Let's Learn to Talk to GC Logs in Java 9

Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Let’s Learn to Talk to GC
Logs in Java 9
Poonam Bajaj Parhar
Consulting Member of Technical Staff
JVM Sustaining, Oracle
March 22, 2017

Safe Harbor Statement
The following is intended to outline our general product direction. It is intended for
information purposes only, and may not be incorporated into any contract. It is not a
commitment to deliver any material, code, or functionality, and should not be relied upon
in making purchasing decisions. The development, release, and timing of any features or
functionality described for Oracle’s products remains at the sole discretion of Oracle.
4

New GC Logging in Java 9
5

Agenda
Unified JVM Logging
Unified GC Logging
-Xlog:gc
Old GC Flags
Examples
1
2
3
4
5
6

Agenda
Unified JVM Logging
Unified GC Logging
-Xlog:gc
Old GC Flags
Examples
1
2
3
4
5
7

Unified JVM Logging
• JEP 158: Unified JVM Logging
– http://openjdk.java.net/jeps/158
• Common command line option for logging from all sub-systems:
-Xlog
• Logging messages categorized using tags
– gc, class, jfr, modules, os, thread etc.
– Messages can have multiple tags (tag-set)
• All logging follows standard and defined levels
– error, warning, info, debug, trace, develop
• Messages can be "decorated". The default decorations are: uptime, level, tags.
• Possible to redirect logging to file
8

Unified JVM Logging
java -Xlog:help
-Xlog Usage: -Xlog[:[what][:[output][:[decorators][:output-options]]]]
where 'what' is a combination of tags and levels on the form
tag1[+tag2...][*][=level][,...]
Unless wildcard (*) is specified, only log messages tagged with exactly the
tags specified will be matched.
9

Copyright © 2017, Oracle and/or its affiliates. All rights reserved. | 10
-Xlog[:option]
option := [<what>][:[<output>][:[<decorators>][:<output-options>]]]
'help'
'disable'
what := <selector>[,...]
selector := <tag-set>[*][=<level>]
tag-set := <tag>[+...]
'all'
tag := name of tag
level := trace
debug
info
warning
error
output := 'stderr'
'stdout'
[file=]<filename>
decorators := <decorator>[,...]
'none'
decorator := time
uptime
timemillis
uptimemillis
timenanos
uptimenanos
pid
tid
level
tags
output-options := <output_option>[,...]
output-option := filecount=<file count>
filesize=<file size in kb>
parameter=value
-Xlog

Agenda
Unified JVM Logging
Unified GC Logging
-Xlog:gc
Old GC Flags
Examples
1
2
3
4
5
11

Unified GC Logging
• JEP 271: Unified GC Logging
– http://openjdk.java.net/jeps/271
• -Xlog:gc
• Old GC logging options removed
• JDK-8145092: Use Unified Logging for the GC logging
12

Agenda
Unified JVM Logging
Unified GC Logging
-Xlog:gc
Old GC Flags
Examples
1
2
3
4
5
13

bash-3.2$ java -Xlog:gc=trace -jar GCBasher.jar -time:30000
[0.041s][trace][gc] MarkStackSize: 4096k MarkStackSizeMax: 16384k
[0.136s][debug][gc] ConcGCThreads: 6
[0.136s][debug][gc] ParallelGCThreads: 23
[0.138s][debug][gc] Initialize mark stack with 4096 chunks, maximum 16384
[0.230s][info ][gc] Using G1
[3.461s][info ][gc] GC(0) Pause Young (G1 Evacuation Pause) 26M->5M(256M) 370.917ms

Agenda
Unified JVM Logging
Unified GC Logging
-Xlog:gc
Old GC Flags
Examples
1
2
3
4
5
15

Popular GC Logging Flags
• -XX:+PrintGC
• -XX:+PrintGCDetails
• -Xloggc
• Mapped to corresponding –Xlog configurations
• Deprecated
16
Old New
-XX:+PrintGC -Xlog:gc
-XX:+PrintGCDetails -Xlog:gc*
-Xloggc:<filename> -Xlog:gc:<filename>

