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 * ---------------------------------------------------------------------------*/
17 #include "OSThreads.h"
20 /* -----------------------------------------------------------------------------
23 ToDo: move this to the wiki when the implementation is done.
25 We're only going to try to parallelise the copying GC for now. The
28 Each thread has a gc_thread structure (see below) which holds its
29 thread-local data. We'll keep a pointer to this in a thread-local
30 variable, or possibly in a register.
32 In the gc_thread structure is a step_workspace for each step. The
33 primary purpose of the step_workspace is to hold evacuated objects;
34 when an object is evacuated, it is copied to the "todo" block in
35 the thread's workspace for the appropriate step. When the todo
36 block is full, it is pushed to the global step->todos list, which
37 is protected by a lock. (in fact we intervene a one-place buffer
38 here to reduce contention).
40 A thread repeatedly grabs a block of work from one of the
41 step->todos lists, scavenges it, and keeps the scavenged block on
42 its own ws->scavd_list (this is to avoid unnecessary contention
43 returning the completed buffers back to the step: we can just
44 collect them all later).
46 When there is no global work to do, we start scavenging the todo
47 blocks in the workspaces. This is where the scan_bd field comes
48 in: we can scan the contents of the todo block, when we have
49 scavenged the contents of the todo block (up to todo_bd->free), we
50 don't want to move this block immediately to the scavd_list,
51 because it is probably only partially full. So we remember that we
52 have scanned up to this point by saving the block in ws->scan_bd,
53 with the current scan pointer in ws->scan. Later, when more
54 objects have been copied to this block, we can come back and scan
55 the rest. When we visit this workspace again in the future,
56 scan_bd may still be the same as todo_bd, or it might be different:
57 if enough objects were copied into this block that it filled up,
58 then we will have allocated a new todo block, but *not* pushed the
59 old one to the step, because it is partially scanned.
61 The reason to leave scanning the todo blocks until last is that we
62 want to deal with full blocks as far as possible.
63 ------------------------------------------------------------------------- */
66 /* -----------------------------------------------------------------------------
69 A step workspace exists for each step for each GC thread. The GC
70 thread takes a block from the todos list of the step into the
71 scanbd and then scans it. Objects referred to by those in the scan
72 block are copied into the todo or scavd blocks of the relevant step.
74 ------------------------------------------------------------------------- */
76 typedef struct step_workspace_ {
77 step * step; // the step for this workspace
78 struct gc_thread_ * my_gct; // the gc_thread that contains this workspace
80 // where objects to be scavenged go
82 StgPtr todo_free; // free ptr for todo_bd
83 StgPtr todo_lim; // lim for todo_bd
86 bdescr * todo_overflow;
89 // where large objects to be scavenged go
90 bdescr * todo_large_objects;
92 // Objects that have already been scavenged.
94 nat n_scavd_blocks; // count of blocks in this list
96 // Partially-full, scavenged, blocks
98 unsigned int n_part_blocks; // count of above
102 } step_workspace ATTRIBUTE_ALIGNED(64);
103 // align so that computing gct->steps[n] is a shift, not a multiply
104 // fails if the size is <64, which is why we need the pad above
106 /* ----------------------------------------------------------------------------
109 Every GC thread has one of these. It contains all the step specific
110 workspaces and other GC thread local information. At some later
111 point it maybe useful to move this other into the TLS store of the
113 ------------------------------------------------------------------------- */
115 typedef struct gc_thread_ {
117 OSThreadId id; // The OS thread that this struct belongs to
120 volatile rtsBool wakeup;
122 nat thread_index; // a zero based index identifying the thread
124 bdescr * free_blocks; // a buffer of free blocks for this thread
125 // during GC without accessing the block
126 // allocators spin lock.
