File: lttng-concepts.7

package info (click to toggle)
ltt-control 2.13.15-2
  • links: PTS, VCS
  • area: main
  • in suites: sid, trixie
  • size: 17,656 kB
  • sloc: ansic: 167,287; sh: 27,018; makefile: 2,828; python: 1,380; yacc: 692; lex: 129; java: 109; perl: 99; cpp: 35; xml: 23
file content (1807 lines) | stat: -rw-r--r-- 45,443 bytes parent folder | download
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
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
'\" t
.\"     Title: lttng-concepts
.\"    Author: [FIXME: author] [see http://www.docbook.org/tdg5/en/html/author]
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\"      Date: 14 June 2021
.\"    Manual: LTTng Manual
.\"    Source: LTTng 2.13.15
.\"  Language: English
.\"
.TH "LTTNG\-CONCEPTS" "7" "14 June 2021" "LTTng 2\&.13\&.15" "LTTng Manual"
.\" -----------------------------------------------------------------
.\" * Define some portability stuff
.\" -----------------------------------------------------------------
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.\" http://bugs.debian.org/507673
.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.ie \n(.g .ds Aq \(aq
.el       .ds Aq '
.\" -----------------------------------------------------------------
.\" * set default formatting
.\" -----------------------------------------------------------------
.\" disable hyphenation
.nh
.\" disable justification (adjust text to left margin only)
.ad l
.\" -----------------------------------------------------------------
.\" * MAIN CONTENT STARTS HERE *
.\" -----------------------------------------------------------------
.SH "NAME"
lttng-concepts \- LTTng concepts
.SH "DESCRIPTION"
.sp
This manual page documents the concepts of LTTng\&.
.sp
Many other LTTng manual pages refer to this one so that you can understand what are the various LTTng objects and how they relate to each other\&.
.sp
The concepts of LTTng\ \&2\&.13\&.15 are:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Instrumentation point, event rule, and event
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Trigger
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Recording session
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Tracing domain
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Channel and ring buffer
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Recording event rule and event record
.RE
.SH "INSTRUMENTATION POINT, EVENT RULE, AND EVENT"
.sp
An \fIinstrumentation point\fR is a point, within a piece of software, which, when executed, creates an LTTng \fIevent\fR\&.
.sp
LTTng offers various types of instrumentation; see the \(lqInstrumentation point types\(rq section below to learn about them\&.
.sp
An \fIevent rule\fR is a set of conditions to match a set of events\&.
.sp
When LTTng creates an event\ \&\fIE\fR, an event rule\ \&\fIER\fR is said to \fImatch\fR\ \&\fIE\fR when\ \&\fIE\fR satisfies \fBall\fR the conditions of\ \&\fIER\fR\&. This concept is similar to a regular expression which matches a set of strings\&.
.sp
When an event rule matches an event, LTTng \fIemits\fR the event, therefore attempting to execute one or more actions\&.
.if n \{\
.sp
.\}
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
\fBImportant\fR
.ps -1
.br
.RS 4
.sp
The event creation and emission processes are documentation concepts to help understand the journey from an instrumentation point to the execution of actions\&.
.sp
The actual creation of an event can be costly because LTTng needs to evaluate the arguments of the instrumentation point\&.
.sp
In practice, LTTng implements various optimizations for the Linux kernel and user space tracing domains (see the \(lqTRACING DOMAIN\(rq section below) to avoid actually creating an event when the tracer knows, thanks to properties which are independent from the event payload and current context, that it would never emit such an event\&. Those properties are:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The instrumentation point type (see the \(lqInstrumentation point types\(rq section below)\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The instrumentation point name\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The instrumentation point log level\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
For a recording event rule (see the \(lqRECORDING EVENT RULE AND EVENT RECORD\(rq section below):
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The status of the rule itself\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The status of the channel (see the \(lqCHANNEL AND RING BUFFER\(rq section below)\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The activity of the recording session (started or stopped; see the \(lqRECORDING SESSION\(rq section below)\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Whether or not the process for which LTTng would create the event is allowed to record events (see
\fBlttng-track\fR(1))\&.
.RE
.RE
.sp
In other words: if, for a given instrumentation point\ \&\fIIP\fR, the LTTng tracer knows that it would never emit an event, executing\ \&\fIIP\fR represents a simple boolean variable check and, for a Linux kernel recording event rule, a few process attribute checks\&.
.sp .5v
.RE
.sp
As of LTTng\ \&2\&.13\&.15, there are two places where you can find an event rule:
.PP
Recording event rule
.RS 4
A specific type of event rule of which the action is to record the matched event as an event record\&.
.sp
See the \(lqRECORDING EVENT RULE AND EVENT RECORD\(rq section below\&.
.sp
Create or enable a recording event rule with the
\fBlttng-enable-event\fR(1)
command\&.
.sp
List the recording event rules of a specific recording session and/or channel with the
\fBlttng-list\fR(1)
and
\fBlttng-status\fR(1)
commands\&.