CMSDumpAtPromotionFailure, CMSPrintEdenSurvivorChunks, G1LogLevel,
G1PrintHeapRegions, G1PrintRegionLivenessInfo, G1SummarizeConcMark,
G1SummarizeRSetStats, G1TraceConcRefinement,
G1TraceEagerReclaimHumongousObjects, G1TraceStringSymbolTableScrubbing,
GCLogFileSize, NumberOfGCLogFiles, PrintAdaptiveSizePolicy,
PrintClassHistogramAfterFullGC, PrintClassHistogramBeforeFullGC,
PrintCMSInitiationStatistics, PrintCMSStatistics, PrintFLSCensus,
PrintFLSStatistics, PrintGCApplicationConcurrentTime,
PrintGCApplicationStoppedTime, PrintGCCause, PrintGCDateStamps, PrintGCID,
PrintGCTaskTimeStamps, PrintGCTimeStamps, PrintHeapAtGC,
PrintHeapAtGCExtended, PrintJNIGCStalls, PrintOldPLAB,
PrintParallelOldGCPhaseTimes, PrintPLAB, PrintPromotionFailure,
PrintReferenceGC, PrintStringDeduplicationStatistics, PrintTaskqueue,
PrintTenuringDistribution, PrintTerminationStats, PrintTLAB,
TraceDynamicGCThreads, TraceMetadataHumongousAllocation, UseGCLogFileRotation,
VerifySilently
43 GC Logging Flags Removed

-void GenCollectedHeap::print_heap_change(size_t prev_used) const {
- if (PrintGCDetails && Verbose) {
- gclog_or_tty->print(" " SIZE_FORMAT
- "->" SIZE_FORMAT
- "(" SIZE_FORMAT ")",
- prev_used, used(), capacity());
- } else {
- gclog_or_tty->print(" " SIZE_FORMAT "K"
- "->" SIZE_FORMAT "K"
- "(" SIZE_FORMAT "K)",
- prev_used / K, used() / K, capacity() / K);
- }
+void GenCollectedHeap::print_heap_change(size_t young_prev_used, size_t old_prev_used) const {
+ log_info(gc, heap)("%s: " SIZE_FORMAT "K->" SIZE_FORMAT "K(" SIZE_FORMAT "K)",
+ _young_gen->short_name(), young_prev_used / K, _young_gen->used() /K,
+ _young_gen->capacity() /K);
+ log_info(gc, heap)("%s: " SIZE_FORMAT "K->" SIZE_FORMAT "K(" SIZE_FORMAT "K)",
+ _old_gen->short_name(), old_prev_used / K, _old_gen->used() /K,
+ _old_gen->capacity() /K);
+ }
GC Logging code change

- if (PrintAdaptiveSizePolicy) {
- ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
- gclog_or_tty->print("AdaptiveSizePolicy::survivor space sizes: "
- "collection: %d "
- "(" SIZE_FORMAT ", " SIZE_FORMAT ") -> "
- "(" SIZE_FORMAT ", " SIZE_FORMAT ") ",
- heap->total_collections(),
- old_from, old_to,
- from_space()->capacity_in_bytes(),
- to_space()->capacity_in_bytes());
- gclog_or_tty->cr();
- }
+ log_trace(gc, ergo)("AdaptiveSizePolicy::survivor space sizes: "
+ "collection: %d "
+ "(" SIZE_FORMAT ", " SIZE_FORMAT ") -> "
+ "(" SIZE_FORMAT ", " SIZE_FORMAT ") ",
+ ParallelScavengeHeap::heap()->total_collections(),
+ old_from, old_to,
+ from_space()->capacity_in_bytes(),
+ to_space()->capacity_in_bytes());
+
GC Logging code change

Agenda
Unified JVM Logging
Unified GC Logging
-Xlog:gc
Old GC Flags
Examples
1
2
3
4
5
20

bash-3.2$ java -XX:+UseG1GC -XX:+PrintGC GCTest
[GC pause (G1 Evacuation Pause) (young) 24M->4527K(256M), 0.0861751 secs]
[GC pause (G1 Evacuation Pause) (young) 20M->9270K(256M), 0.0630344 secs]
[GC pause (G1 Evacuation Pause) (young) 35M->16M(256M), 0.0797056 secs]
[GC pause (G1 Evacuation Pause) (young) (initial-mark) 213M->168M(256M), 0.0964933 secs]
[GC concurrent-root-region-scan-start]
[GC concurrent-root-region-scan-end, 0.0372045 secs]
[GC concurrent-mark-start]
[GC concurrent-mark-end, 0.1160973 secs]
[GC remark, 0.0143419 secs]
[GC cleanup 176M->88M(256M), 0.0049707 secs]
[GC concurrent-cleanup-start]
[GC concurrent-cleanup-end, 0.0010084 secs]
[GC pause (G1 Evacuation Pause) (mixed) 102M->78M(256M), 0.0414904 secs]
[GC pause (G1 Evacuation Pause) (mixed) 88M->72M(256M), 0.0424158 secs]
[…]
-XX:+PrintGC

