aboutsummaryrefslogtreecommitdiffstats
path: root/erts/emulator/beam/time.c
blob: a7e5a64b22af1771f5acf15f8288a2b54a395c6b (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
/*
 * %CopyrightBegin%
 * 
 * Copyright Ericsson AB 1996-2016. All Rights Reserved.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * 
 * %CopyrightEnd%
 */
 
/*
 * TIMER WHEEL
 *
 *
 * The time scale used for timers is Erlang monotonic time. The
 * time unit used is ERTS specific clock ticks. A clock tick is
 * currently defined to 1 millisecond. That is, the resolution of
 * timers triggered by the runtime system is 1 millisecond.
 * 
 * When a timer is set, it is determined at what Erlang monotonic
 * time, in clock ticks, it should be triggered.
 *
 * The 'pos' field of the wheel corresponds to current time of
 * the wheel. That is, it corresponds to Erlang monotonic time in
 * clock tick time unit. The 'pos' field of the wheel is
 * monotonically increased when erts_bump_timers() is called. All
 * timers in the wheel that have a time less than or equal to
 * 'pos' are triggered by the bump operation. The bump operation
 * may however be spread over multiple calls to erts_bump_timers()
 * if there are a lots of timers to trigger.
 *
 * Each scheduler thread maintains its own timer wheel. The timer
 * wheel of a scheduler, however, actually consists of two wheels.
 * A soon wheel and a later wheel.
 *
 *
 * -- The Soon Wheel --
 *
 * The soon wheel contain timers that should be triggered soon.
 * That is, they are soon to be triggered. Each slot in the soon
 * wheel is 1 clock tick wide. The number of slots in the soon
 * wheel is currently 2¹⁴. That is, it contains timers in the
 * range ('pos', 'pos' + 2¹⁴] which corresponds to a bit more
 * than 16 seconds.
 *
 * When the bump operation is started, 'pos' is moved forward to a
 * position that corresponds to current Erlang monotonic time. Then
 * all timers that are in the range (old 'pos', new 'pos'] are
 * triggered. During a bump operation, the soon wheel may contain
 * timers in the two, possibly overlapping, ranges (old 'pos',
 * old 'pos' + 2¹⁴], and (new 'pos', new 'pos' + 2¹⁴]. This may
 * occur even if the bump operation doesn't yield, due to timeout
 * callbacks inserting new timers.
 *
 *
 * -- The Later Wheel --
 *
 * The later wheel contain timers that are further away from 'pos'
 * than the width of the soon timer wheel. That is, currently
 * timers further away from 'pos' than 2¹⁴ clock ticks. The width
 * of each slot in the later wheel is half the width of the soon
 * wheel. That is, each slot is currently 2¹³ clock ticks wide
 * which corresponds to about 8 seconds. If three timers of the
 * times 'pos' + 17000, 'pos' + 18000, and 'pos' + 19000 are
 * inserted, they will all end up in the same slot in the later
 * wheel.
 *
 * The number of slots in the later wheel is currently the same as
 * in the soon wheel, i.e. 2¹⁴. That is, one revolution of the later
 * wheel currently corresponds to 2¹⁴×2¹³ clock ticks which is
 * almost 37 ½ hour. Timers even further away than that are put in
 * the later slot identified by their time modulo the size of the later
 * wheel. Such timers are however very uncommon. Most timers used
 * by the runtime system will utilize the high level timer API.
 * The high level timer implementation will not insert timers
 * further away then one revolution into the later wheel. It will
 * instead keep such timers in a tree of very long timers. The
 * high level timer implementation utilize one timer wheel timer
 * for the management of this tree of timers. This timer is set to
 * the closest timeout in the tree. This timer may however be
 * further away than one revolution in the later wheel.
 *
 * The 'later.pos' field identifies next position in the later wheel.
 * 'later.pos' is always increased by the width of a later wheel slot.
 * That is, currently 2¹³ clock ticks. When 'pos' is moved (during
 * a bump operation) closer to 'later.pos' than the width of a later
 * wheel slot, i.e. currently when 'pos' + 2¹³ ≥ 'later.pos', we
 * inspect the slot identified by 'later.pos' and then move 'later.pos'
 * forward. When inspecting the later slot we move all timers in the
 * slot, that are in the soon wheel range, from the later wheel to
 * the soon wheel. Timers one or more revolutions of the later wheel
 * away are kept in the slot.
 *
 * During normal operation, timers originally located in the later
 * wheel will currently be moved into the soon wheel about 8 to
 * 16 seconds before they should be triggered. During extremely
 * heavy load, the scheduler might however be heavily delayed, so
 * the code must be prepared for situations where time for
 * triggering the timer has passed when we inspect the later wheel
 * slot, and then trigger the timer immediately. We must also be
 * prepared to inspect multiple later wheel slots at once due to the
 * delay.
 *
 *
 * -- Slot Management --
 *
 * All timers of a slot are placed in a circular double linked
 * list. This makes insertion and removal of a timer O(1).
 *
 * While bumping timers in a slot, we move the circular list
 * away from the slot, and refer to it from the 'sentinel'
 * field. The list will stay there until we are done with it
 * even if the bump operation should yield. The cancel operation
 * can remove the timer from this position as well as from the
 * slot position by just removing it from the circular double
 * linked list that it is in.
 *
 * -- At Once Slot --
 *
 * If a timer is set that has a time earlier or equal to 'pos',
 * it is not inserted into the wheel. It is instead inserted,
 * into a circular double linked list referred to by the "at
 * once" slot. When the bump operation is performed these timers
 * will be triggered at once. The circular list of the slot will
 * be moved to the 'sentinel' field while bumping these timers
 * as when bumping an ordinary wheel slot. A yielding bump
 * operation and cancelation of timers is handled the same way
 * as if the timer was in a wheel slot.
 *
 * -- Searching for Next Timeout --
 *
 * In order to limit the amount of work needed in order to find
 * next timeout, we keep track of total amount of timers in the
 * wheels, total amount of timers in the later wheel, total amount
 * of timers in soon wheel, and the total amount of timers in
 * each range of slots. Each slot range currently contain 512
 * slots.
 *
 * When next timeout is less than the soon wheel width away we
 * determine the exact timeout. Due to the timer counts of
 * slot ranges, we currently at most need to search 1024 slots
 * in the soon wheel. This besides inspecting slot range counts
 * and two slots in the later wheel which potentially might trigger
 * timeouts for moving timers from the later wheel to the soon wheel
 * earlier than timeouts in the soon wheel. We also keep track
 * of latest known minimum timeout position in each wheel which
 * makes it possible to avoid scanning from current position
 * each time.
 *
 * When next timeout is further away than the soon wheel width
 * we settle for the earliest possible timeout in the first
 * non-empty slot range. The further away the next timeout is, the
 * more likely it is that the next timeout change before we
 * actually get there. That is, a change due to another timer is
 * set to an earlier time and/or the timer is cancelled. It is
 * therefore in this case no point determining next timeout
 * exactly. If the state should not change, we will wake up a bit
 * early and do a recalculation of next timeout and eventually
 * we will be so close to it that we determine it exactly.
 *
 */

#ifdef HAVE_CONFIG_H
#  include "config.h"
#endif

#include "sys.h"
#include "erl_vm.h"
#include "global.h"
#define ERTS_WANT_TIMER_WHEEL_API
#include "erl_time.h"

#define ERTS_MAX_CLKTCKS \
    ERTS_MONOTONIC_TO_CLKTCKS(ERTS_MONOTONIC_TIME_MAX)

#define ERTS_CLKTCKS_WEEK \
    ERTS_MONOTONIC_TO_CLKTCKS(ERTS_SEC_TO_MONOTONIC(7*60*60*24))