.RE
.PP
\(lqEvent rule matches\(rq trigger condition (since LTTng\ \&2\&.13)
.RS 4
When the event rule of the trigger condition matches an event, LTTng can execute user\-defined actions such as sending an LTTng notification, starting a recording session, and more\&.
.sp
See
\fBlttng-add-trigger\fR(1)
and
\fBlttng-event-rule\fR(7)\&.
.RE
.sp
For LTTng to emit an event\ \&\fIE\fR,\ \&\fIE\fR must satisfy \fBall\fR the basic conditions of an event rule\ \&\fIER\fR, that is:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The instrumentation point from which LTTng creates\ \&\fIE\fR
has a specific type\&.
.sp
See the \(lqInstrumentation point types\(rq section below\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
A pattern matches the name of\ \&\fIE\fR
while another pattern doesn\(cqt\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The log level of the instrumentation point from which LTTng creates\ \&\fIE\fR
is at least as severe as some value, or is exactly some value\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The fields of the payload of\ \&\fIE\fR
and the current context fields satisfy a filter expression\&.
.RE
.sp
A recording event rule has additional, implicit conditions to satisfy\&. See the \(lqRECORDING EVENT RULE AND EVENT RECORD\(rq section below to learn more\&.
.SS "Instrumentation point types"
.sp
As of LTTng\ \&2\&.13\&.15, the available instrumentation point types are, depending on the tracing domain (see the \(lqTRACING DOMAIN\(rq section below):
.PP
Linux kernel
.RS 4
.PP
LTTng tracepoint
.RS 4
A statically defined point in the source code of the kernel image or of a kernel module using the LTTng\-modules macros\&.
.sp
List the available Linux kernel tracepoints with
\fBlttng list --kernel\fR\&. See
\fBlttng-list\fR(1)
to learn more\&.
.RE
.PP
Linux kernel system call
.RS 4
Entry, exit, or both of a Linux kernel system call\&.
.sp
List the available Linux kernel system call instrumentation points with
\fBlttng list --kernel --syscall\fR\&. See
\fBlttng-list\fR(1)
to learn more\&.
.RE
.PP
Linux kprobe
.RS 4
A single probe dynamically placed in the compiled kernel code\&.
.sp
When you create such an instrumentation point, you set its memory address or symbol name\&.
.RE
.PP
Linux user space probe
.RS 4
A single probe dynamically placed at the entry of a compiled user space application/library function through the kernel\&.
.sp
When you create such an instrumentation point, you set:
.PP
With the ELF method
.RS 4
Its application/library path and its symbol name\&.
.RE
.PP
With the USDT method
.RS 4
Its application/library path, its provider name, and its probe name\&.
.sp
\(lqUSDT\(rq stands for SystemTap User\-level Statically Defined Tracing, a DTrace\-style marker\&.
.RE
.sp
As of LTTng\ \&2\&.13\&.15, LTTng only supports USDT probes which are NOT reference\-counted\&.
.RE
.PP
Linux kretprobe
.RS 4
Entry, exit, or both of a Linux kernel function\&.
.sp
When you create such an instrumentation point, you set the memory address or symbol name of its function\&.
.RE
.RE
.PP
User space
.RS 4
.PP
LTTng tracepoint
.RS 4
A statically defined point in the source code of a C/C++ application/library using the LTTng\-UST macros\&.
.sp
List the available Linux kernel tracepoints with
\fBlttng list --userspace\fR\&. See
\fBlttng-list\fR(1)
to learn more\&.
.RE
.RE
.PP
\fBjava.util.logging\fR, Apache log4j, and Python
.RS 4
.PP
Java or Python logging statement
.RS 4
A method call on a Java or Python logger attached to an LTTng\-UST handler\&.
.sp
List the available Java and Python loggers with
\fBlttng list --jul\fR,
\fBlttng list --log4j\fR, and
\fBlttng list --python\fR\&. See
\fBlttng-list\fR(1)
to learn more\&.
.RE
.RE
.SH "TRIGGER"
.sp
A \fItrigger\fR associates a condition to one or more actions\&.
.sp
When the condition of a trigger is satisfied, LTTng attempts to execute its actions\&.
.sp
As of LTTng\ \&2\&.13\&.15, the available trigger conditions and actions are:
.PP
Conditions
.RS 4
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The consumed buffer size of a given recording session (see the \(lqRECORDING SESSION\(rq section below) becomes greater than some value\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The buffer usage of a given channel (see the \(lqCHANNEL AND RING BUFFER\(rq section below) becomes greater than some value\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The buffer usage of a given channel becomes less than some value\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
There\(cqs an ongoing recording session rotation (see the \(lqRecording session rotation\(rq section below)\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
A recording session rotation becomes completed\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
An event rule matches an event\&.
.sp
As of LTTng\ \&2\&.13\&.15, this is the only available condition when you add a trigger with the
\fBlttng-add-trigger\fR(1)
command\&. The other ones are available through the liblttng\-ctl C\ \&API\&.