bash-3.2$ java -Xlog:gc GCTest
[0.222s][info][gc] Using G1
[2.825s][info][gc] GC(0) Pause Young (G1 Evacuation Pause) 26M->5M(256M) 355.623ms
[…]
-Xlog:gc

bash-3.2$ java -XX:+UseG1GC -XX:+PrintGCDetails GCTest
[GC pause (G1 Evacuation Pause) (young), 0.0858830 secs]
[Parallel Time: 73.2 ms, GC Workers: 23]
[GC Worker Start (ms): Min: 2141.5, Avg: 2141.9, Max: 2142.3, Diff: 0.8]
[Ext Root Scanning (ms): Min: 0.4, Avg: 1.3, Max: 17.6, Diff: 17.2, Sum: 31.0]
[Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
[Processed Buffers: Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0]
[Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.4]
[Code Root Scanning (ms): Min: 0.0, Avg: 0.3, Max: 4.4, Diff: 4.4, Sum: 7.1]
[Object Copy (ms): Min: 52.2, Avg: 67.6, Max: 69.9, Diff: 17.7, Sum: 1555.0]
[Termination (ms): Min: 0.0, Avg: 1.1, Max: 2.3, Diff: 2.3, Sum: 26.0]
[Termination Attempts: Min: 1, Avg: 5.3, Max: 10, Diff: 9, Sum: 123]
[GC Worker Other (ms): Min: 1.2, Avg: 1.6, Max: 2.0, Diff: 0.8, Sum: 36.6]
[GC Worker Total (ms): Min: 71.6, Avg: 72.0, Max: 72.4, Diff: 0.9, Sum: 1656.3]
[GC Worker End (ms): Min: 2213.9, Avg: 2213.9, Max: 2214.0, Diff: 0.2]
[Code Root Fixup: 1.2 ms]
[Code Root Purge: 0.1 ms]
[Clear CT: 1.2 ms]
[Other: 10.3 ms]
[Choose CSet: 0.1 ms]
[Ref Proc: 5.1 ms]
[Ref Enq: 0.1 ms]
[Redirty Cards: 1.1 ms]
[Humongous Register: 1.0 ms]
[Humongous Reclaim: 0.0 ms]
[Free CSet: 0.5 ms]
[Eden: 24.0M(24.0M)->0.0B(16.0M) Survivors: 0.0B->3072.0K Heap: 24.0M(256.0M)->4529.0K(256.0M)]
[Times: user=0.46 sys=0.92, real=0.09 secs]
-XX:+PrintGCDetails

bash-3.2$ java -Xlog:gc* GCTest
[0.233s][info][gc,heap] Heap region size: 1M
[0.383s][info][gc ] Using G1
[0.383s][info][gc,heap,coops] Heap address: 0xfffffffe50400000, size: 4064 MB, Compressed Oops mode: Non-zero based:
0xfffffffe50000000, Oop shift amount: 3
[3.064s][info][gc,start ] GC(0) Pause Young (G1 Evacuation Pause)
[3.064s][info][gc,task ] GC(0) Using 23 workers of 23 for evacuation
[3.420s][info][gc,phases ] GC(0) Pre Evacuate Collection Set: 0.2ms
[3.421s][info][gc,phases ] GC(0) Evacuate Collection Set: 348.0ms
[3.421s][info][gc,phases ] GC(0) Post Evacuate Collection Set: 6.2ms
[3.421s][info][gc,phases ] GC(0) Other: 2.8ms
[3.421s][info][gc,heap ] GC(0) Eden regions: 24->0(9)
[3.421s][info][gc,heap ] GC(0) Survivor regions: 0->3(3)
[3.421s][info][gc,heap ] GC(0) Old regions: 0->2
[3.421s][info][gc,heap ] GC(0) Humongous regions: 2->1
[3.421s][info][gc,metaspace ] GC(0) Metaspace: 4719K->4719K(1056768K)
[3.421s][info][gc ] GC(0) Pause Young (G1 Evacuation Pause) 26M->5M(256M) 357.743ms
[3.422s][info][gc,cpu ] GC(0) User=0.70s Sys=5.13s Real=0.36s
[3.699s][info][gc,phases ] GC(1) Evacuate Collection Set: 45.6ms
[3.699s][info][gc,phases ] GC(1) Post Evacuate Collection Set: 3.4ms
[3.699s][info][gc,phases ] GC(1) Other: 1.7ms
[3.699s][info][gc,heap ] GC(1) Eden regions: 9->0(10)
[3.699s][info][gc,heap ] GC(1) Survivor regions: 3->2(2)
[3.699s][info][gc,heap ] GC(1) Old regions: 2->5
[3.700s][info][gc,heap ] GC(1) Humongous regions: 1->1
[3.700s][info][gc,metaspace ] GC(1) Metaspace: 4726K->4726K(1056768K)
[3.700s][info][gc ] GC(1) Pause Young (G1 Evacuation Pause) 14M->7M(256M) 51.872ms
[3.700s][info][gc,cpu ] GC(1) User=0.56s Sys=0.46s Real=0.05s
[…]
-Xlog:gc*