#ifdef ERTS_ENABLE_LOCK_CHECK
#define ASSERT_NO_LOCKED_LOCKS		erts_lc_check_exact(NULL, 0)
#else
#define ASSERT_NO_LOCKED_LOCKS
#endif

#if 0
#  define ERTS_TW_HARD_DEBUG
#endif

#if defined(ERTS_TW_HARD_DEBUG) && !defined(ERTS_TW_DEBUG)
#  define ERTS_TW_DEBUG
#endif
#if defined(DEBUG) && !defined(ERTS_TW_DEBUG)
#  define ERTS_TW_DEBUG
#endif

#undef ERTS_TW_ASSERT
#if defined(ERTS_TW_DEBUG) 
#  define ERTS_TW_ASSERT(E) ERTS_ASSERT(E)
#else
#  define ERTS_TW_ASSERT(E) ((void) 1)
#endif

#ifdef ERTS_TW_DEBUG
#  define ERTS_TWHEEL_BUMP_YIELD_LIMIT        500
#else
#  define ERTS_TWHEEL_BUMP_YIELD_LIMIT        10000
#endif
#define ERTS_TW_COST_SLOT                     1
#define ERTS_TW_COST_SLOT_MOVE                5
#define ERTS_TW_COST_TIMEOUT                  100

/*
 * Every slot in the soon wheel is a clock tick (as defined
 * by ERTS) wide. A clock tick is currently 1 milli second.
 */

#define ERTS_TW_SOON_WHEEL_FIRST_SLOT 0
#define ERTS_TW_SOON_WHEEL_END_SLOT \
    (ERTS_TW_SOON_WHEEL_FIRST_SLOT + ERTS_TW_SOON_WHEEL_SIZE)

#define ERTS_TW_SOON_WHEEL_MASK (ERTS_TW_SOON_WHEEL_SIZE-1)

/*
 * Every slot in the later wheel is as wide as half the size
 * of the soon wheel.
 */

#define ERTS_TW_LATER_WHEEL_SHIFT (ERTS_TW_SOON_WHEEL_BITS - 1)
#define ERTS_TW_LATER_WHEEL_SLOT_SIZE \
    ((ErtsMonotonicTime) (1 << ERTS_TW_LATER_WHEEL_SHIFT))
#define ERTS_TW_LATER_WHEEL_POS_MASK \
    (~((ErtsMonotonicTime) (1 << ERTS_TW_LATER_WHEEL_SHIFT)-1))

#define ERTS_TW_LATER_WHEEL_FIRST_SLOT ERTS_TW_SOON_WHEEL_SIZE
#define ERTS_TW_LATER_WHEEL_END_SLOT \
    (ERTS_TW_LATER_WHEEL_FIRST_SLOT + ERTS_TW_LATER_WHEEL_SIZE)

#define ERTS_TW_LATER_WHEEL_MASK (ERTS_TW_LATER_WHEEL_SIZE-1)

#define ERTS_TW_SCNT_BITS 9
#define ERTS_TW_SCNT_SHIFT 
#define ERTS_TW_SCNT_SIZE \
    ((ERTS_TW_SOON_WHEEL_SIZE + ERTS_TW_LATER_WHEEL_SIZE) \
     >> ERTS_TW_SCNT_BITS)

#ifdef __GNUC__
#if ERTS_TW_SOON_WHEEL_BITS < ERTS_TW_SCNT_BITS
#  warning Consider larger soon timer wheel
#endif
#if ERTS_TW_SOON_WHEEL_BITS < ERTS_TW_SCNT_BITS
#  warning Consider larger later timer wheel
#endif
#endif

/* Actual interval time chosen by sys_init_time() */

#if SYS_CLOCK_RESOLUTION == 1
#  define TIW_ITIME 1
#  define TIW_ITIME_IS_CONSTANT
#else
static int tiw_itime; /* Constant after init */
#  define TIW_ITIME tiw_itime
#endif

const int etp_tw_soon_wheel_size = ERTS_TW_SOON_WHEEL_SIZE;
const ErtsMonotonicTime etp_tw_soon_wheel_mask = ERTS_TW_SOON_WHEEL_MASK;
const int etp_tw_soon_wheel_first_slot = ERTS_TW_SOON_WHEEL_FIRST_SLOT;

const int etp_tw_later_wheel_size = ERTS_TW_LATER_WHEEL_SIZE;
const ErtsMonotonicTime etp_tw_later_wheel_slot_size = ERTS_TW_LATER_WHEEL_SLOT_SIZE;
const int etp_tw_later_wheel_shift = ERTS_TW_LATER_WHEEL_SHIFT;
const ErtsMonotonicTime etp_tw_later_wheel_mask = ERTS_TW_LATER_WHEEL_MASK;
const ErtsMonotonicTime etp_tw_later_wheel_pos_mask = ERTS_TW_LATER_WHEEL_POS_MASK;
const int etp_tw_later_wheel_first_slot = ERTS_TW_LATER_WHEEL_FIRST_SLOT;

struct ErtsTimerWheel_ {
    ErtsTWheelTimer *slots[1                            /* At Once Slot */
                           + ERTS_TW_SOON_WHEEL_SIZE    /* Soon Wheel Slots */
                           + ERTS_TW_LATER_WHEEL_SIZE]; /* Later Wheel Slots */
    ErtsTWheelTimer **w;
    Sint scnt[ERTS_TW_SCNT_SIZE];
    Sint bump_scnt[ERTS_TW_SCNT_SIZE];
    ErtsMonotonicTime pos;
    Uint nto;
    struct {
	Uint nto;
    } at_once;
    struct {
        ErtsMonotonicTime min_tpos;
        Uint nto;
    } soon;
    struct {
        ErtsMonotonicTime min_tpos;
        int min_tpos_slot;
        ErtsMonotonicTime pos;
        Uint nto;
    } later;
    int yield_slot;
    int yield_slots_left;
    ErtsTWheelTimer sentinel;
    int true_next_timeout_time;
    ErtsMonotonicTime next_timeout_pos;
    ErtsMonotonicTime next_timeout_time;
};

#define ERTS_TW_SLOT_AT_ONCE (-1)

#define ERTS_TW_BUMP_LATER_WHEEL(TIW) \
    ((tiw)->pos + ERTS_TW_LATER_WHEEL_SLOT_SIZE >= (TIW)->later.pos)

static int bump_later_wheel(ErtsTimerWheel *tiw, int *yield_count_p);

#ifdef ERTS_TW_DEBUG
#define ERTS_TW_DBG_VERIFY_EMPTY_SOON_SLOTS(TIW, TO_POS) \
    dbg_verify_empty_soon_slots((TIW), (TO_POS))
#define ERTS_TW_DBG_VERIFY_EMPTY_LATER_SLOTS(TIW, TO_POS) \
    dbg_verify_empty_later_slots((TIW), (TO_POS))
void dbg_verify_empty_soon_slots(ErtsTimerWheel *, ErtsMonotonicTime);
void dbg_verify_empty_later_slots(ErtsTimerWheel *, ErtsMonotonicTime);
#else
#define ERTS_TW_DBG_VERIFY_EMPTY_SOON_SLOTS(TIW, TO_POS)
#define ERTS_TW_DBG_VERIFY_EMPTY_LATER_SLOTS(TIW, TO_POS)
#endif

static ERTS_INLINE int
scnt_get_ix(int slot)
{
    return slot >> ERTS_TW_SCNT_BITS;
}

static ERTS_INLINE void
scnt_inc(Sint *scnt, int slot)
{
    scnt[slot >> ERTS_TW_SCNT_BITS]++;
}

