1 /* -----------------------------------------------------------------------------
3 * (c) The GHC Team 1998-2008
5 * Generational garbage collector
7 * Documentation on the architecture of the Garbage Collector can be
8 * found in the online commentary:
10 * http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/GC
12 * ---------------------------------------------------------------------------*/
21 /* -----------------------------------------------------------------------------
24 ToDo: move this to the wiki when the implementation is done.
26 We're only going to try to parallelise the copying GC for now. The
29 Each thread has a gc_thread structure (see below) which holds its
30 thread-local data. We'll keep a pointer to this in a thread-local
31 variable, or possibly in a register.
33 In the gc_thread structure is a step_workspace for each step. The
34 primary purpose of the step_workspace is to hold evacuated objects;
35 when an object is evacuated, it is copied to the "todo" block in
36 the thread's workspace for the appropriate step. When the todo
37 block is full, it is pushed to the global step->todos list, which
38 is protected by a lock. (in fact we intervene a one-place buffer
39 here to reduce contention).
41 A thread repeatedly grabs a block of work from one of the
42 step->todos lists, scavenges it, and keeps the scavenged block on
43 its own ws->scavd_list (this is to avoid unnecessary contention
44 returning the completed buffers back to the step: we can just
45 collect them all later).
47 When there is no global work to do, we start scavenging the todo
48 blocks in the workspaces. This is where the scan_bd field comes
49 in: we can scan the contents of the todo block, when we have
50 scavenged the contents of the todo block (up to todo_bd->free), we
51 don't want to move this block immediately to the scavd_list,
52 because it is probably only partially full. So we remember that we
53 have scanned up to this point by saving the block in ws->scan_bd,
54 with the current scan pointer in ws->scan. Later, when more
55 objects have been copied to this block, we can come back and scan
56 the rest. When we visit this workspace again in the future,
57 scan_bd may still be the same as todo_bd, or it might be different:
58 if enough objects were copied into this block that it filled up,
59 then we will have allocated a new todo block, but *not* pushed the
60 old one to the step, because it is partially scanned.
62 The reason to leave scanning the todo blocks until last is that we
63 want to deal with full blocks as far as possible.
64 ------------------------------------------------------------------------- */
67 /* -----------------------------------------------------------------------------
70 A step workspace exists for each step for each GC thread. The GC
71 thread takes a block from the todos list of the step into the
72 scanbd and then scans it. Objects referred to by those in the scan
73 block are copied into the todo or scavd blocks of the relevant step.
75 ------------------------------------------------------------------------- */
77 typedef struct step_workspace_ {
78 step * step; // the step for this workspace
79 struct gc_thread_ * my_gct; // the gc_thread that contains this workspace
81 // where objects to be scavenged go
83 StgPtr todo_free; // free ptr for todo_bd
84 StgPtr todo_lim; // lim for todo_bd
87 bdescr * todo_overflow;
90 // where large objects to be scavenged go
91 bdescr * todo_large_objects;
93 // Objects that have already been scavenged.
95 nat n_scavd_blocks; // count of blocks in this list
97 // Partially-full, scavenged, blocks
99 unsigned int n_part_blocks; // count of above
103 } step_workspace ATTRIBUTE_ALIGNED(64);
104 // align so that computing gct->steps[n] is a shift, not a multiply
105 // fails if the size is <64, which is why we need the pad above
107 /* ----------------------------------------------------------------------------
110 Every GC thread has one of these. It contains all the step specific
111 workspaces and other GC thread local information. At some later
112 point it maybe useful to move this other into the TLS store of the
114 ------------------------------------------------------------------------- */
116 typedef struct gc_thread_ {
118 OSThreadId id; // The OS thread that this struct belongs to
121 volatile rtsBool wakeup;
123 nat thread_index; // a zero based index identifying the thread
125 bdescr * free_blocks; // a buffer of free blocks for this thread
126 // during GC without accessing the block
127 // allocators spin lock.