128 StgClosure* static_objects; // live static objects
129 StgClosure* scavenged_static_objects; // static objects scavenged so far
131 lnat gc_count; // number of GCs this thread has done
133 // block that is currently being scanned
136 // Remembered sets on this CPU. Each GC thread has its own
137 // private per-generation remembered sets, so it can add an item
138 // to the remembered set without taking a lock. The mut_lists
139 // array on a gc_thread is the same as the one on the
140 // corresponding Capability; we stash it here too for easy access
141 // during GC; see recordMutableGen_GC().
144 // --------------------
147 step *evac_step; // Youngest generation that objects
148 // should be evacuated to in
149 // evacuate(). (Logically an
150 // argument to evacuate, but it's
151 // static a lot of the time so we
152 // optimise it into a per-thread
155 rtsBool failed_to_evac; // failure to evacuate an object typically
156 // Causes it to be recorded in the mutable
159 rtsBool eager_promotion; // forces promotion to the evac gen
160 // instead of the to-space
161 // corresponding to the object
163 lnat thunk_selector_depth; // ummm.... not used as of now
169 // -------------------
178 // -------------------
181 // array of workspaces, indexed by stp->abs_no. This is placed
182 // directly at the end of the gc_thread structure so that we can get from
183 // the gc_thread pointer to a workspace using only pointer
184 // arithmetic, no memory access. This happens in the inner loop
185 // of the GC, see Evac.c:alloc_for_copy().
186 step_workspace steps[];
190 extern nat n_gc_threads;
192 /* -----------------------------------------------------------------------------
193 The gct variable is thread-local and points to the current thread's
194 gc_thread structure. It is heavily accessed, so we try to put gct
195 into a global register variable if possible; if we don't have a
196 register then use gcc's __thread extension to create a thread-local
199 Even on x86 where registers are scarce, it is worthwhile using a
200 register variable here: I measured about a 2-5% slowdown with the
202 -------------------------------------------------------------------------- */
204 extern gc_thread **gc_threads;
206 #if defined(THREADED_RTS)
208 #define GLOBAL_REG_DECL(type,name,reg) register type name REG(reg);
210 #define SET_GCT(to) gct = (to)
214 #if (defined(i386_HOST_ARCH) && defined(linux_HOST_OS))
215 // Using __thread is better than stealing a register on x86/Linux, because
216 // we have too few registers available. In my tests it was worth
217 // about 5% in GC performance, but of course that might change as gcc
218 // improves. -- SDM 2009/04/03
220 extern __thread gc_thread* gct;
221 #define DECLARE_GCT __thread gc_thread* gct;
224 #elif defined(sparc_TARGET_ARCH)
225 // On SPARC we can't pin gct to a register. Names like %l1 are just offsets
226 // into the register window, which change on each function call.
228 // There are eight global (non-window) registers, but they're used for other purposes.
229 // %g0 -- always zero
230 // %g1 -- volatile over function calls, used by the linker
231 // %g2-%g3 -- used as scratch regs by the C compiler (caller saves)
232 // %g4 -- volatile over function calls, used by the linker
233 // %g5-%g7 -- reserved by the OS
235 extern __thread gc_thread* gct;
236 #define DECLARE_GCT __thread gc_thread* gct;
239 #elif defined(REG_Base) && !defined(i386_HOST_ARCH)
240 // on i386, REG_Base is %ebx which is also used for PIC, so we don't
243 GLOBAL_REG_DECL(gc_thread*, gct, REG_Base)
244 #define DECLARE_GCT /* nothing */
247 #elif defined(REG_R1)
249 GLOBAL_REG_DECL(gc_thread*, gct, REG_R1)
250 #define DECLARE_GCT /* nothing */
253 #elif defined(__GNUC__)
255 extern __thread gc_thread* gct;
256 #define DECLARE_GCT __thread gc_thread* gct;
260 #error Cannot find a way to declare the thread-local gct
264 #else // not the threaded RTS
266 extern StgWord8 the_gc_thread[];
268 #define gct ((gc_thread*)&the_gc_thread)
269 #define SET_GCT(to) /*nothing*/
270 #define DECLARE_GCT /*nothing*/