#ifdef ERTS_TW_HARD_DEBUG

static ERTS_INLINE void
scnt_ix_inc(Sint *scnt, int six)
{
    scnt[six]++;
}

#endif

static ERTS_INLINE void
scnt_dec(Sint *scnt, int slot)
{
    scnt[slot >> ERTS_TW_SCNT_BITS]--;
    ERTS_TW_ASSERT(scnt[slot >> ERTS_TW_SCNT_BITS] >= 0);
}

static ERTS_INLINE void
scnt_ix_dec(Sint *scnt, int six)
{
    scnt[six]--;
    ERTS_TW_ASSERT(scnt[six] >= 0);
}

static ERTS_INLINE void
scnt_wheel_next(int *slotp, int *leftp, ErtsMonotonicTime *posp,
                int *sixp, Sint *scnt, int first_slot,
                int end_slot, ErtsMonotonicTime slot_sz)
{
    int slot = *slotp;
    int left = *leftp;
    int ix;

    ERTS_TW_ASSERT(*leftp >= 0);

    left--;
    slot++;
    if (slot == end_slot)
        slot = first_slot;
    ix = slot >> ERTS_TW_SCNT_BITS;

    while (!scnt[ix] && left > 0) {
        int diff, old_slot = slot;
        ix++;
        slot = (ix << ERTS_TW_SCNT_BITS);
        diff = slot - old_slot;
        if (left < diff) {
            slot = old_slot + left;
            diff = left;
        }
        if (slot < end_slot)
            left -= diff;
        else {
            left -= end_slot - old_slot;
            slot = first_slot;
            ix = slot >> ERTS_TW_SCNT_BITS;
        }
    }

    ERTS_TW_ASSERT(left >= -1);

    if (posp)
        *posp += slot_sz * ((ErtsMonotonicTime) (*leftp - left));
    if (sixp)
        *sixp = slot >> ERTS_TW_SCNT_BITS;
    *leftp = left;
    *slotp = slot;
}


static ERTS_INLINE void
scnt_soon_wheel_next(int *slotp, int *leftp, ErtsMonotonicTime *posp,
                    int *sixp, Sint *scnt)
{
    scnt_wheel_next(slotp, leftp, posp, sixp, scnt,
                    ERTS_TW_SOON_WHEEL_FIRST_SLOT,
                    ERTS_TW_SOON_WHEEL_END_SLOT, 1);
}

static ERTS_INLINE void
scnt_later_wheel_next(int *slotp, int *leftp, ErtsMonotonicTime *posp,
                    int *sixp, Sint *scnt)
{
    scnt_wheel_next(slotp, leftp, posp, sixp, scnt,
                    ERTS_TW_LATER_WHEEL_FIRST_SLOT,
                    ERTS_TW_LATER_WHEEL_END_SLOT,
                    ERTS_TW_LATER_WHEEL_SLOT_SIZE);
}


static ERTS_INLINE int
soon_slot(ErtsMonotonicTime soon_pos)
{
    ErtsMonotonicTime slot = soon_pos;
    slot &= ERTS_TW_SOON_WHEEL_MASK;

    ERTS_TW_ASSERT(ERTS_TW_SOON_WHEEL_FIRST_SLOT <= slot);
    ERTS_TW_ASSERT(slot < ERTS_TW_SOON_WHEEL_END_SLOT);

    return (int) slot;
}

static ERTS_INLINE int
later_slot(ErtsMonotonicTime later_pos)
{
    ErtsMonotonicTime slot = later_pos;
    slot >>= ERTS_TW_LATER_WHEEL_SHIFT;
    slot &= ERTS_TW_LATER_WHEEL_MASK;
    slot += ERTS_TW_LATER_WHEEL_FIRST_SLOT;

    ERTS_TW_ASSERT(ERTS_TW_LATER_WHEEL_FIRST_SLOT <= slot);
    ERTS_TW_ASSERT(slot < ERTS_TW_LATER_WHEEL_END_SLOT);

    return (int) slot;
}

#ifdef ERTS_TW_HARD_DEBUG
#define ERTS_HARD_DBG_CHK_WHEELS(TIW, CHK_MIN_TPOS) \
    hrd_dbg_check_wheels((TIW), (CHK_MIN_TPOS))
static void hrd_dbg_check_wheels(ErtsTimerWheel *tiw, int check_min_tpos);
#else
#define ERTS_HARD_DBG_CHK_WHEELS(TIW, CHK_MIN_TPOS)
#endif

static ErtsMonotonicTime
find_next_timeout(ErtsSchedulerData *esdp, ErtsTimerWheel *tiw)
{
    int slot, slots;
    int true_min_timeout = 0;
    ErtsMonotonicTime min_timeout_pos;

    ERTS_TW_ASSERT(tiw->pos + ERTS_TW_LATER_WHEEL_SLOT_SIZE < tiw->later.pos
                   && tiw->later.pos <= tiw->pos + ERTS_TW_SOON_WHEEL_SIZE);

    ERTS_HARD_DBG_CHK_WHEELS(tiw, 0);

    ERTS_TW_ASSERT(tiw->yield_slot == ERTS_TW_SLOT_INACTIVE);

    if (tiw->nto == 0) { /* no timeouts in wheel */
        ErtsMonotonicTime curr_time = erts_get_monotonic_time(esdp);
        tiw->pos = min_timeout_pos = ERTS_MONOTONIC_TO_CLKTCKS(curr_time);
        tiw->later.pos = min_timeout_pos + ERTS_TW_SOON_WHEEL_SIZE;
        tiw->later.pos &= ERTS_TW_LATER_WHEEL_POS_MASK;
	min_timeout_pos += ERTS_CLKTCKS_WEEK;
	goto done;
    }

    ERTS_TW_ASSERT(tiw->soon.nto || tiw->later.nto);

    if (!tiw->soon.nto) {
        ErtsMonotonicTime tpos, min_tpos;

        /* Search later wheel... */

        min_tpos = tiw->later.min_tpos & ERTS_TW_LATER_WHEEL_POS_MASK;

        if (min_tpos <= tiw->later.pos) {
            tpos = tiw->later.pos;
            slots = ERTS_TW_LATER_WHEEL_SIZE;
        }
        else {
            ErtsMonotonicTime tmp;
            /* Don't inspect slots we know are empty... */
            tmp = min_tpos - tiw->later.pos;
            tmp /= ERTS_TW_LATER_WHEEL_SLOT_SIZE;
            if (tmp >= ERTS_TW_LATER_WHEEL_SIZE) {
                /* Timeout more than one revolution ahead... */

                /* Pre-timeout for move from later to soon wheel... */
                min_timeout_pos = min_tpos - ERTS_TW_LATER_WHEEL_SLOT_SIZE;
                goto done;
            }
            tpos = min_tpos;
            ERTS_TW_DBG_VERIFY_EMPTY_LATER_SLOTS(tiw, min_tpos);
            slots = ERTS_TW_LATER_WHEEL_SIZE - ((int) tmp);
        }

        slot = later_slot(tpos);

        /*
         * We never search for an exact timeout in the
         * later wheel, but instead settle for the first
         * scnt range used.
         */
        if (tiw->w[slot])
            true_min_timeout = 1;
        else
            scnt_later_wheel_next(&slot, &slots, &tpos, NULL, tiw->scnt);

        tiw->later.min_tpos = tpos;
        tiw->later.min_tpos_slot = slot;
        ERTS_TW_ASSERT(slot == later_slot(tpos));

        /* Pre-timeout for move from later to soon wheel... */
        tpos -= ERTS_TW_LATER_WHEEL_SLOT_SIZE;
        min_timeout_pos = tpos;
    }
    else {
        ErtsMonotonicTime tpos;
        /* Search soon wheel... */

        min_timeout_pos = tiw->pos + ERTS_TW_SOON_WHEEL_SIZE;