.RE
.RE
.PP
Actions
.RS 4
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Send a notification to a user application\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Start a given recording session, like
\fBlttng-start\fR(1)
would do\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Stop a given recording session, like
\fBlttng-stop\fR(1)
would do\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Archive the current trace chunk of a given recording session (rotate), like
\fBlttng-rotate\fR(1)
would do\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Take a snapshot of a given recording session, like
\fBlttng-snapshot\fR(1)
would do\&.
.RE
.RE
.sp
A trigger belongs to a session daemon (see \fBlttng-sessiond\fR(8)), not to a specific recording session\&. For a given session daemon, each Unix user has its own, private triggers\&. Note, however, that the \fBroot\fR Unix user may, for the root session daemon:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Add a trigger as another Unix user\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
List all the triggers, regardless of their owner\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Remove a trigger which belongs to another Unix user\&.
.RE
.sp
For a given session daemon and Unix user, a trigger has a unique name\&.
.sp
Add a trigger to a session daemon with the \fBlttng-add-trigger\fR(1) command\&.
.sp
List the triggers of your Unix user (or of all users if your Unix user is \fBroot\fR) with the \fBlttng-list-triggers\fR(1) command\&.
.sp
Remove a trigger with the \fBlttng-remove-trigger\fR(1) command\&.
.SH "RECORDING SESSION"
.sp
A \fIrecording session\fR (named \(lqtracing session\(rq prior to LTTng\ \&2\&.13) is a stateful dialogue between you and a session daemon (see \fBlttng-sessiond\fR(8)) for everything related to event recording\&.
.sp
Everything that you do when you control LTTng tracers to record events happens within a recording session\&. In particular, a recording session:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Has its own name, unique for a given session daemon\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Has its own set of trace files, if any\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Has its own state of activity (started or stopped)\&.
.sp
An active recording session is an implicit recording event rule condition (see the \(lqRECORDING EVENT RULE AND EVENT RECORD\(rq section below)\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Has its own mode (local, network streaming, snapshot, or live)\&.
.sp
See the \(lqRecording session modes\(rq section below to learn more\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Has its own channels (see the \(lqCHANNEL AND RING BUFFER\(rq section below) to which are attached their own recording event rules\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Has its own process attribute inclusion sets (see
\fBlttng-track\fR(1))\&.
.RE
.sp
Those attributes and objects are completely isolated between different recording sessions\&.
.sp
A recording session is like an ATM session: the operations you do on the banking system through the ATM don\(cqt alter the data of other users of the same system\&. In the case of the ATM, a session lasts as long as your bank card is inside\&. In the case of LTTng, a recording session lasts from the \fBlttng-create\fR(1) command to the \fBlttng-destroy\fR(1) command\&.
.sp
A recording session belongs to a session daemon (see \fBlttng-sessiond\fR(8))\&. For a given session daemon, each Unix user has its own, private recording sessions\&. Note, however, that the \fBroot\fR Unix user may operate on or destroy another user\(cqs recording session\&.
.sp
Create a recording session with the \fBlttng-create\fR(1) command\&.
.sp
List the recording sessions of the connected session daemon with the \fBlttng-list\fR(1) command\&.
.sp
Start and stop a recording session with the \fBlttng-start\fR(1) and \fBlttng-stop\fR(1) commands\&.
.sp
Save and load a recording session with the \fBlttng-save\fR(1) and \fBlttng-load\fR(1) commands\&.
.sp
Archive the current trace chunk of (rotate) a recording session with the \fBlttng-rotate\fR(1) command\&.
.sp
Destroy a recording session with the \fBlttng-destroy\fR(1) command\&.
.SS "Current recording session"
.sp
When you run the \fBlttng-create\fR(1) command, LTTng creates the \fB$LTTNG_HOME/.lttngrc\fR file if it doesn\(cqt exist (\fB$LTTNG_HOME\fR defaults to \fB$HOME\fR)\&.
.sp
\fB$LTTNG_HOME/.lttngrc\fR contains the name of the \fIcurrent recording session\fR\&.
.sp
When you create a new recording session with the \fBcreate\fR command, LTTng updates the current recording session\&.
.sp
The following \fBlttng\fR(1) commands select the current recording session if you don\(cqt specify one:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-add-context\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-clear\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-destroy\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-disable-channel\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-disable-event\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-disable-rotation\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-enable-channel\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-enable-event\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-enable-rotation\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-regenerate\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-rotate\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-save\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-snapshot\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-start\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-status\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-stop\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-track\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-untrack\fR(1)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fBlttng-view\fR(1)
.RE
.sp
Set the current recording session manually with the \fBlttng-set-session\fR(1) command, without having to edit the \fB.lttngrc\fR file\&.
.SS "Recording session modes"
.sp
LTTng offers four recording session modes:
.PP
Local mode
.RS 4
Write the trace data to the local file system\&.
.RE
.PP
Network streaming mode
.RS 4
Send the trace data over the network to a listening relay daemon (see
\fBlttng-relayd\fR(8))\&.
.RE
.PP
Snapshot mode
.RS 4
Only write the trace data to the local file system or send it to a listening relay daemon (\fBlttng-relayd\fR(8)) when LTTng takes a snapshot\&.
