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/* -----------------------------------------------------------------------------
*
* (c) The GHC Team 1998-2008
*
* Generational garbage collector
*
* Documentation on the architecture of the Garbage Collector can be
* found in the online commentary:
*
* https://gitlab.haskell.org/ghc/ghc/wikis/commentary/rts/storage/gc
*
* ---------------------------------------------------------------------------*/
#pragma once
#include "WSDeque.h"
#include "GetTime.h" // for Ticks
#include "BeginPrivate.h"
/* -----------------------------------------------------------------------------
General scheme
ToDo: move this to the wiki when the implementation is done.
We're only going to try to parallelise the copying GC for now. The
Plan is as follows.
Each thread has a gc_thread structure (see below) which holds its
thread-local data. We'll keep a pointer to this in a thread-local
variable, or possibly in a register.
In the gc_thread structure is a gen_workspace for each generation. The
primary purpose of the gen_workspace is to hold evacuated objects;
when an object is evacuated, it is copied to the "todo" block in
the thread's workspace for the appropriate generation. When the todo
block is full, it is pushed to the global gen->todos list, which
is protected by a lock. (in fact we intervene a one-place buffer
here to reduce contention).
A thread repeatedly grabs a block of work from one of the
gen->todos lists, scavenges it, and keeps the scavenged block on
its own ws->scavd_list (this is to avoid unnecessary contention
returning the completed buffers back to the generation: we can just
collect them all later).
When there is no global work to do, we start scavenging the todo
blocks in the workspaces. This is where the scan_bd field comes
in: we can scan the contents of the todo block, when we have
scavenged the contents of the todo block (up to todo_bd->free), we
don't want to move this block immediately to the scavd_list,
because it is probably only partially full. So we remember that we
have scanned up to this point by saving the block in ws->scan_bd,
with the current scan pointer in ws->scan. Later, when more
objects have been copied to this block, we can come back and scan
the rest. When we visit this workspace again in the future,
scan_bd may still be the same as todo_bd, or it might be different:
if enough objects were copied into this block that it filled up,
then we will have allocated a new todo block, but *not* pushed the
old one to the generation, because it is partially scanned.
The reason to leave scanning the todo blocks until last is that we
want to deal with full blocks as far as possible.
------------------------------------------------------------------------- */
/* -----------------------------------------------------------------------------
Generation Workspace
A generation workspace exists for each generation for each GC
thread. The GC thread takes a block from the todos list of the
generation into the scanbd and then scans it. Objects referred to
by those in the scan block are copied into the todo or scavd blocks
of the relevant generation.
------------------------------------------------------------------------- */
typedef struct gen_workspace_ {
generation * gen; // the gen for this workspace
struct gc_thread_ * my_gct; // the gc_thread that contains this workspace
// where objects to be scavenged go
bdescr * todo_bd;
StgPtr todo_free; // free ptr for todo_bd
StgPtr todo_lim; // lim for todo_bd
WSDeque * todo_q;
bdescr * todo_overflow;
uint32_t n_todo_overflow;
// where large objects to be scavenged go
bdescr * todo_large_objects;
// Objects that have already been scavenged.
bdescr * scavd_list;
StgWord n_scavd_blocks; // count of blocks in this list
StgWord n_scavd_words;
// Partially-full, scavenged, blocks
bdescr * part_list;
StgWord n_part_blocks; // count of above
StgWord n_part_words;
StgWord pad[1];
} gen_workspace ATTRIBUTE_ALIGNED(64);
// align so that computing gct->gens[n] is a shift, not a multiply
// fails if the size is <64, which is why we need the pad above
/* ----------------------------------------------------------------------------
GC thread object
Every GC thread has one of these. It contains all the generation
specific workspaces and other GC thread local information. At some
later point it maybe useful to move this other into the TLS store
of the GC threads
------------------------------------------------------------------------- */
/* values for the wakeup field */
#define GC_THREAD_INACTIVE 0
#define GC_THREAD_STANDING_BY 1
#define GC_THREAD_RUNNING 2
#define GC_THREAD_WAITING_TO_CONTINUE 3
typedef struct gc_thread_ {
Capability *cap;
#if defined(THREADED_RTS)
OSThreadId id; // The OS thread that this struct belongs to
SpinLock gc_spin;
SpinLock mut_spin;
volatile StgWord wakeup; // NB not StgWord8; only StgWord is guaranteed atomic
#endif
uint32_t thread_index; // a zero based index identifying the thread
bdescr * free_blocks; // a buffer of free blocks for this thread
// during GC without accessing the block
// allocators spin lock.
// These two lists are chained through the STATIC_LINK() fields of static
// objects. Pointers are tagged with the current static_flag, so before
// following a pointer, untag it with UNTAG_STATIC_LIST_PTR().
StgClosure* static_objects; // live static objects
StgClosure* scavenged_static_objects; // static objects scavenged so far
W_ gc_count; // number of GCs this thread has done
// block that is currently being scanned
bdescr * scan_bd;
// Remembered sets on this CPU. Each GC thread has its own
// private per-generation remembered sets, so it can add an item
// to the remembered set without taking a lock. The mut_lists
// array on a gc_thread is the same as the one on the
// corresponding Capability; we stash it here too for easy access
// during GC; see recordMutableGen_GC().
bdescr ** mut_lists;
// --------------------
// evacuate flags
uint32_t evac_gen_no; // Youngest generation that objects
// should be evacuated to in
// evacuate(). (Logically an
// argument to evacuate, but it's
// static a lot of the time so we
// optimise it into a per-thread
// variable).
bool failed_to_evac; // failure to evacuate an object typically
// Causes it to be recorded in the mutable
// object list
bool eager_promotion; // forces promotion to the evac gen
// instead of the to-space
// corresponding to the object
W_ thunk_selector_depth; // used to avoid unbounded recursion in
// evacuate() for THUNK_SELECTOR
// -------------------
// stats
W_ copied;
W_ scanned;
W_ any_work;
W_ no_work;
W_ scav_find_work;
Time gc_start_cpu; // process CPU time
Time gc_sync_start_elapsed; // start of GC sync
Time gc_start_elapsed; // process elapsed time
W_ gc_start_faults;
// -------------------
// workspaces
// array of workspaces, indexed by gen->abs_no. This is placed
// directly at the end of the gc_thread structure so that we can get from
// the gc_thread pointer to a workspace using only pointer
// arithmetic, no memory access. This happens in the inner loop
// of the GC, see Evac.c:alloc_for_copy().
gen_workspace gens[];
} gc_thread;
extern uint32_t n_gc_threads;
extern gc_thread **gc_threads;
#if defined(THREADED_RTS) && defined(CC_LLVM_BACKEND)
extern ThreadLocalKey gctKey;
#endif
#include "EndPrivate.h"
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