        /*
         * Besides inspecting the soon wheel we
         * may also have to inspect two slots in the
         * later wheel which potentially can trigger
         * timeouts before timeouts in soon wheel...
         */
        if (tiw->later.min_tpos > (tiw->later.pos
                                   + 2*ERTS_TW_LATER_WHEEL_SLOT_SIZE)) {
            ERTS_TW_DBG_VERIFY_EMPTY_LATER_SLOTS(
                tiw, 2*ERTS_TW_LATER_WHEEL_SLOT_SIZE);
        }
        else {
            int fslot;
            tpos = tiw->later.pos;
            tpos -= ERTS_TW_LATER_WHEEL_SLOT_SIZE;
            fslot = later_slot(tiw->later.pos);
            if (tiw->w[fslot])
                min_timeout_pos = tpos;
            else {
                tpos += ERTS_TW_LATER_WHEEL_SLOT_SIZE;
                if (tpos < min_timeout_pos) {
                    fslot++;
                    if (fslot == ERTS_TW_LATER_WHEEL_END_SLOT)
                        fslot = ERTS_TW_LATER_WHEEL_FIRST_SLOT;
                    if (tiw->w[fslot])
                        min_timeout_pos = tpos;
                }
            }
        }

        if (tiw->soon.min_tpos <= tiw->pos) {
            tpos = tiw->pos;
            slots = ERTS_TW_SOON_WHEEL_SIZE;
        }
        else {
            ErtsMonotonicTime tmp;
            /* Don't inspect slots we know are empty... */
            tmp = tiw->soon.min_tpos - tiw->pos;
            ERTS_TW_ASSERT(ERTS_TW_SOON_WHEEL_SIZE > tmp);
            ERTS_TW_DBG_VERIFY_EMPTY_SOON_SLOTS(tiw, tiw->soon.min_tpos);
            slots = ERTS_TW_SOON_WHEEL_SIZE - ((int) tmp);
            tpos = tiw->soon.min_tpos;
        }

        slot = soon_slot(tpos);

        /* find next non-empty slot */
        while (tpos < min_timeout_pos) {
            if (tiw->w[slot]) {
                ERTS_TW_ASSERT(tiw->w[slot]->timeout_pos == tpos);
                min_timeout_pos = tpos;
                break;
            }
            scnt_soon_wheel_next(&slot, &slots, &tpos, NULL, tiw->scnt);
        }

        tiw->soon.min_tpos = min_timeout_pos;
        true_min_timeout = 1;
    }

done: {
        ErtsMonotonicTime min_timeout;

        min_timeout = ERTS_CLKTCKS_TO_MONOTONIC(min_timeout_pos);
        tiw->next_timeout_pos = min_timeout_pos;
        tiw->next_timeout_time = min_timeout;
        tiw->true_next_timeout_time = true_min_timeout;

        ERTS_HARD_DBG_CHK_WHEELS(tiw, 1);

        return min_timeout;
    }
}

static ERTS_INLINE void
insert_timer_into_slot(ErtsTimerWheel *tiw, int slot, ErtsTWheelTimer *p)
{
    ERTS_TW_ASSERT(ERTS_TW_SLOT_AT_ONCE <= slot
                   && slot < ERTS_TW_LATER_WHEEL_END_SLOT);
    p->slot = slot;
    if (!tiw->w[slot]) {
	tiw->w[slot] = p;
	p->next = p;
	p->prev = p;
    }
    else {
	ErtsTWheelTimer *next, *prev;
	next = tiw->w[slot];
	prev = next->prev;
	p->next = next;
	p->prev = prev;
	prev->next = p;
	next->prev = p;
    }
    if (slot == ERTS_TW_SLOT_AT_ONCE)
	tiw->at_once.nto++;
    else {
        ErtsMonotonicTime tpos = p->timeout_pos;
        if (slot < ERTS_TW_SOON_WHEEL_END_SLOT) {
            ERTS_TW_ASSERT(p->timeout_pos < tiw->pos + ERTS_TW_SOON_WHEEL_SIZE);
            tiw->soon.nto++;
            if (tiw->soon.min_tpos > tpos)
                tiw->soon.min_tpos = tpos;
        }
        else {
            ERTS_TW_ASSERT(p->timeout_pos >= tiw->pos + ERTS_TW_SOON_WHEEL_SIZE);
            tiw->later.nto++;
            if (tiw->later.min_tpos > tpos) {
                tiw->later.min_tpos = tpos;
                tiw->later.min_tpos_slot = slot;
            }
        }
        scnt_inc(tiw->scnt, slot);
    }
}

static ERTS_INLINE void
remove_timer(ErtsTimerWheel *tiw, ErtsTWheelTimer *p)
{
    int slot = p->slot;
    int empty_slot;
    ERTS_TW_ASSERT(slot != ERTS_TW_SLOT_INACTIVE);

    /*
     * Timer is in circular list either referred to
     * by at once slot, slot in soon wheel, slot
     * in later wheel, or by sentinel (timers currently
     * being triggered).
     */
    ERTS_TW_ASSERT(ERTS_TW_SLOT_AT_ONCE <= slot
                   && slot < ERTS_TW_LATER_WHEEL_END_SLOT);

    if (p->next == p) {
        /* Cannot be referred by sentinel, i.e. must be referred by slot... */
        ERTS_TW_ASSERT(tiw->w[slot] == p);
        tiw->w[slot] = NULL;
        empty_slot = 1;
    }
    else {
        if (tiw->w[slot] == p)
            tiw->w[slot] = p->next;
        p->prev->next = p->next;
        p->next->prev = p->prev;
        empty_slot = 0;
    }
    if (slot == ERTS_TW_SLOT_AT_ONCE) {
	ERTS_TW_ASSERT(tiw->at_once.nto > 0);
	tiw->at_once.nto--;
    }
    else {
        scnt_dec(tiw->scnt, slot);
        if (slot < ERTS_TW_SOON_WHEEL_END_SLOT) {
            if (empty_slot
                && tiw->true_next_timeout_time
                && p->timeout_pos == tiw->next_timeout_pos) {
                tiw->true_next_timeout_time = 0;
            }
            if (--tiw->soon.nto == 0)
                tiw->soon.min_tpos = ERTS_MAX_CLKTCKS;
        }
        else {
            if (empty_slot
                && tiw->true_next_timeout_time
                && tiw->later.min_tpos_slot == slot) {
                ErtsMonotonicTime tpos = tiw->later.min_tpos;
                tpos &= ERTS_TW_LATER_WHEEL_POS_MASK;
                tpos -= ERTS_TW_LATER_WHEEL_SLOT_SIZE;
                if (tpos == tiw->next_timeout_pos)
                    tiw->true_next_timeout_time = 0;
            }
            if (--tiw->later.nto == 0) {
                tiw->later.min_tpos = ERTS_MAX_CLKTCKS;
                tiw->later.min_tpos_slot = ERTS_TW_LATER_WHEEL_END_SLOT;
            }
        }
    }
    p->slot = ERTS_TW_SLOT_INACTIVE;
}

ErtsMonotonicTime
erts_check_next_timeout_time(ErtsSchedulerData *esdp)
{
    ErtsTimerWheel *tiw = esdp->timer_wheel;
    ErtsMonotonicTime time;
    ERTS_MSACC_DECLARE_CACHE_X();
    ERTS_TW_ASSERT(tiw->next_timeout_time
                   == ERTS_CLKTCKS_TO_MONOTONIC(tiw->next_timeout_pos));
    if (tiw->true_next_timeout_time)
	return tiw->next_timeout_time; /* known timeout... */
    if (tiw->next_timeout_pos > tiw->pos + ERTS_TW_SOON_WHEEL_SIZE)
        return tiw->next_timeout_time; /* sufficiently later away... */
    ERTS_MSACC_PUSH_AND_SET_STATE_CACHED_X(ERTS_MSACC_STATE_TIMERS);
    time = find_next_timeout(esdp, tiw);
    ERTS_MSACC_POP_STATE_M_X();
    return time;
}

static ERTS_INLINE void
timeout_timer(ErtsTWheelTimer *p)
{
    ErlTimeoutProc timeout;
    void *arg;
    p->slot = ERTS_TW_SLOT_INACTIVE;
    timeout = p->timeout;
    arg = p->arg;
    (*timeout)(arg);
    ASSERT_NO_LOCKED_LOCKS;
}

void
erts_bump_timers(ErtsTimerWheel *tiw, ErtsMonotonicTime curr_time)
{
    int slot, restarted, yield_count, slots, scnt_ix;
    ErtsMonotonicTime bump_to;
    Sint *scnt, *bump_scnt;
    ERTS_MSACC_PUSH_AND_SET_STATE_M_X(ERTS_MSACC_STATE_TIMERS);

    yield_count = ERTS_TWHEEL_BUMP_YIELD_LIMIT;

    scnt = &tiw->scnt[0];
    bump_scnt = &tiw->bump_scnt[0];