bash-3.2$ java -Xlog:gc*,gc+phases=debug GCTest [0.115s][info][gc,heap] Heap region size: 1M
[0.227s][info][gc ] Using G1
[0.227s][info][gc,heap,coops] Heap address: 0xfffffffe50400000, size: 4064 MB, Compressed Oops mode: Non-zero based: 0xfffffffe50000000, Oop shift amount: 3
[2.797s][debug][gc,phases ] GC(0) Choose Collection Set: 0.0ms
[2.797s][debug][gc,phases ] GC(0) Humongous Register: 0.1ms
[2.797s][info ][gc,phases ] GC(0) Evacuate Collection Set: 341.6ms
[2.797s][debug][gc,phases ] GC(0) Ext Root Scanning (ms): Min: 0.0, Avg: 9.2, Max: 205.8, Diff: 205.8, Sum: 212.0, Workers: 23
[2.797s][debug][gc,phases ] GC(0) Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1, Workers: 23
[2.797s][debug][gc,phases ] GC(0) Processed Buffers: Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0, Workers: 23
[2.797s][debug][gc,phases ] GC(0) Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.2, Workers: 23
[2.797s][debug][gc,phases ] GC(0) Code Root Scanning (ms): Min: 0.0, Avg: 0.8, Max: 4.6, Diff: 4.6, Sum: 17.7, Workers: 23
[2.797s][debug][gc,phases ] GC(0) Object Copy (ms): Min: 135.2, Avg: 330.2, Max: 340.7, Diff: 205.6, Sum: 7594.1, Workers: 23
[2.797s][debug][gc,phases ] GC(0) Termination (ms): Min: 0.0, Avg: 0.8, Max: 1.6, Diff: 1.5, Sum: 18.7, Workers: 23
[2.798s][debug][gc,phases ] GC(0) Termination Attempts: Min: 1, Avg: 4.0, Max: 8, Diff: 7, Sum: 91, Workers: 23
[2.798s][debug][gc,phases ] GC(0) GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.1, Sum: 1.2, Workers: 23
[2.798s][debug][gc,phases ] GC(0) GC Worker Total (ms): Min: 340.6, Avg: 341.0, Max: 341.5, Diff: 0.8, Sum: 7844.1, Workers: 23
[2.798s][info ][gc,phases ] GC(0) Post Evacuate Collection Set: 4.7ms
[2.798s][debug][gc,phases ] GC(0) Code Roots Fixup: 0.0ms
[2.798s][debug][gc,phases ] GC(0) Preserve CM Refs: 0.0ms
[2.798s][debug][gc,phases ] GC(0) Reference Processing: 3.1ms
[2.798s][debug][gc,phases ] GC(0) Clear Card Table: 0.1ms
[2.798s][debug][gc,phases ] GC(0) Reference Enqueuing: 0.1ms
[2.798s][debug][gc,phases ] GC(0) Merge Per-Thread State: 0.2ms
[2.798s][debug][gc,phases ] GC(0) Code Roots Purge: 0.1ms
[2.798s][debug][gc,phases ] GC(0) Redirty Cards: 0.3ms
[2.798s][debug][gc,phases ] GC(0) Free Collection Set: 0.6ms
[2.798s][debug][gc,phases ] GC(0) Humongous Reclaim: 0.1ms
[2.799s][debug][gc,phases ] GC(0) Expand Heap After Collection: 0.0ms
[2.799s][info ][gc,phases ] GC(0) Other: 1.9ms
[2.799s][info ][gc,heap ] GC(0) Eden regions: 24->0(9)
[2.799s][info ][gc,heap ] GC(0) Survivor regions: 0->3(3)
[2.799s][info ][gc,heap ] GC(0) Old regions: 0->2
[2.799s][info ][gc,heap ] GC(0) Humongous regions: 2->1
[2.799s][info ][gc,metaspace ] GC(0) Metaspace: 4719K->4719K(1056768K)
[2.799s][info ][gc ] GC(0) Pause Young (G1 Evacuation Pause) 26M->5M(256M) 350.839ms
[2.799s][info ][gc,cpu ] GC(0) User=0.61s Sys=5.00s Real=0.35s
[…]