.sp
LTTng forces all the channels (see the \(lqCHANNEL AND RING BUFFER\(rq section below) to be created to be configured to be snapshot\-ready\&.
.sp
LTTng takes a snapshot of such a recording session when:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
You run the
\fBlttng-snapshot\fR(1)
command\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
LTTng executes a
\fBsnapshot-session\fR
trigger action (see the \(lqTRIGGER\(rq section above)\&.
.RE
.RE
.PP
Live mode
.RS 4
Send the trace data over the network to a listening relay daemon (see
\fBlttng-relayd\fR(8)) for live reading\&.
.sp
An LTTng live reader (for example,
\fBbabeltrace2\fR(1)) can connect to the same relay daemon to receive trace data while the recording session is active\&.
.RE
.SS "Recording session rotation"
.sp
A \fIrecording session rotation\fR is the action of archiving the current trace chunk of the recording session to the file system\&.
.sp
Once LTTng archives a trace chunk, it does NOT manage it anymore: you can read it, modify it, move it, or remove it\&.
.sp
An \fIarchived trace chunk\fR is a collection of metadata and data stream files which form a self\-contained LTTng trace\&. See the \(lqTrace chunk naming\(rq section below to learn how LTTng names a trace chunk archive directory\&.
.sp
The \fIcurrent trace chunk\fR of a given recording session includes:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The stream files which LTTng already wrote to the file system, and which are not part of a previously archived trace chunk, since the most recent event amongst:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The first time the recording session was started, either with the
\fBlttng-start\fR(1)
command or with a
\fBstart-session\fR
trigger action (see the \(lqTRIGGER\(rq section above)\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The last rotation, performed with:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
An
\fBlttng-rotate\fR(1)
command\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
A rotation schedule previously set with
\fBlttng-enable-rotation\fR(1)\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
An executed
\fBrotate-session\fR
trigger action (see the \(lqTRIGGER\(rq section above)\&.
.RE
.RE
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The content of all the non\-flushed sub\-buffers of the channels of the recording session\&.
.RE
.SS "Trace chunk archive naming"
.sp
A trace chunk archive is a subdirectory of the \fBarchives\fR subdirectory within the output directory of a recording session (see the \fB--output\fR option of the \fBlttng-create\fR(1) command and of \fBlttng-relayd\fR(8))\&.
.sp
A trace chunk archive contains, through tracing domain and possibly UID/PID subdirectories, metadata and data stream files\&.
.sp
A trace chunk archive is, at the same time:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
A self\-contained LTTng trace\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
A member of a set of trace chunk archives which form the complete trace of a recording session\&.
.RE
.sp
In other words, an LTTng trace reader can read both the recording session output directory (all the trace chunk archives), or a single trace chunk archive\&.
.sp
When LTTng performs a recording session rotation, it names the resulting trace chunk archive as such, relative to the output directory of the recording session:
.sp
.if n \{\
.RS 4
.\}
.nf
archives/\fIBEGIN\fR\-\fIEND\fR\-\fIID\fR
.fi
.if n \{\
.RE
.\}
.PP
\fIBEGIN\fR
.RS 4
Date and time of the beginning of the trace chunk archive with the ISO\ \&8601\-compatible
\fIYYYYmmddTHHMMSS\(+-HHMM\fR
form, where
\fIYYYYmmdd\fR
is the date and
\fIHHMMSS\(+-HHMM\fR
is the time with the time zone offset from UTC\&.
.sp
Example:
\fB20171119T152407-0500\fR
.RE
.PP
\fIEND\fR
.RS 4
Date and time of the end of the trace chunk archive with the ISO\ \&8601\-compatible
\fIYYYYmmddTHHMMSS\(+-HHMM\fR
form, where
\fIYYYYmmdd\fR
is the date and
\fIHHMMSS\(+-HHMM\fR
is the time with the time zone offset from UTC\&.
.sp
Example:
\fB20180118T152407+0930\fR
.RE
.PP
\fIID\fR
.RS 4
Unique numeric identifier of the trace chunk within its recording session\&.
.RE
.sp
Trace chunk archive name example:
.sp
.if n \{\
.RS 4
.\}
.nf
archives/20171119T152407\-0500\-20171119T151422\-0500\-3
.fi
.if n \{\
.RE
.\}
.SH "TRACING DOMAIN"
.sp
A \fItracing domain\fR identifies a type of LTTng tracer\&.
.sp
A tracing domain has its own properties and features\&.
.sp
There are currently five available tracing domains:
.TS
allbox tab(:);
ltB ltB ltB.
T{
Tracing domain
T}:T{
\(lqEvent rule matches\(rq trigger condition option
T}:T{
Option for other CLI commands
T}
.T&
lt lt lt
lt lt lt
lt lt lt
lt lt lt
lt lt lt.
