cpp: Use #pragma once instead of #ifndef guards
[ghc.git] / rts / sm / GCThread.h
1 /* -----------------------------------------------------------------------------
2 *
3 * (c) The GHC Team 1998-2008
4 *
5 * Generational garbage collector
6 *
7 * Documentation on the architecture of the Garbage Collector can be
8 * found in the online commentary:
9 *
10 * http://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/GC
11 *
12 * ---------------------------------------------------------------------------*/
13
14 #pragma once
15
16 #include "WSDeque.h"
17 #include "GetTime.h" // for Ticks
18
19 #include "BeginPrivate.h"
20
21 /* -----------------------------------------------------------------------------
22 General scheme
23
24 ToDo: move this to the wiki when the implementation is done.
25
26 We're only going to try to parallelise the copying GC for now. The
27 Plan is as follows.
28
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.
32
33 In the gc_thread structure is a gen_workspace for each generation. The
34 primary purpose of the gen_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 generation. When the todo
37 block is full, it is pushed to the global gen->todos list, which
38 is protected by a lock. (in fact we intervene a one-place buffer
39 here to reduce contention).
40
41 A thread repeatedly grabs a block of work from one of the
42 gen->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 generation: we can just
45 collect them all later).
46
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 generation, because it is partially scanned.
61
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 ------------------------------------------------------------------------- */
65
66
67 /* -----------------------------------------------------------------------------
68 Generation Workspace
69
70 A generation workspace exists for each generation for each GC
71 thread. The GC thread takes a block from the todos list of the
72 generation into the scanbd and then scans it. Objects referred to
73 by those in the scan block are copied into the todo or scavd blocks
74 of the relevant generation.
75
76 ------------------------------------------------------------------------- */
77
78 typedef struct gen_workspace_ {
79 generation * gen; // the gen for this workspace
80 struct gc_thread_ * my_gct; // the gc_thread that contains this workspace
81
82 // where objects to be scavenged go
83 bdescr * todo_bd;
84 StgPtr todo_free; // free ptr for todo_bd
85 StgPtr todo_lim; // lim for todo_bd
86
87 WSDeque * todo_q;
88 bdescr * todo_overflow;
89 uint32_t n_todo_overflow;
90
91 // where large objects to be scavenged go
92 bdescr * todo_large_objects;
93
94 // Objects that have already been scavenged.
95 bdescr * scavd_list;
96 StgWord n_scavd_blocks; // count of blocks in this list
97 StgWord n_scavd_words;
98
99 // Partially-full, scavenged, blocks
100 bdescr * part_list;
101 StgWord n_part_blocks; // count of above
102 StgWord n_part_words;
103
104 StgWord pad[1];
105
106 } gen_workspace ATTRIBUTE_ALIGNED(64);
107 // align so that computing gct->gens[n] is a shift, not a multiply
108 // fails if the size is <64, which is why we need the pad above
109
110 /* ----------------------------------------------------------------------------
111 GC thread object
112
113 Every GC thread has one of these. It contains all the generation
114 specific workspaces and other GC thread local information. At some
115 later point it maybe useful to move this other into the TLS store
116 of the GC threads
117 ------------------------------------------------------------------------- */
118
119 typedef struct gc_thread_ {
120 Capability *cap;
121
122 #ifdef THREADED_RTS
123 OSThreadId id; // The OS thread that this struct belongs to
124 SpinLock gc_spin;
125 SpinLock mut_spin;
126 volatile StgWord wakeup; // NB not StgWord8; only StgWord is guaranteed atomic
127 #endif
128 uint32_t thread_index; // a zero based index identifying the thread
129
130 bdescr * free_blocks; // a buffer of free blocks for this thread
131 // during GC without accessing the block
132 // allocators spin lock.
133
134 // These two lists are chained through the STATIC_LINK() fields of static
135 // objects. Pointers are tagged with the current static_flag, so before
136 // following a pointer, untag it with UNTAG_STATIC_LIST_PTR().
137 StgClosure* static_objects; // live static objects
138 StgClosure* scavenged_static_objects; // static objects scavenged so far
139
140 W_ gc_count; // number of GCs this thread has done
141
142 // block that is currently being scanned
143 bdescr * scan_bd;
144
145 // Remembered sets on this CPU. Each GC thread has its own
146 // private per-generation remembered sets, so it can add an item
147 // to the remembered set without taking a lock. The mut_lists
148 // array on a gc_thread is the same as the one on the
149 // corresponding Capability; we stash it here too for easy access
150 // during GC; see recordMutableGen_GC().
151 bdescr ** mut_lists;
152
153 // --------------------
154 // evacuate flags
155
156 uint32_t evac_gen_no; // Youngest generation that objects
157 // should be evacuated to in
158 // evacuate(). (Logically an
159 // argument to evacuate, but it's
160 // static a lot of the time so we
161 // optimise it into a per-thread
162 // variable).
163
164 bool failed_to_evac; // failure to evacuate an object typically
165 // Causes it to be recorded in the mutable
166 // object list
167
168 bool eager_promotion; // forces promotion to the evac gen
169 // instead of the to-space
170 // corresponding to the object
171
172 W_ thunk_selector_depth; // used to avoid unbounded recursion in
173 // evacuate() for THUNK_SELECTOR
174
175 // -------------------
176 // stats
177
178 W_ copied;
179 W_ scanned;
180 W_ any_work;
181 W_ no_work;
182 W_ scav_find_work;
183
184 Time gc_start_cpu; // process CPU time
185 Time gc_sync_start_elapsed; // start of GC sync
186 Time gc_start_elapsed; // process elapsed time
187 W_ gc_start_faults;
188
189 // -------------------
190 // workspaces
191
192 // array of workspaces, indexed by gen->abs_no. This is placed
193 // directly at the end of the gc_thread structure so that we can get from
194 // the gc_thread pointer to a workspace using only pointer
195 // arithmetic, no memory access. This happens in the inner loop
196 // of the GC, see Evac.c:alloc_for_copy().
197 gen_workspace gens[];
198 } gc_thread;
199
200
201 extern uint32_t n_gc_threads;
202
203 extern gc_thread **gc_threads;
204
205 #if defined(THREADED_RTS) && defined(llvm_CC_FLAVOR)
206 extern ThreadLocalKey gctKey;
207 #endif
208
209 #include "EndPrivate.h"