129 StgClosure* static_objects; // live static objects
130 StgClosure* scavenged_static_objects; // static objects scavenged so far
132 lnat gc_count; // number of GCs this thread has done
134 // block that is currently being scanned
137 // Remembered sets on this CPU. Each GC thread has its own
138 // private per-generation remembered sets, so it can add an item
139 // to the remembered set without taking a lock. The mut_lists
140 // array on a gc_thread is the same as the one on the
141 // corresponding Capability; we stash it here too for easy access
142 // during GC; see recordMutableGen_GC().
145 // --------------------
148 step *evac_step; // Youngest generation that objects
149 // should be evacuated to in
150 // evacuate(). (Logically an
151 // argument to evacuate, but it's
152 // static a lot of the time so we
153 // optimise it into a per-thread
156 rtsBool failed_to_evac; // failure to evacuate an object typically
157 // Causes it to be recorded in the mutable
160 rtsBool eager_promotion; // forces promotion to the evac gen
161 // instead of the to-space
162 // corresponding to the object
164 lnat thunk_selector_depth; // ummm.... not used as of now
170 // -------------------
179 // -------------------
182 // array of workspaces, indexed by stp->abs_no. This is placed
183 // directly at the end of the gc_thread structure so that we can get from
184 // the gc_thread pointer to a workspace using only pointer
185 // arithmetic, no memory access. This happens in the inner loop
186 // of the GC, see Evac.c:alloc_for_copy().
187 step_workspace steps[];
191 extern nat n_gc_threads;
193 /* -----------------------------------------------------------------------------
194 The gct variable is thread-local and points to the current thread's
195 gc_thread structure. It is heavily accessed, so we try to put gct
196 into a global register variable if possible; if we don't have a
197 register then use gcc's __thread extension to create a thread-local
200 Even on x86 where registers are scarce, it is worthwhile using a
201 register variable here: I measured about a 2-5% slowdown with the
203 -------------------------------------------------------------------------- */
205 extern gc_thread **gc_threads;
207 #if defined(THREADED_RTS)
209 #define GLOBAL_REG_DECL(type,name,reg) register type name REG(reg);
211 #define SET_GCT(to) gct = (to)
215 #if (defined(i386_HOST_ARCH) && defined(linux_HOST_OS))
216 // Using __thread is better than stealing a register on x86/Linux, because
217 // we have too few registers available. In my tests it was worth
218 // about 5% in GC performance, but of course that might change as gcc
219 // improves. -- SDM 2009/04/03
221 // We ought to do the same on MacOS X, but __thread is not
222 // supported there yet (gcc 4.0.1).
224 extern __thread gc_thread* gct;
225 #define DECLARE_GCT __thread gc_thread* gct;
228 #elif defined(sparc_TARGET_ARCH)
229 // On SPARC we can't pin gct to a register. Names like %l1 are just offsets
230 // into the register window, which change on each function call.
232 // There are eight global (non-window) registers, but they're used for other purposes.
233 // %g0 -- always zero
234 // %g1 -- volatile over function calls, used by the linker
235 // %g2-%g3 -- used as scratch regs by the C compiler (caller saves)
236 // %g4 -- volatile over function calls, used by the linker
237 // %g5-%g7 -- reserved by the OS
239 extern __thread gc_thread* gct;
240 #define DECLARE_GCT __thread gc_thread* gct;
243 #elif defined(REG_Base) && !defined(i386_HOST_ARCH)
244 // on i386, REG_Base is %ebx which is also used for PIC, so we don't
247 GLOBAL_REG_DECL(gc_thread*, gct, REG_Base)
248 #define DECLARE_GCT /* nothing */
251 #elif defined(REG_R1)
253 GLOBAL_REG_DECL(gc_thread*, gct, REG_R1)
254 #define DECLARE_GCT /* nothing */
257 #elif defined(__GNUC__)
259 extern __thread gc_thread* gct;
260 #define DECLARE_GCT __thread gc_thread* gct;
264 #error Cannot find a way to declare the thread-local gct
268 #else // not the threaded RTS
270 extern StgWord8 the_gc_thread[];
272 #define gct ((gc_thread*)&the_gc_thread)
273 #define SET_GCT(to) /*nothing*/
274 #define DECLARE_GCT /*nothing*/
280 #endif // SM_GCTHREAD_H