    /*
     * In order to be fair we always continue with work
     * where we left off when restarting after a yield.
     */

    slot = tiw->yield_slot;
    restarted = slot != ERTS_TW_SLOT_INACTIVE;
    if (restarted) {
	bump_to = tiw->pos;
        if (slot >= ERTS_TW_LATER_WHEEL_FIRST_SLOT)
            goto restart_yielded_later_slot;
        tiw->yield_slot = ERTS_TW_SLOT_INACTIVE;
        if (slot == ERTS_TW_SLOT_AT_ONCE)
            goto restart_yielded_at_once_slot;
        scnt_ix = scnt_get_ix(slot);
	slots = tiw->yield_slots_left;
        ASSERT(0 <= slots && slots <= ERTS_TW_SOON_WHEEL_SIZE);
        goto restart_yielded_soon_slot;
    }

    do {

        restarted = 0;
	bump_to = ERTS_MONOTONIC_TO_CLKTCKS(curr_time);
        tiw->true_next_timeout_time = 1;
        tiw->next_timeout_pos = bump_to;
        tiw->next_timeout_time = ERTS_CLKTCKS_TO_MONOTONIC(bump_to);

	while (1) {
	    ErtsTWheelTimer *p;

	    if (tiw->nto == 0) {
	    empty_wheel:
                ERTS_TW_DBG_VERIFY_EMPTY_SOON_SLOTS(tiw, bump_to);
                ERTS_TW_DBG_VERIFY_EMPTY_LATER_SLOTS(tiw, bump_to);
		tiw->true_next_timeout_time = 0;
                tiw->next_timeout_pos = bump_to + ERTS_CLKTCKS_WEEK;
		tiw->next_timeout_time = ERTS_CLKTCKS_TO_MONOTONIC(tiw->next_timeout_pos);;
		tiw->pos = bump_to;
                tiw->later.pos = bump_to + ERTS_TW_SOON_WHEEL_SIZE;
                tiw->later.pos &= ERTS_TW_LATER_WHEEL_POS_MASK;
		tiw->yield_slot = ERTS_TW_SLOT_INACTIVE;
                ERTS_MSACC_POP_STATE_M_X();
		return;
	    }

	    p = tiw->w[ERTS_TW_SLOT_AT_ONCE];

	    if (p) {

                if (p->next == p) {
                    ERTS_TW_ASSERT(tiw->sentinel.next == &tiw->sentinel);
                    ERTS_TW_ASSERT(tiw->sentinel.prev == &tiw->sentinel);
                }
                else {
                    tiw->sentinel.next = p->next;
                    tiw->sentinel.prev = p->prev;
                    tiw->sentinel.next->prev = &tiw->sentinel;
                    tiw->sentinel.prev->next = &tiw->sentinel;
                }
                tiw->w[ERTS_TW_SLOT_AT_ONCE] = NULL;

                while (1) {
                    ERTS_TW_ASSERT(tiw->nto > 0);
                    ERTS_TW_ASSERT(tiw->at_once.nto > 0);
                    tiw->nto--;
                    tiw->at_once.nto--;

                    timeout_timer(p);

                    yield_count -= ERTS_TW_COST_TIMEOUT;

                restart_yielded_at_once_slot:

                    p = tiw->sentinel.next;
                    if (p == &tiw->sentinel) {
                        ERTS_TW_ASSERT(tiw->sentinel.prev == &tiw->sentinel);
                        break;
                    }

                    if (yield_count <= 0) {
                        ERTS_TW_ASSERT(tiw->nto > 0);
                        ERTS_TW_ASSERT(tiw->at_once.nto > 0);
                        tiw->yield_slot = ERTS_TW_SLOT_AT_ONCE;
                        ERTS_MSACC_POP_STATE_M_X();
                        return; /* Yield! */
                    }

                    tiw->sentinel.next = p->next;
                    p->next->prev = &tiw->sentinel;
                }

	    }

	    if (tiw->pos >= bump_to) {
                ERTS_MSACC_POP_STATE_M_X();
		break;
            }

	    if (tiw->nto == 0)
		goto empty_wheel;

            /*
             * Save slot counts in bump operation local
             * array.
             *
             * The amount of timers to trigger (or move)
             * will only decrease from now until we have
             * completed this bump operation (even if we
             * yield in the middle of it).
             *
             * The amount of timers in the wheels may
             * however increase due to timers being set
             * by timeout callbacks.
             */
            sys_memcpy((void *) bump_scnt, (void *) scnt,
                       sizeof(Sint) * ERTS_TW_SCNT_SIZE);

	    if (tiw->soon.min_tpos > tiw->pos) {
		ErtsMonotonicTime skip_until_pos = tiw->soon.min_tpos;

		/*
		 * No need inspecting slots where we know no timeouts
		 * to trigger should reside.
		 */

		if (skip_until_pos > bump_to)
		    skip_until_pos = bump_to;

		skip_until_pos--;

		if (skip_until_pos > tiw->pos) {
                    ERTS_TW_DBG_VERIFY_EMPTY_SOON_SLOTS(tiw, skip_until_pos);
		    tiw->pos = skip_until_pos;
		}
	    }

            {
                ErtsMonotonicTime tmp_slots = bump_to - tiw->pos;
                tmp_slots = (bump_to - tiw->pos);
                if (tmp_slots < ERTS_TW_SOON_WHEEL_SIZE)
                    slots = (int) tmp_slots;
                else
                    slots = ERTS_TW_SOON_WHEEL_SIZE;
            }

            slot = soon_slot(tiw->pos+1);
	    tiw->pos = bump_to;

            tiw->next_timeout_pos = bump_to;
            tiw->next_timeout_time = ERTS_CLKTCKS_TO_MONOTONIC(bump_to);

            scnt_ix = scnt_get_ix(slot);

            /* Timeout timers in soon wheel */
	    while (slots > 0) {

                yield_count -= ERTS_TW_COST_SLOT;

		p = tiw->w[slot];
		if (p) {
                    /* timeout callback need tiw->pos to be up to date */
		    if (p->next == p) {
			ERTS_TW_ASSERT(tiw->sentinel.next == &tiw->sentinel);
			ERTS_TW_ASSERT(tiw->sentinel.prev == &tiw->sentinel);
		    }
		    else {
			tiw->sentinel.next = p->next;
			tiw->sentinel.prev = p->prev;
			tiw->sentinel.next->prev = &tiw->sentinel;
			tiw->sentinel.prev->next = &tiw->sentinel;
		    }
		    tiw->w[slot] = NULL;

		    while (1) {

                        ERTS_TW_ASSERT(ERTS_TW_SOON_WHEEL_FIRST_SLOT <= p->slot
                                       && p->slot < ERTS_TW_SOON_WHEEL_END_SLOT);
                        if (--tiw->soon.nto == 0)
                            tiw->soon.min_tpos = ERTS_MAX_CLKTCKS;
                        scnt_ix_dec(scnt, scnt_ix);
                        if (p->timeout_pos <= bump_to) {
                            timeout_timer(p);
                            tiw->nto--;
                            scnt_ix_dec(bump_scnt, scnt_ix);
                            yield_count -= ERTS_TW_COST_TIMEOUT;
                        }
                        else {
                            /* uncommon case */
                            insert_timer_into_slot(tiw, slot, p);
                            yield_count -= ERTS_TW_COST_SLOT_MOVE;
                        }