T{
.sp
Linux kernel
T}:T{
.sp
\fB--type\fR option starts with \fBkernel:\fR
T}:T{
.sp
\fB--kernel\fR
T}
T{
.sp
User space
T}:T{
.sp
\fB--type\fR option starts with \fBuser:\fR
T}:T{
.sp
\fB--userspace\fR
T}
T{
.sp
\fBjava.util.logging\fR (JUL)
T}:T{
.sp
\fB--type\fR option starts with \fBjul:\fR
T}:T{
.sp
\fB--jul\fR
T}
T{
.sp
Apache log4j
T}:T{
.sp
\fB--type\fR option starts with \fBlog4j:\fR
T}:T{
.sp
\fB--log4j\fR
T}
T{
.sp
Python
T}:T{
.sp
\fB--type\fR option starts with \fBpython:\fR
T}:T{
.sp
\fB--python\fR
T}
.TE
.sp 1
.sp
You must specify a tracing domain to target a type of LTTng tracer when using some \fBlttng\fR(1) commands to avoid ambiguity\&. For example, because the Linux kernel and user space tracing domains support named tracepoints as instrumentation points (see the \(lqINSTRUMENTATION POINT, EVENT RULE, AND EVENT\(rq section above), you need to specify a tracing domain when you create an event rule because both tracing domains could have tracepoints sharing the same name\&.
.sp
You can create channels (see the \(lqCHANNEL AND RING BUFFER\(rq section below) in the Linux kernel and user space tracing domains\&. The other tracing domains have a single, default channel\&.
.SH "CHANNEL AND RING BUFFER"
.sp
A \fIchannel\fR is an object which is responsible for a set of ring buffers\&.
.sp
Each ring buffer is divided into multiple \fIsub\-buffers\fR\&. When a recording event rule (see the \(lqRECORDING EVENT RULE AND EVENT RECORD\(rq section below) matches an event, LTTng can record it to one or more sub\-buffers of one or more channels\&.
.sp
When you create a channel with the \fBlttng-enable-channel\fR(1) command, you set its final attributes, that is:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Its buffering scheme\&.
.sp
See the \(lqBuffering scheme\(rq section below\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
What to do when there\(cqs no space left for a new event record because all sub\-buffers are full\&.
.sp
See the \(lqEvent record loss mode\(rq section below\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The size of each ring buffer and how many sub\-buffers a ring buffer has\&.
.sp
See the \(lqSub\-buffer size and count\(rq section below\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The size of each trace file LTTng writes for this channel and the maximum count of trace files\&.
.sp
See the \(lqMaximum trace file size and count\(rq section below\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The periods of its read, switch, and monitor timers\&.
.sp
See the \(lqTimers\(rq section below\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
For a Linux kernel channel: its output type (\fBmmap\fR(2)
or
\fBsplice\fR(2))\&.
.sp
See the
\fB--output\fR
option of the
\fBlttng-enable-channel\fR(1)
command\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
For a user space channel: the value of its blocking timeout\&.
.sp
See the
\fB--blocking-timeout\fR
option of the
\fBlttng-enable-channel\fR(1)
command\&.
.RE
.sp
Note that the \fBlttng-enable-event\fR(1) command can automatically create a default channel with sane defaults when no channel exists for the provided tracing domain\&.
.sp
A channel is always associated to a tracing domain (see the \(lqTRACING DOMAIN\(rq section below)\&. The \fBjava.util.logging\fR (JUL), log4j, and Python tracing domains each have a default channel which you can\(cqt configure\&.
.sp
A channel owns recording event rules\&.
.sp
List the channels of a given recording session with the \fBlttng-list\fR(1) and \fBlttng-status\fR(1) commands\&.
.sp
Disable an enabled channel with the \fBlttng-disable-channel\fR(1) command\&.
.SS "Buffering scheme"
.sp
A channel has at least one ring buffer per CPU\&. LTTng always records an event to the ring buffer dedicated to the CPU which emits it\&.
.sp
The buffering scheme of a user space channel determines what has its own set of per\-CPU ring buffers:
.PP
Per\-user buffering (\fB--buffers-uid\fR option of the \fBlttng-enable-channel\fR(1) command)
.RS 4
Allocate one set of ring buffers (one per CPU) shared by all the instrumented processes of:
.PP
If your Unix user is \fBroot\fR
.RS 4
Each Unix user\&.
.RE
.PP
Otherwise
.RS 4
Your Unix user\&.
.RE
.RE
.PP
Per\-process buffering (\fB--buffers-pid\fR option of the \fBlttng-enable-channel\fR(1) command)
.RS 4
Allocate one set of ring buffers (one per CPU) for each instrumented process of:
.PP
If your Unix user is \fBroot\fR
.RS 4
All Unix users\&.
.RE
.PP
Otherwise
.RS 4
Your Unix user\&.
.RE
.RE
.sp
The per\-process buffering scheme tends to consume more memory than the per\-user option because systems generally have more instrumented processes than Unix users running instrumented processes\&. However, the per\-process buffering scheme ensures that one process having a high event throughput won\(cqt fill all the shared sub\-buffers of the same Unix user, only its own\&.