{Heap before GC invocations=0 (full 0):
garbage-first heap total 262144K, used 24576K [0xfffffffe60400000, 0xfffffffe60500800, 0xffffffff5e400000)
region size 1024K, 24 young (24576K), 0 survivors (0K)
Metaspace used 2835K, capacity 4596K, committed 4864K, reserved 1056768K
class space used 301K, capacity 424K, committed 512K, reserved 1048576K
Heap after GC invocations=1 (full 0):
}
{Heap before GC invocations=1 (full 0):
Heap after GC invocations=2 (full 0):
}
-XX:+PrintHeapAtGC

bash-3.2$ java -Xlog:gc+heap=debug GCTest
[0.113s][info][gc,heap] Heap region size: 1M
[0.113s][debug][gc,heap] Minimum heap 8388608 Initial heap 268435456 Maximum heap 4261412864
[2.529s][debug][gc,heap] GC(0) Heap before GC invocations=0 (full 0):
[2.529s][debug][gc,heap] GC(0) garbage-first heap total 262144K, used 26624K [0xfffffffe50400000, 0xfffffffe50500800,
0xffffffff4e400000)
[2.529s][debug][gc,heap] GC(0) region size 1024K, 24 young (24576K), 0 survivors (0K)
[2.530s][debug][gc,heap] GC(0) Metaspace used 4719K, capacity 4844K, committed 5120K, reserved 1056768K
[2.530s][debug][gc,heap] GC(0) class space used 413K, capacity 464K, committed 512K, reserved 1048576K
[2.892s][info ][gc,heap] GC(0) Eden regions: 24->0(9)
[2.892s][info ][gc,heap] GC(0) Survivor regions: 0->3(3)
[2.892s][info ][gc,heap] GC(0) Old regions: 0->2
[2.892s][info ][gc,heap] GC(0) Humongous regions: 2->1
[2.893s][debug][gc,heap] GC(0) Heap after GC invocations=1 (full 0):
0xffffffff4e400000)
[3.107s][debug][gc,heap] GC(1) Heap before GC invocations=1 (full 0):
0xffffffff4e400000)
[3.158s][info ][gc,heap] GC(1) Eden regions: 9->0(10)
[3.158s][info ][gc,heap] GC(1) Survivor regions: 3->2(2)
[…]
-Xlog:gc+heap=debug

bash-3.2$ java -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCApplicationConcurrentTime -XX:+UseG1GC GCTest
Application time: 0.6692265 seconds
Total time for which application threads were stopped: 0.0006422 seconds, Stopping threads took: 0.0003064 seconds
[…]
-XX:+PrintGCApplicationConcurrentTime
-XX:+PrintGCApplicationStoppedTime

bash-3.2$ java -Xlog:safepoint GCTest
[1.376s][info][safepoint] Application time: 0.3091519 seconds
[1.377s][info][safepoint] Total time for which application threads were stopped: 0.0004600 seconds, Stopping threads took:
0.0002648 seconds
0.0002297 seconds
0.0002130 seconds
0.0002285 seconds
0.0002681 seconds
0.0002210 seconds
0.0002244 seconds
0.0002243 seconds
[…]
-Xlog:safepoint

