interp *interpreter)

Unblock the GC sweep phase for the passed interpreter, but do not unblock the
mark phase if it has been blocked by C<Parrot_block_GC_mark>.

=item Parrot_is_blocked_GC_mark(Interp *interpreter)

Test whether the mark phase has been blocked. Notice that the sweep phase can
be locked independently and cannot be determined using this function.

=item Parrot_is_blocked_GC_sweep(Interp *interpreter)

Test whether the sweep phase has been blocked. Notice that the mark phase can
be locked independently and cannot be determined using this function.

=back

=head3 PMC/Buffer API

=head4 Flags

For PMCs and Buffers to be collected properly, you must set the appropriate
flags on them. Directly manipulating these flags is not recommended because
the exact values can be changed over time. A series of macros have been
created in F<include/parrot/pobject.h> that set and check for these flags.
Always use these provided macros when you need to test or set these flags.

=over 4

=item PObj_custom_destroy_FLAG

The PMC has some sort of active destructor, and will have that destructor
called when the PMC is destroyed. The destructor is typically called from
within C<src/gc/api.c:Parrot_gc_free_pmc>.

=item PObj_custom_mark_FLAG

The C<mark> vtable slot will be called during the GC mark phase. The mark
function must call C<Parrot_gc_mark_PObj_alive> for all non-NULL objects
(Buffers and PMCs) that PMC refers to. This flag is typically tested and the
custom mark VTABLE function called from C<src/gc/api.c:mark_special>.

=item PObj_external_FLAG

Set if the buffer points to memory that came from outside Parrot's memory
system.

=item PObj_sysmem_FLAG

Set if the memory came from the system malloc. When the buffer is considered
dead, the memory will be freed back to the system.

=item PObj_COW_FLAG

The buffer's memory is copy on write. Any changes to the buffer must first
have the buffer's memory copied. The COW flag should then be removed.

=back

The following flags can be used by the GC subsystem:

=over 4

=item PObj_live_FLAG

The system considers the object to be alive for collection purposes. Objects
with this flag set should never be collected, freed, destroyed, or put on the
free list.

=item PObj_on_free_list_FLAG

The object is unused, and on the free list for later allocation.

=item PObj_custom_GC_FLAG

Mark the buffer as needing GC.

=back

=head2 References

"Uniprocessor Garbage Collection Techniques"
L<http://www.cs.rice.edu/~javaplt/311/Readings/wilson92uniprocessor.pdf>

"A unified theory of garbage collection":
L<http://portal.acm.org/citation.cfm?id=1028982>

"Scalable Locality-Conscious Multithreaded Memory Allocation":
L<http://people.cs.vt.edu/~scschnei/papers/ismm06.pdf>

"Parallel and concurrent garbage collectors":
L<http://chaoticjava.com/posts/parallel-and-concurrent-garbage-collectors/>

"Region-Based Memory Management":
L<http://www.irisa.fr/prive/talpin/papers/ic97.pdf>

Dan's first musings on the GC subsystem:
L<http://www.mail-archive.com/perl6-all@perl.org/msg14072.html>

Semi-timely and ordered destruction:
L<http://www.sidhe.org/~dan/blog/archives/000199.html>

=cut

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