.sp
The buffering scheme of a Linux kernel channel is always to allocate a single set of ring buffers for the whole system\&. This scheme is similar to the per\-user option, but with a single, global user \(lqrunning\(rq the kernel\&.
.SS "Event record loss mode"
.sp
When LTTng emits an event, LTTng can record it to a specific, available sub\-buffer within the ring buffers of specific channels\&. When there\(cqs no space left in a sub\-buffer, the tracer marks it as consumable and another, available sub\-buffer starts receiving the following event records\&. An LTTng consumer daemon eventually consumes the marked sub\-buffer, which returns to the available state\&.
.sp
In an ideal world, sub\-buffers are consumed faster than they are filled\&. In the real world, however, all sub\-buffers can be full at some point, leaving no space to record the following events\&.
.sp
By default, LTTng\-modules and LTTng\-UST are \fInon\-blocking\fR tracers: when there\(cqs no available sub\-buffer to record an event, it\(cqs acceptable to lose event records when the alternative would be to cause substantial delays in the execution of the instrumented application\&. LTTng privileges performance over integrity; it aims at perturbing the instrumented application as little as possible in order to make the detection of subtle race conditions and rare interrupt cascades possible\&.
.sp
Since LTTng\ \&2\&.10, the LTTng user space tracer, LTTng\-UST, supports a \fIblocking mode\fR\&. See the \fB--blocking-timeout\fR of the \fBlttng-enable-channel\fR(1) command to learn how to use the blocking mode\&.
.sp
When it comes to losing event records because there\(cqs no available sub\-buffer, or because the blocking timeout of the channel is reached, the \fIevent record loss mode\fR of the channel determines what to do\&. The available event record loss modes are:
.PP
Discard mode
.RS 4
Drop the newest event records until a sub\-buffer becomes available\&.
.sp
This is the only available mode when you specify a blocking timeout\&.
.sp
With this mode, LTTng increments a count of lost event records when an event record is lost and saves this count to the trace\&. A trace reader can use the saved discarded event record count of the trace to decide whether or not to perform some analysis even if trace data is known to be missing\&.
.RE
.PP
Overwrite mode
.RS 4
Clear the sub\-buffer containing the oldest event records and start writing the newest event records there\&.
.sp
This mode is sometimes called
\fIflight recorder mode\fR
because it\(cqs similar to a
flight recorder <https://en.wikipedia.org/wiki/Flight_recorder>: always keep a fixed amount of the latest data\&. It\(cqs also similar to the roll mode of an oscilloscope\&.
.sp
Since LTTng\ \&2\&.8, with this mode, LTTng writes to a given sub\-buffer its sequence number within its data stream\&. With a local, network streaming, or live recording session (see the \(lqRecording session modes\(rq section above), a trace reader can use such sequence numbers to report lost packets\&. A trace reader can use the saved discarded sub\-buffer (packet) count of the trace to decide whether or not to perform some analysis even if trace data is known to be missing\&.
.sp
With this mode, LTTng doesn\(cqt write to the trace the exact number of lost event records in the lost sub\-buffers\&.
.RE
.sp
Which mechanism you should choose depends on your context: prioritize the newest or the oldest event records in the ring buffer?
.sp
Beware that, in overwrite mode, the tracer abandons a \fIwhole sub\-buffer\fR as soon as a there\(cqs no space left for a new event record, whereas in discard mode, the tracer only discards the event record that doesn\(cqt fit\&.
.sp
Set the event record loss mode of a channel with the \fB--discard\fR and \fB--overwrite\fR options of the \fBlttng-enable-channel\fR(1) command\&.
.sp
There are a few ways to decrease your probability of losing event records\&. The \(lqSub\-buffer size and count\(rq section below shows how to fine\-tune the sub\-buffer size and count of a channel to virtually stop losing event records, though at the cost of greater memory usage\&.
.SS "Sub\-buffer size and count"
.sp
A channel has one or more ring buffer for each CPU of the target system\&.
.sp
See the \(lqBuffering scheme\(rq section above to learn how many ring buffers of a given channel are dedicated to each CPU depending on its buffering scheme\&.
.sp
Set the size of each sub\-buffer the ring buffers of a channel contain with the \fB--subbuf-size\fR option of the \fBlttng-enable-channel\fR(1) command\&.
.sp
Set the number of sub\-buffers each ring buffer of a channel contains with the \fB--num-subbuf\fR option of the \fBlttng-enable-channel\fR(1) command\&.
.sp
Note that LTTng switching the current sub\-buffer of a ring buffer (marking a full one as consumable and switching to an available one for LTTng to record the next events) introduces noticeable CPU overhead\&. Knowing this, the following list presents a few practical situations along with how to configure the sub\-buffer size and count for them:
.PP
High event throughput
.RS 4
In general, prefer large sub\-buffers to lower the risk of losing event records\&.
.sp
Having larger sub\-buffers also ensures a lower sub\-buffer switching frequency (see the \(lqTimers\(rq section below)\&.