		    restart_yielded_soon_slot:

			p = tiw->sentinel.next;
			if (p == &tiw->sentinel) {
			    ERTS_TW_ASSERT(tiw->sentinel.prev == &tiw->sentinel);
			    break;
			}

			if (yield_count <= 0) {
			    tiw->yield_slot = slot;
			    tiw->yield_slots_left = slots;
                            ERTS_MSACC_POP_STATE_M_X();
			    return; /* Yield! */
			}

			tiw->sentinel.next = p->next;
			p->next->prev = &tiw->sentinel;
		    }
		}

                scnt_soon_wheel_next(&slot, &slots, NULL, &scnt_ix, bump_scnt);
	    }

            if (ERTS_TW_BUMP_LATER_WHEEL(tiw)) {
            restart_yielded_later_slot:
                if (bump_later_wheel(tiw, &yield_count))
                    return; /* Yield! */
            }
	}

    } while (restarted);

    tiw->true_next_timeout_time = 0;
    ERTS_TW_ASSERT(tiw->next_timeout_pos == bump_to);

    (void) find_next_timeout(NULL, tiw);
    ERTS_MSACC_POP_STATE_M_X();
}

static int
bump_later_wheel(ErtsTimerWheel *tiw, int *ycount_p)
{
    ErtsMonotonicTime cpos = tiw->pos;
    ErtsMonotonicTime later_pos = tiw->later.pos;
    int ycount = *ycount_p;
    int slots, fslot, scnt_ix;
    Sint *scnt, *bump_scnt;

    scnt = &tiw->scnt[0];
    bump_scnt = &tiw->bump_scnt[0];

    ERTS_HARD_DBG_CHK_WHEELS(tiw, 0);

    if (tiw->yield_slot >= ERTS_TW_LATER_WHEEL_FIRST_SLOT) {
        fslot = tiw->yield_slot;
        scnt_ix = scnt_get_ix(fslot);
        slots = tiw->yield_slots_left;
        ASSERT(0 <= slots && slots <= ERTS_TW_LATER_WHEEL_SIZE);
        tiw->yield_slot = ERTS_TW_SLOT_INACTIVE;
        goto restart_yielded_slot;
    }
    else {
        ErtsMonotonicTime end_later_pos, tmp_slots, min_tpos;

        min_tpos = tiw->later.min_tpos & ERTS_TW_LATER_WHEEL_POS_MASK;
        end_later_pos = cpos + ERTS_TW_SOON_WHEEL_SIZE;
        end_later_pos &= ERTS_TW_LATER_WHEEL_POS_MASK;

        /* Skip known empty slots... */
        if (min_tpos > later_pos) {
            if (min_tpos > end_later_pos) {
                ERTS_TW_DBG_VERIFY_EMPTY_LATER_SLOTS(tiw, end_later_pos);
                tiw->later.pos = end_later_pos;
                goto done;
            }
            later_pos = min_tpos;
            ERTS_TW_DBG_VERIFY_EMPTY_LATER_SLOTS(tiw, later_pos);
        }

        tmp_slots = end_later_pos;
        tmp_slots -= later_pos;
        tmp_slots /= ERTS_TW_LATER_WHEEL_SLOT_SIZE;
        if (tmp_slots < ERTS_TW_LATER_WHEEL_SIZE)
            slots = (int) tmp_slots;
        else
            slots = ERTS_TW_LATER_WHEEL_SIZE;

        fslot = later_slot(later_pos);
        scnt_ix = scnt_get_ix(fslot);

        tiw->later.pos = end_later_pos;
    }

    while (slots > 0) {
        ErtsTWheelTimer *p;

        ycount -= ERTS_TW_COST_SLOT;

        p = tiw->w[fslot];

        if (p) {

            if (p->next == p) {
                ERTS_TW_ASSERT(tiw->sentinel.next == &tiw->sentinel);
                ERTS_TW_ASSERT(tiw->sentinel.prev == &tiw->sentinel);
            }
            else {
                tiw->sentinel.next = p->next;
                tiw->sentinel.prev = p->prev;
                tiw->sentinel.next->prev = &tiw->sentinel;
                tiw->sentinel.prev->next = &tiw->sentinel;
            }
            tiw->w[fslot] = NULL;

            while (1) {
                ErtsMonotonicTime tpos = p->timeout_pos;

                ERTS_TW_ASSERT(p->slot == fslot);

                if (--tiw->later.nto == 0) {
                    tiw->later.min_tpos = ERTS_MAX_CLKTCKS;
                    tiw->later.min_tpos_slot = ERTS_TW_LATER_WHEEL_END_SLOT;
                }
                scnt_ix_dec(scnt, scnt_ix);

                if (tpos >= tiw->later.pos + ERTS_TW_LATER_WHEEL_SLOT_SIZE) {
                    /* keep in later slot; very uncommon... */
                    insert_timer_into_slot(tiw, fslot, p);
                    ycount -= ERTS_TW_COST_SLOT_MOVE;
                }
                else {
                    scnt_ix_dec(bump_scnt, scnt_ix);
                    ERTS_TW_ASSERT(tpos < cpos + ERTS_TW_SOON_WHEEL_SIZE);
                    if (tpos > cpos) {
                        /* move into soon wheel */
                        insert_timer_into_slot(tiw, soon_slot(tpos), p);
                        ycount -= ERTS_TW_COST_SLOT_MOVE;
                    }
                    else {
                        /* trigger at once */
                        timeout_timer(p);
                        tiw->nto--;
                        ycount -= ERTS_TW_COST_TIMEOUT;
                    }
                }

            restart_yielded_slot:

                p = tiw->sentinel.next;
                if (p == &tiw->sentinel) {
                    ERTS_TW_ASSERT(tiw->sentinel.prev == &tiw->sentinel);
                    break;
                }

                if (ycount < 0) {
                    tiw->yield_slot = fslot;
                    tiw->yield_slots_left = slots;
                    *ycount_p = 0;
                    ERTS_HARD_DBG_CHK_WHEELS(tiw, 0);
                    return 1; /* Yield! */
                }

                tiw->sentinel.next = p->next;
                p->next->prev = &tiw->sentinel;
            }
        }

        scnt_later_wheel_next(&fslot, &slots, NULL, &scnt_ix, bump_scnt);
    }

done:

    ERTS_HARD_DBG_CHK_WHEELS(tiw, 0);

    *ycount_p = ycount;

    return 0;
}

Uint
erts_timer_wheel_memory_size(void)
{
    return sizeof(ErtsTimerWheel)*erts_no_schedulers;
}

ErtsTimerWheel *
erts_create_timer_wheel(ErtsSchedulerData *esdp)
{
    ErtsMonotonicTime mtime;
    int i;
    ErtsTimerWheel *tiw;

    /* Some compile time sanity checks... */

    /* Slots... */
    ERTS_CT_ASSERT(ERTS_TW_SLOT_AT_ONCE == -1);
    ERTS_CT_ASSERT(ERTS_TW_SLOT_INACTIVE < ERTS_TW_SLOT_AT_ONCE);
    ERTS_CT_ASSERT(ERTS_TW_SLOT_AT_ONCE + 1 == ERTS_TW_SOON_WHEEL_FIRST_SLOT);
    ERTS_CT_ASSERT(ERTS_TW_SOON_WHEEL_FIRST_SLOT < ERTS_TW_SOON_WHEEL_END_SLOT);
    ERTS_CT_ASSERT(ERTS_TW_SOON_WHEEL_END_SLOT == ERTS_TW_LATER_WHEEL_FIRST_SLOT);    
    ERTS_CT_ASSERT(ERTS_TW_LATER_WHEEL_FIRST_SLOT < ERTS_TW_LATER_WHEEL_END_SLOT);