bash-3.2$ java -XX:+PrintAdaptiveSizePolicy -XX:+UseG1GC GCTest
0.323: [G1Ergonomics (Heap Sizing) expand the heap, requested expansion amount: 268435456 bytes, attempted expansion
amount: 268435456 bytes]
2.544: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 0, predicted base time: 10.00 ms, remaining
time: 190.00 ms, target pause time: 200.00 ms]
2.544: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 24 regions, survivors: 0 regions, predicted
young region time: 367.19 ms]
2.545: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 24 regions, survivors: 0 regions, old: 0 regions,
predicted pause time: 377.19 ms, target pause time: 200.00 ms]
3.013: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 6159, predicted base time: 33.53 ms,
remaining time: 166.47 ms, target pause time: 200.00 ms]
3.723: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 6657, predicted base time: 27.44 ms,
remaining time: 172.56 ms, target pause time: 200.00 ms]
[…]
-XX:+PrintAdaptiveSizePolicy

bash-3.2$ java -Xlog:gc+ergo*=trace GCTest [0.122s][debug][gc,ergo,refine] Initial Refinement Zones: green: 23, yellow:
69, red: 115, min yellow size: 46
[0.142s][debug][gc,ergo,heap ] Expand the heap. requested expansion amount:268435456B expansion amount:268435456B
[2.475s][trace][gc,ergo,cset ] GC(0) Start choosing CSet. pending cards: 0 predicted base time: 10.00ms remaining time:
190.00ms target pause time: 200.00ms
[2.476s][trace][gc,ergo,cset ] GC(0) Add young regions to CSet. eden: 24 regions, survivors: 0 regions, predicted young
region time: 367.19ms, target pause time: 200.00ms
[2.476s][debug][gc,ergo,cset ] GC(0) Finish choosing CSet. old: 0 regions, predicted old region time: 0.00ms, time
remaining: 0.00
[2.826s][debug][gc,ergo ] GC(0) Running G1 Clear Card Table Task using 1 workers for 1 units of work for 24 regions.
[2.827s][debug][gc,ergo ] GC(0) Running G1 Free Collection Set using 1 workers for collection set length 24
[2.828s][trace][gc,ergo,refine] GC(0) Updating Refinement Zones: update_rs time: 0.004ms, update_rs buffers: 0, update_rs
goal time: 19.999ms
[2.829s][debug][gc,ergo,refine] GC(0) Updated Refinement Zones: green: 23, yellow: 69, red: 115
[3.045s][trace][gc,ergo,cset ] GC(1) Start choosing CSet. pending cards: 5898 predicted base time: 26.69ms remaining
time: 173.31ms target pause time: 200.00ms
[3.045s][trace][gc,ergo,cset ] GC(1) Add young regions to CSet. eden: 9 regions, survivors: 3 regions, predicted young
region time: 457.38ms, target pause time: 200.00ms
[3.045s][debug][gc,ergo,cset ] GC(1) Finish choosing CSet. old: 0 regions, predicted old region time: 0.00ms, time
remaining: 0.00
[3.090s][debug][gc,ergo ] GC(1) Running G1 Clear Card Table Task using 1 workers for 1 units of work for 12 regions.
[3.091s][debug][gc,ergo ] GC(1) Running G1 Free Collection Set using 1 workers for collection set length 12
[3.093s][trace][gc,ergo,refine] GC(1) Updating Refinement Zones: update_rs time: 2.510ms, update_rs buffers: 25, update_rs
goal time: 19.999ms
[3.093s][debug][gc,ergo,refine] GC(1) Updated Refinement Zones: green: 25, yellow: 75, red: 125
[…]
-Xlog:gc+ergo*=trace

bash-3.2$ java -XX:+PrintTenuringDistribution -XX:MaxTenuringThreshold=15 -XX:+UseG1GC
GCTest
Desired survivor size 1048576 bytes, new threshold 15 (max 15)
- age 1: 143280 bytes, 143280 total
- age 1: 1679496 bytes, 1679496 total
- age 2: 115912 bytes, 1795408 total
- age 1: 17444896 bytes, 17444896 total
- age 2: 787568 bytes, 18232464 total
- age 3: 89512 bytes, 18321976 total
- age 1: 17357168 bytes, 17357168 total
-XX:+PrintTenuringDistribution

bash-3.2$ java -Xlog:gc+age=trace GCTest
[2.406s][debug][gc,age] GC(0) Desired survivor size 1572864 bytes, new threshold 15 (max threshold 15)
[2.745s][trace][gc,age] GC(0) Age table with threshold 15 (max threshold 15)
[2.745s][trace][gc,age] GC(0) - age 1: 3100640 bytes, 3100640 total
[…]
-Xlog:gc+age=trace