.sp
The sub\-buffer count is only meaningful if you create the channel in overwrite mode (see the \(lqEvent record loss mode\(rq section above): in this case, if LTTng overwrites a sub\-buffer, then the other sub\-buffers are left unaltered\&.
.RE
.PP
Low event throughput
.RS 4
In general, prefer smaller sub\-buffers since the risk of losing event records is low\&.
.sp
Because LTTng emits events less frequently, the sub\-buffer switching frequency should remain low and therefore the overhead of the tracer shouldn\(cqt be a problem\&.
.RE
.PP
Low memory system
.RS 4
If your target system has a low memory limit, prefer fewer first, then smaller sub\-buffers\&.
.sp
Even if the system is limited in memory, you want to keep the sub\-buffers as large as possible to avoid a high sub\-buffer switching frequency\&.
.RE
.sp
Note that LTTng uses CTF <https://diamon.org/ctf/> as its trace format, which means event record data is very compact\&. For example, the average LTTng kernel event record weights about 32\ \&bytes\&. Therefore, a sub\-buffer size of 1\ \&MiB is considered large\&.
.sp
The previous scenarios highlight the major trade\-off between a few large sub\-buffers and more, smaller sub\-buffers: sub\-buffer switching frequency vs\&. how many event records are lost in overwrite mode\&. Assuming a constant event throughput and using the overwrite mode, the two following configurations have the same ring buffer total size:
.PP
Two sub\-buffers of 4\ \&MiB each
.RS 4
Expect a very low sub\-buffer switching frequency, but if LTTng ever needs to overwrite a sub\-buffer, half of the event records so far (4\ \&MiB) are definitely lost\&.
.RE
.PP
Eight sub\-buffers of 1\ \&MiB each
.RS 4
Expect four times the tracer overhead of the configuration above, but if LTTng needs to overwrite a sub\-buffer, only the eighth of event records so far (1\ \&MiB) are definitely lost\&.
.RE
.sp
In discard mode, the sub\-buffer count parameter is pointless: use two sub\-buffers and set their size according to your requirements\&.
.SS "Maximum trace file size and count"
.sp
By default, trace files can grow as large as needed\&.
.sp
Set the maximum size of each trace file that LTTng writes of a given channel with the \fB--tracefile-size\fR option of the \fBlttng-enable-channel\fR(1) command\&.
.sp
When the size of a trace file reaches the fixed maximum size of the channel, LTTng creates another file to contain the next event records\&. LTTng appends a file count to each trace file name in this case\&.
.sp
If you set the trace file size attribute when you create a channel, the maximum number of trace files that LTTng creates is \fIunlimited\fR by default\&. To limit them, use the \fB--tracefile-count\fR option of \fBlttng-enable-channel\fR(1)\&. When the number of trace files reaches the fixed maximum count of the channel, LTTng overwrites the oldest trace file\&. This mechanism is called \fItrace file rotation\fR\&.
.if n \{\
.sp
.\}
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
\fBImportant\fR
.ps -1
.br
.RS 4
.sp
Even if you don\(cqt limit the trace file count, always assume that LTTng manages all the trace files of the recording session\&.
.sp
In other words, there\(cqs no safe way to know if LTTng still holds a given trace file open with the trace file rotation feature\&.
.sp
The only way to obtain an unmanaged, self\-contained LTTng trace before you destroy the recording session is with the recording session rotation feature (see the \(lqRecording session rotation\(rq section above), which is available since LTTng\ \&2\&.11\&.
.sp .5v
.RE
.SS "Timers"
.sp
Each channel can have up to three optional timers:
.PP
Switch timer
.RS 4
When this timer expires, a sub\-buffer switch happens: for each ring buffer of the channel, LTTng marks the current sub\-buffer as consumable and switches to an available one to record the next events\&.
.sp
A switch timer is useful to ensure that LTTng consumes and commits trace data to trace files or to a distant relay daemon (\fBlttng-relayd\fR(8)) periodically in case of a low event throughput\&.
.sp
Such a timer is also convenient when you use large sub\-buffers (see the \(lqSub\-buffer size and count\(rq section above) to cope with a sporadic high event throughput, even if the throughput is otherwise low\&.
.sp
Set the period of the switch timer of a channel, or disable the timer altogether, with the
\fB--switch-timer\fR
option of the
\fBlttng-enable-channel\fR(1)
command\&.
.RE
.PP
Read timer
.RS 4
When this timer expires, LTTng checks for full, consumable sub\-buffers\&.
.sp
By default, the LTTng tracers use an asynchronous message mechanism to signal a full sub\-buffer so that a consumer daemon can consume it\&.
.sp
When such messages must be avoided, for example in real\-time applications, use this timer instead\&.
.sp
Set the period of the read timer of a channel, or disable the timer altogether, with the
\fB--read-timer\fR
option of the
\fBlttng-enable-channel\fR(1)
command\&.