    /* Both wheel sizes should be a powers of 2 */
    ERTS_CT_ASSERT(ERTS_TW_SOON_WHEEL_SIZE
                   && !(ERTS_TW_SOON_WHEEL_SIZE & (ERTS_TW_SOON_WHEEL_SIZE-1)));
    ERTS_CT_ASSERT(ERTS_TW_LATER_WHEEL_SIZE
                   && !(ERTS_TW_LATER_WHEEL_SIZE & (ERTS_TW_LATER_WHEEL_SIZE-1)));

    tiw = erts_alloc_permanent_cache_aligned(ERTS_ALC_T_TIMER_WHEEL,
					     sizeof(ErtsTimerWheel));
    tiw->w = &tiw->slots[1];
    for(i = ERTS_TW_SLOT_AT_ONCE; i < ERTS_TW_LATER_WHEEL_END_SLOT; i++)
	tiw->w[i] = NULL;

    for (i = 0; i < ERTS_TW_SCNT_SIZE; i++)
        tiw->scnt[i] = 0;

    mtime = erts_get_monotonic_time(esdp);
    tiw->pos = ERTS_MONOTONIC_TO_CLKTCKS(mtime);
    tiw->nto = 0;
    tiw->at_once.nto = 0;
    tiw->soon.min_tpos = ERTS_MAX_CLKTCKS;
    tiw->soon.nto = 0;
    tiw->later.min_tpos = ERTS_MAX_CLKTCKS;
    tiw->later.min_tpos_slot = ERTS_TW_LATER_WHEEL_END_SLOT;
    tiw->later.pos = tiw->pos + ERTS_TW_SOON_WHEEL_SIZE;
    tiw->later.pos &= ERTS_TW_LATER_WHEEL_POS_MASK;
    tiw->later.nto = 0;
    tiw->yield_slot = ERTS_TW_SLOT_INACTIVE;
    tiw->true_next_timeout_time = 0;
    tiw->next_timeout_pos = tiw->pos + ERTS_CLKTCKS_WEEK;
    tiw->next_timeout_time = ERTS_CLKTCKS_TO_MONOTONIC(tiw->next_timeout_pos);
    tiw->sentinel.next = &tiw->sentinel;
    tiw->sentinel.prev = &tiw->sentinel;
    tiw->sentinel.timeout = NULL;
    tiw->sentinel.arg = NULL;
    return tiw;
}

ErtsNextTimeoutRef
erts_get_next_timeout_reference(ErtsTimerWheel *tiw)
{
    return (ErtsNextTimeoutRef) &tiw->next_timeout_time;
}


/* this routine links the time cells into a free list at the start
   and sets the time queue as empty */
void
erts_init_time(int time_correction, ErtsTimeWarpMode time_warp_mode)
{
    int itime;

    /* system dependent init; must be done before do_time_init()
       if timer thread is enabled */
    itime = erts_init_time_sup(time_correction, time_warp_mode);
#ifdef TIW_ITIME_IS_CONSTANT 
    if (itime != TIW_ITIME) {
	erts_exit(ERTS_ABORT_EXIT, "timer resolution mismatch %d != %d", itime, TIW_ITIME);
    }
#else
    tiw_itime = itime;
#endif
}

void
erts_twheel_set_timer(ErtsTimerWheel *tiw,
		      ErtsTWheelTimer *p, ErlTimeoutProc timeout,
		      void *arg, ErtsMonotonicTime timeout_pos)
{
    int slot;
    ERTS_MSACC_PUSH_AND_SET_STATE_M_X(ERTS_MSACC_STATE_TIMERS);

    p->timeout = timeout;
    p->arg = arg;

    ERTS_TW_ASSERT(p->slot == ERTS_TW_SLOT_INACTIVE);

    tiw->nto++;

    /* calculate slot */
    if (timeout_pos <= tiw->pos) {
        /* at once */
        p->timeout_pos = timeout_pos = tiw->pos;
        slot = ERTS_TW_SLOT_AT_ONCE;
    }
    else if (timeout_pos < tiw->pos + ERTS_TW_SOON_WHEEL_SIZE) {
        /* soon wheel */
        p->timeout_pos = timeout_pos;
        slot = soon_slot(timeout_pos);
        if (tiw->soon.min_tpos > timeout_pos)
            tiw->soon.min_tpos = timeout_pos;
    }
    else {
        /* later wheel */
        p->timeout_pos = timeout_pos;
        slot = later_slot(timeout_pos);

        /*
         * Next timeout due to this timeout
         * should be in good time before the
         * actual timeout (one later wheel slot
         * size). This, in order to move it
         * from the later wheel to the soon
         * wheel.
         */
        timeout_pos &= ERTS_TW_LATER_WHEEL_POS_MASK;
        timeout_pos -= ERTS_TW_LATER_WHEEL_SLOT_SIZE;
    }

    insert_timer_into_slot(tiw, slot, p);

    if (timeout_pos <= tiw->next_timeout_pos) {
	tiw->true_next_timeout_time = 1;
        if (timeout_pos < tiw->next_timeout_pos) {
            tiw->next_timeout_pos = timeout_pos;
            tiw->next_timeout_time = ERTS_CLKTCKS_TO_MONOTONIC(timeout_pos);
        }
    }
    ERTS_MSACC_POP_STATE_M_X();
}

void
erts_twheel_cancel_timer(ErtsTimerWheel *tiw, ErtsTWheelTimer *p)
{
    if (p->slot != ERTS_TW_SLOT_INACTIVE) {
        ERTS_MSACC_PUSH_AND_SET_STATE_M_X(ERTS_MSACC_STATE_TIMERS);
	remove_timer(tiw, p);
        tiw->nto--;
        ERTS_MSACC_POP_STATE_M_X();
    }
}

void
erts_twheel_debug_foreach(ErtsTimerWheel *tiw,
			  void (*tclbk)(void *),
			  void (*func)(void *,
				       ErtsMonotonicTime,
				       void *),
			  void *arg)
{
    ErtsTWheelTimer *tmr;
    int ix;

    tmr = tiw->sentinel.next;
    while (tmr != &tiw->sentinel) {
	if (tmr->timeout == tclbk)
	    (*func)(arg, tmr->timeout_pos, tmr->arg);
	tmr = tmr->next;
    }

    for (ix = ERTS_TW_SLOT_AT_ONCE; ix < ERTS_TW_LATER_WHEEL_END_SLOT; ix++) {
	tmr = tiw->w[ix];
	if (tmr) {
	    do {
		if (tmr->timeout == tclbk)
		    (*func)(arg, tmr->timeout_pos, tmr->arg);
		tmr = tmr->next;
	    } while (tmr != tiw->w[ix]);
	}
    }
}