-Xloggc:<file>, UseGCLogFileRotation,
NumberOfGCLogFiles, GCLogFileSize
• bash-3.2$ java -Xloggc:gc.old.log -XX:+UseGCLogFileRotation -
XX:NumberOfGCLogFiles=3 -XX:+UseG1GC GCTest
• Log files
– gc.old.log.0
– gc.old.log.1
– gc.old.log.2.current
34

-Xlog:gc*:file=<file>::filecount=<count>,filesize=<filesize in
kb>
• bash-3.2$ java -Xlog:gc*:file=gc.new.log::filecount=3,filesize=1024 GCTest
• Log Files:
– gc.new.log
– gc.new.log.1
– gc.new.log.0
– gc.new.log.2
35

Other Useful Tags
• -Xlog:gc*=trace (all of gc logging at the lowest level)
• -Xlog:all=trace (all the JVM logging at the lowest level)
• -Xlog:disable (disable all logging)
• -Xlog:help (syntax and examples)
• -Xlog:logging=trace (logging for the logging framework)
36

bash-3.2$ java -Xlog:logging=trace GCTest
[0.604s][info][logging] Log configuration fully initialized.
[0.604s][debug][logging] Available log levels: off, trace, debug, info, warning, error
[0.604s][debug][logging] Available log decorators: time (t), utctime (utc), uptime (u),
timemillis (tm), uptimemillis (um), timenanos (tn), uptimenanos (un), hostname (hn), pid (p)
, tid (ti), level (l), tags (tg)
[0.604s][debug][logging] Available log tags: add, age, alloc, aot, annotation, arguments,
attach, barrier, biasedlocking, blocks, bot, breakpoint, census, class, classhisto, cleanu
p, compaction, constraints, constantpool, coops, cpu, cset, data, defaultmethods, dump, ergo,
exceptions, exit, fingerprint, freelist, gc, hashtables, heap, humongous, ihop, iklass
, init, itables, jni, jvmti, liveness, load, loader, logging, mark, marking, methodcomparator,
metadata, metaspace, mmu, modules, monitorinflation, monitormismatch, nmethod, normal
ize, objecttagging, obsolete, oopmap, os, pagesize, patch, path, phases, plab, promotion,
preorder, protectiondomain, ref, redefine, refine, region, remset, purge, resolve, safepoi
nt, scavenge, scrub, stacktrace, stackwalk, start, startuptime, state, stats, stringdedup,
stringtable, stackmap, subclass, survivor, sweep, task, thread, tlab, time, timer, update
, unload, verification, verify, vmoperation, vtables, workgang, jfr, system, parser, bytecode,
setting, event
[0.604s][debug][logging] Described tag combinations:
[0.604s][debug][logging] logging: Logging for the log framework itself
[0.605s][debug][logging] Log output configuration:

[0.605s][debug][logging] #0: stdout
os+cpu=warning,vmoperation=warning,verification=warning,ref=warning,modules+startuptime=warnin
g,monitorinflation=warning,stackwalk=warning,safepoint+cleanup=warning,gc+ref+start=warning,gc
+ref=warning,ergo=warning,gc+compaction=warning,os+thread=warning,safepoint=warning,pagesize=w
arning,os=warning,redefine+class+oopmap=warning,redefine+class+nmethod=warning,modules+patch=w
arning,redefine+class+methodcomparator=warning,startuptime=warning,gc+metaspace+freelist+block
s=warning,gc+metaspace+alloc=warning,gc+metaspace=warning,gc+metaspace+freelist=warning,class+
loader+constraints=warning,vtables=warning,itables=warning,redefine+class+update+itables=warni
ng,redefine+class+obsolete+metadata=warning,redefine+class+timer=warning,redefine+class+normal
ize=warning,redefine+class+load=warning,redefine+class+load+exceptions=warning,redefine+class+
constantpool=warning,redefine+class+annotation=warning,redefine+class+stackmap=warning,redefin
e+class+obsolete+mark=warning,redefine+class+subclass=warning,redefine+class=warning,redefine+
class+dump=warning,redefine+class+breakpoint=warning,jvmti+objecttagging=warning,jvmti=warning
,jfr+system+event=warning,jfr+system+setting=warning,jfr+system+bytecode=warning,jfr+system+pa
rser=warning,jfr+system+metadata=warning,jfr+metadata=warning,jfr+event=warning,jfr+setting=wa
rning,jfr=warning,jfr+system=warning,stacktrace=warning,gc+heap+exit=warning,redefine+class+ob
solete=warning,class+init=warning,modules=warning,redefine+class+update+vtables=warning,redefi
ne+class+iklass+purge=warning,redefine+class+iklass+add=warning,monitormismatch=warning,gc+cpu
=warning,gc+task+thread=warning,gc+task+time=warning,gc+jni=warning,gc+stringdedup=warning,gc+
marking+start=warning,gc+remset=warning,gc+remset+exit=warning,gc+verify+start=warning,gc+ihop
=warning,gc+phases+task=warning,gc+stats=warning,gc+ergo+cset=warning,gc+stringtable=warning,g
c+heap+region=warning,gc+task+stats=warning,gc+humongous=warning,gc+region=warning,protectiond
omain=warning,gc+ergo+ihop=warning,gc+scavenge=warning,gc+mmu=warning,defaultmethods=warning,g
c+ergo+heap=warning,redefine+class+update=warning
[…]