.RE
.PP
Monitor timer
.RS 4
When this timer expires, the consumer daemon samples some channel statistics to evaluate the following trigger conditions:
.sp
.RS 4
.ie n \{\
\h'-04' 1.\h'+01'\c
.\}
.el \{\
.sp -1
.IP "  1." 4.2
.\}
The consumed buffer size of a given recording session becomes greater than some value\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 2.\h'+01'\c
.\}
.el \{\
.sp -1
.IP "  2." 4.2
.\}
The buffer usage of a given channel becomes greater than some value\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 3.\h'+01'\c
.\}
.el \{\
.sp -1
.IP "  3." 4.2
.\}
The buffer usage of a given channel becomes less than some value\&.
.RE
.sp
If you disable the monitor timer of a channel\ \&\fIC\fR:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The consumed buffer size value of the recording session of\ \&\fIC\fR
could be wrong for trigger condition type\ \&1: the consumed buffer size of\ \&\fIC\fR
won\(cqt be part of the grand total\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The buffer usage trigger conditions (types\ \&2 and\ \&3) for\ \&\fIC\fR
will never be satisfied\&.
.RE
.sp
See the \(lqTRIGGER\(rq section above to learn more about triggers\&.
.sp
Set the period of the monitor timer of a channel, or disable the timer altogether, with the
\fB--monitor-timer\fR
option of the
\fBlttng-enable-channel\fR(1)
command\&.
.RE
.SH "RECORDING EVENT RULE AND EVENT RECORD"
.sp
A \fIrecording event rule\fR is a specific type of event rule (see the \(lqINSTRUMENTATION POINT, EVENT RULE, AND EVENT\(rq section above) of which the action is to serialize and record the matched event as an \fIevent record\fR\&.
.sp
Set the explicit conditions of a recording event rule when you create it with the \fBlttng-enable-event\fR(1) command\&. A recording event rule also has the following implicit conditions:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The recording event rule itself is enabled\&.
.sp
A recording event rule is enabled on creation\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The channel to which the recording event rule is attached is enabled\&.
.sp
A channel is enabled on creation\&.
.sp
See the \(lqCHANNEL AND RING BUFFER\(rq section above\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The recording session of the recording event rule is active (started)\&.
.sp
A recording session is inactive (stopped) on creation\&.
.sp
See the \(lqRECORDING SESSION\(rq section above\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The process for which LTTng creates an event to match is allowed to record events\&.
.sp
All processes are allowed to record events on recording session creation\&.
.sp
Use the
\fBlttng-track\fR(1)
and
\fBlttng-untrack\fR(1)
commands to select which processes are allowed to record events based on specific process attributes\&.
.RE
.sp
You always attach a recording event rule to a channel, which belongs to a recording session, when you create it\&.
.sp
When a recording event rule\ \&\fIER\fR matches an event\ \&\fIE\fR, LTTng attempts to serialize and record\ \&\fIE\fR to one of the available sub\-buffers of the channel to which\ \&\fIE\fR is attached\&.
.sp
When multiple matching recording event rules are attached to the same channel, LTTng attempts to serialize and record the matched event \fIonce\fR\&. In the following example, the second recording event rule is redundant when both are enabled:
.sp
.if n \{\
.RS 4
.\}
.nf
$ lttng enable\-event \-\-userspace hello:world
$ lttng enable\-event \-\-userspace hello:world \-\-loglevel=INFO
.fi
.if n \{\
.RE
.\}
.sp
List the recording event rules of a specific recording session and/or channel with the \fBlttng-list\fR(1) and \fBlttng-status\fR(1) commands\&.
.sp
Disable a recording event rule with the \fBlttng-disable-event\fR(1) command\&.
.sp
As of LTTng\ \&2\&.13\&.15, you cannot remove a recording event rule: it exists as long as its recording session exists\&.
.SH "RESOURCES"
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
LTTng project website <https://lttng.org>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
LTTng documentation <https://lttng.org/docs>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
LTTng bug tracker <https://bugs.lttng.org>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Git repositories <https://git.lttng.org>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
GitHub organization <https://github.com/lttng>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Continuous integration <https://ci.lttng.org/>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Mailing list <https://lists.lttng.org/>
for support and development:
\fBlttng-dev@lists.lttng.org\fR
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
IRC channel <irc://irc.oftc.net/lttng>:
\fB#lttng\fR
on
\fBirc.oftc.net\fR
.RE
.SH "COPYRIGHT"
.sp
This program is part of the LTTng\-tools project\&.
.sp
LTTng\-tools is distributed under the GNU General Public License version\ \&2 <http://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html>\&. See the \fBLICENSE\fR <https://github.com/lttng/lttng-tools/blob/master/LICENSE> file for details\&.
.SH "THANKS"
.sp
Special thanks to Michel Dagenais and the DORSAL laboratory <http://www.dorsal.polymtl.ca/> at \('Ecole Polytechnique de Montr\('eal for the LTTng journey\&.
.sp
Also thanks to the Ericsson teams working on tracing which helped us greatly with detailed bug reports and unusual test cases\&.
.SH "SEE ALSO"
.sp
\fBlttng\fR(1), \fBlttng-relayd\fR(8), \fBlttng-sessiond\fR(8)