#ifdef ERTS_TW_DEBUG

void
dbg_verify_empty_soon_slots(ErtsTimerWheel *tiw, ErtsMonotonicTime to_pos)
{
    int ix;
    ErtsMonotonicTime tmp;

    ix = soon_slot(tiw->pos);
    tmp = to_pos;
    if (tmp > tiw->pos) {
        int slots;
        tmp -= tiw->pos;
        ERTS_TW_ASSERT(tmp > 0);
        if (tmp < (ErtsMonotonicTime) ERTS_TW_SOON_WHEEL_SIZE)
            slots = (int) tmp;
        else
            slots = ERTS_TW_SOON_WHEEL_SIZE;

        while (slots > 0) {
            ERTS_TW_ASSERT(!tiw->w[ix]);
            ix++;
            if (ix == ERTS_TW_SOON_WHEEL_END_SLOT)
                ix = ERTS_TW_SOON_WHEEL_FIRST_SLOT;
            slots--;
        }
    }    
}

void
dbg_verify_empty_later_slots(ErtsTimerWheel *tiw, ErtsMonotonicTime to_pos)
{
    int ix;
    ErtsMonotonicTime tmp;

    ix = later_slot(tiw->later.pos);
    tmp = to_pos;
    tmp &= ERTS_TW_LATER_WHEEL_POS_MASK;
    if (tmp > tiw->later.pos) {
        ErtsMonotonicTime pos_min;
        int slots;
        tmp -= tiw->later.pos;
        tmp /= ERTS_TW_LATER_WHEEL_SLOT_SIZE;
        ERTS_TW_ASSERT(tmp > 0);

        pos_min = tiw->later.pos;

        if (tmp < (ErtsMonotonicTime) ERTS_TW_LATER_WHEEL_SIZE)
            slots = (int) tmp;
        else {
            pos_min += ((tmp / ERTS_TW_LATER_WHEEL_SIZE)
                        * ERTS_TW_LATER_WHEEL_SLOT_SIZE);
            slots = ERTS_TW_LATER_WHEEL_SIZE;
        }

        while (slots > 0) {
            ErtsTWheelTimer *tmr = tiw->w[ix];
            pos_min += ERTS_TW_LATER_WHEEL_SLOT_SIZE;
            if (tmr) {
                ErtsTWheelTimer *end = tmr;
                do {
                    ERTS_TW_ASSERT(tmr->timeout_pos >= pos_min);
                    tmr = tmr->next;
                } while (tmr != end);
            }
            ix++;
            if (ix == ERTS_TW_LATER_WHEEL_END_SLOT)
                ix = ERTS_TW_LATER_WHEEL_FIRST_SLOT;
            slots--;
        }
    }    
}

#endif /* ERTS_TW_DEBUG */

#ifdef ERTS_TW_HARD_DEBUG

static void
hrd_dbg_check_wheels(ErtsTimerWheel *tiw, int check_min_tpos)
{
    int ix, six, soon_tmo, later_tmo, at_once_tmo,
        scnt_slot, scnt_slots, scnt_six;
    ErtsMonotonicTime min_tpos;
    Sint scnt[ERTS_TW_SCNT_SIZE];
    ErtsTWheelTimer *p;

    for (six = 0; six < ERTS_TW_SCNT_SIZE; six++)
        scnt[six] = 0;

    min_tpos = ERTS_MONOTONIC_TO_CLKTCKS(tiw->next_timeout_time);

    at_once_tmo = 0;
    p = tiw->w[ERTS_TW_SLOT_AT_ONCE];
    if (p) {
        ErtsTWheelTimer *first = p;
        do {
            at_once_tmo++;
            ERTS_TW_ASSERT(p->slot == ERTS_TW_SLOT_AT_ONCE);
            ERTS_TW_ASSERT(p->timeout_pos <= tiw->pos);
            ERTS_TW_ASSERT(!check_min_tpos || tiw->pos >= min_tpos);
            ERTS_TW_ASSERT(p->next->prev == p);
            p = p->next;
        } while (p != first);
    }

    soon_tmo = 0;
    scnt_slot = ERTS_TW_SOON_WHEEL_END_SLOT-1;
    scnt_slots = ERTS_TW_SOON_WHEEL_SIZE;
    scnt_six = 0;
    scnt_soon_wheel_next(&scnt_slot, &scnt_slots,
                         NULL, &scnt_six, tiw->scnt);
    for (ix = ERTS_TW_SOON_WHEEL_FIRST_SLOT;
         ix < ERTS_TW_SOON_WHEEL_END_SLOT;
         ix++) {
        p = tiw->w[ix];
        six = scnt_get_ix(ix);
        ERTS_TW_ASSERT(!p || six == scnt_six);
        if (p) {
            ErtsTWheelTimer *first = p;
            do {
                ErtsMonotonicTime tpos = p->timeout_pos;
                soon_tmo++;
                scnt_ix_inc(scnt, six);
                ERTS_TW_ASSERT(p->slot == ix);
                ERTS_TW_ASSERT(ix == soon_slot(tpos));
                ERTS_TW_ASSERT(p->timeout_pos < tiw->pos + ERTS_TW_SOON_WHEEL_SIZE);
                ERTS_TW_ASSERT(!check_min_tpos || tpos >= min_tpos);
                ERTS_TW_ASSERT(p->next->prev == p);
                p = p->next;
            } while (p != first);
        }
        if (ix == scnt_slot)
            scnt_soon_wheel_next(&scnt_slot, &scnt_slots,
                                 NULL, &scnt_six, tiw->scnt);
    }

    later_tmo = 0;
    scnt_slot = ERTS_TW_SOON_WHEEL_END_SLOT-1;
    scnt_slots = ERTS_TW_SOON_WHEEL_SIZE;
    scnt_six = 0;
    scnt_later_wheel_next(&scnt_slot, &scnt_slots,
                         NULL, &scnt_six, tiw->scnt);
    for (ix = ERTS_TW_LATER_WHEEL_FIRST_SLOT;
         ix < ERTS_TW_LATER_WHEEL_END_SLOT;
         ix++) {
        p = tiw->w[ix];
        six = scnt_get_ix(ix);
        ERTS_TW_ASSERT(!p || six == scnt_six);
        if (p) {
            ErtsTWheelTimer *first = p;
            six = scnt_get_ix(ix);
            do {
                ErtsMonotonicTime tpos = p->timeout_pos;
                later_tmo++;
                scnt_ix_inc(scnt, six);
                ERTS_TW_ASSERT(p->slot == ix);
                ERTS_TW_ASSERT(later_slot(tpos) == ix);
                tpos &= ERTS_TW_LATER_WHEEL_POS_MASK;
                tpos -= ERTS_TW_LATER_WHEEL_SLOT_SIZE;
                ERTS_TW_ASSERT(!check_min_tpos || tpos >= min_tpos);
                ERTS_TW_ASSERT(p->next->prev == p);
                p = p->next;
            } while (p != first);
        }
        if (ix == scnt_slot)
            scnt_later_wheel_next(&scnt_slot, &scnt_slots,
                                NULL, &scnt_six, tiw->scnt);
    }

    if (tiw->yield_slot != ERTS_TW_SLOT_INACTIVE) {
        p = tiw->sentinel.next;
        ix = tiw->yield_slot;
        while (p != &tiw->sentinel) {
            ErtsMonotonicTime tpos = p->timeout_pos;
            ERTS_TW_ASSERT(ix == p->slot);
            if (ix == ERTS_TW_SLOT_AT_ONCE)
                at_once_tmo++;
            else {
                scnt_inc(scnt, ix);
                if (ix >= ERTS_TW_LATER_WHEEL_FIRST_SLOT) {
                    later_tmo++;
                    ERTS_TW_ASSERT(ix == later_slot(tpos));
                }
                else {
                    soon_tmo++;
                    ERTS_TW_ASSERT(ix == (tpos & ERTS_TW_SOON_WHEEL_MASK));
                    ERTS_TW_ASSERT(tpos < tiw->pos + ERTS_TW_SOON_WHEEL_SIZE);
                }
                p = p->next;
            }
        }
    }


    ERTS_TW_ASSERT(tiw->at_once.nto == at_once_tmo);
    ERTS_TW_ASSERT(tiw->soon.nto == soon_tmo);
    ERTS_TW_ASSERT(tiw->later.nto == later_tmo);
    ERTS_TW_ASSERT(tiw->nto == soon_tmo + later_tmo + at_once_tmo);

    for (six = 0; six < ERTS_TW_SCNT_SIZE; six++)
        ERTS_TW_ASSERT(scnt[six] == tiw->scnt[six]);
}

#endif /* ERTS_TW_HARD_DEBUG */