bash-3.2$ jcmd 29688 VM.log
29688:
Syntax : VM.log [options]
Options: (options must be specified using the <key> or <key>=<value> syntax)
output : [optional] The name or index (#<index>) of output to configure.
(STRING, no default value)
output_options : [optional] Options for the output. (STRING, no default value)
what : [optional] Configures what tags to log. (STRING, no default value)
decorators : [optional] Configures which decorators to use. Use 'none' or an
empty value to remove all. (STRING, no default value)
disable : [optional] Turns off all logging and clears the log configuration.
(BOOLEAN, no default value)
list : [optional] Lists current log configuration. (BOOLEAN, no default value)
rotate : [optional] Rotates all logs. (BOOLEAN, no default value)
bash-3.2$ jcmd 29688 VM.log what=gc*
4460:
Command executed successfully
Runtime Configuration of -Xlog

Old GC logging option Equivalent –Xlog:gc configuration
G1PrintHeapRegions -Xlog:gc+region=trace
G1PrintRegionLivenessInfo -Xlog:gc+liveness=trace
G1SummarizeConcMark -Xlog:gc+marking=trace
G1SummarizeRSetStats -Xlog:gc+remset*=trace
GCLogFileSize, NumberOfGCLogFiles,
UseGCLogFileRotation
-Xlog:gc*:file=<file>::filecount=<count>,filesize=<filesize
in kb>
PrintAdaptiveSizePolicy -Xlog:gc+ergo*=trace
PrintClassHistogramAfterFullGC -Xlog:classhisto*=trace
PrintClassHistogramBeforeFullGC -Xlog:classhisto*=trace
PrintGCApplicationConcurrentTime -Xlog:safepoint
PrintGCApplicationStoppedTime -Xlog:safepoint
PrintGCCause Always logged
PrintGCDateStamps ‘time’ decorator
PrintGCID Always logged

PrintGCTaskTimeStamps -Xlog:gc+task=trace
PrintGCTimeStamps ‘uptime’ decorator
PrintHeapAtGC -Xlog:gc+heap=debug
PrintHeapAtGCExtended -Xlog:gc+heap=trace
PrintJNIGCStalls -Xlog:gc+jni=debug
PrintOldPLAB -Xlog:gc+plab=trace
PrintParallelOldGCPhaseTimes -Xlog:gc+phases=trace
PrintPLAB -Xlog:gc+plab=trace
PrintPromotionFailure -Xlog:gc+promotion=debug
PrintReferenceGC -Xlog:gc+ref=debug
PrintStringDeduplicationStatistics -Xlog:gc+stringdedup
PrintTaskqueue -Xlog:gc+task+stats=trace
PrintTenuringDistribution -Xlog:gc+age=trace

PrintTerminationStats -Xlog:gc+task+stats=debug
PrintTLAB -Xlog:gc+tlab=trace
TraceAdaptiveGCBoundary -Xlog:heap+ergo=debug
TraceDynamicGCThreads -Xlog:gc+task=trace
TraceMetadataHumongousAllocation -Xlog:gc+metaspace+alloc=debug
G1TraceConcRefinement -Xlog:gc+refine=debug
G1TraceEagerReclaimHumongousObjects -Xlog:gc+humongous=debug
G1TraceStringSymbolTableScrubbing -Xlog:gc+stringtable=trace

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