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Garbage First Garbage
Collector: Where The
Rubber Meets The Road!
-By Monica Beckwith
Code Karam LLC
@mon_beck; monica@cod...
©2017 CodeKaram
About me
• Java performance engineer
• President @Code Karam LLC
• Partner @Extreme Performance Experts
• ...
©2017 CodeKaram
Agenda
• Background on G1 GC
• Heap regions - Regular and Humongous
• Additional data structures
• G1 GC p...
©2017 CodeKaram
Agenda
• Tuning considerations with G1 GC
• Taming mixed GCs
• Components of a G1 GC pause
• Humongous obj...
G1 GC Background
5
Regionalized Heap
6
©2017 CodeKaram
Traditional Java Heap
7
Contiguous Java Heap
Eden S0 S1
Old
Generation
©2017 CodeKaram
Garbage First GC - Heap
Regions
8
Contiguous Java Heap
Free
Region
Non-Free
Region
©2017 CodeKaram
G1 GC Heap Regions
• Young Regions - Regions that contain objects in
the Eden and Survivor Spaces
• Old Re...
©2017 CodeKaram
Eden
Old Old
Eden
Old
Survivor
Humongous
10
Garbage First GC - Heap
Regions
Humongous Objects
11
©2017 CodeKaram
Humongous Objects
12
An old generation region
A young generation region
©2017 CodeKaram
Humongous Objects
13
Object < 50% of
G1 region size
Object >= 50% of
G1 region size
Object > G1
region siz...
©2017 CodeKaram14
Object < 50% of
G1 region size
Humongous Objects
©2017 CodeKaram15
Object >= 50% of
G1 region size
Humongous Objects
©2017 CodeKaram16
Object > G1
region size
Humongous Objects
©2017 CodeKaram17
Object NOT Humongous
Object Humongous
Object Humongous ->
Needs Contiguous Regions
Humongous Objects
G1 GC Maintenance
18
Collection Set
19
©2017 CodeKaram
Collection Set
• A young collection set (CSet) will incorporate all
the young regions
• A mixed collection...
©2017 CodeKaram
Eden
Old Old
Eden
Old
Surv
ivor
CSet during a young collection -
21
Collection Set
©2017 CodeKaram
Old Old
Old
CSet during a mixed collection -
Sur
viv
or
Ol
d
Eden Eden
22
Collection Set
Remembered Sets
23
©2017 CodeKaram
Remembered Sets
• Additional data structures to help with
maintenance
• Add a slight footprint overhead (~...
©2017 CodeKaram
• Maintain and track incoming references into its
region
• old-to-young references
• old-to-old references...
©2017 CodeKaram26
Figure 2.3 Remembered sets with incoming object references**
<insert G1-Deep-Dive-Figure-3.tif>
empty re...
G1 GC Phases
27
©2017 CodeKaram
G1 GC - Pause Histogram
28
Pausetimeinmilliseconds
0
30
60
90
120
Timestamps
3415 3416.3 3417.2 3418.4 341...
A Young Collection
29
©2017 CodeKaram
The Garbage First Collector
Eden
Old Old
Eden
Old
Surv
ivor
E.g.: Current heap configuration -
30
©2017 CodeKaram
The Garbage First Collector
Eden
Old Old
Eden
Old
Surv
ivor
E.g.: During a young collection -
31
©2017 CodeKaram
The Garbage First Collector
Old Old
Old
E.g.: After a young collection -
Sur
viv
or
Ol
d
32
©2017 CodeKaram
Old Old
Old
Sur
viv
or
Ol
d
Eden Eden
Old
The Garbage First Collector
33
E.g.: Current heap configuration -
Marking Initiation
34
©2017 CodeKaram
Initiating Heap Occupancy
Percent
35
• Threshold to start the concurrent marking cycle
to identify candida...
©2017 CodeKaram
Initiating Heap Occupancy
Percent - Helpful Options
36
• -XX:InitiatingHeapOccupancyPercent=<p>
• -XX:Conc...
The Concurrent
Marking Stages
37
©2017 CodeKaram
Stages of Concurrent
Marking
38
• Initial-mark
• Root region scan
• Concurrent mark
• Remark / Final mark
...
©2017 CodeKaram
Class Unloading with
Concurrent Mark
• With JDK 8 update 40, you have
ClassUnloadingWithConcurrentMark, an...
©2017 CodeKaram
Old Old
Old
E.g.: Reclamation of a garbage-filled region
during the cleanup phase -
Sur
viv
or
Ol
d
Eden E...
©2017 CodeKaram
Old Old
Old
E.g.: Reclamation of a garbage-filled region
during the cleanup phase -
Sur
viv
or
Ol
d
Eden E...
Incremental Compaction
aka Mixed Collection
42
©2017 CodeKaram
Old Old
Old
E.g.: Current heap configuration -
Sur
viv
or
Ol
d
Eden Eden
The Garbage First Collector
43
©2017 CodeKaram
Old Old
Old
E.g.: During a mixed collection -
Sur
viv
or
Ol
d
Eden Eden
The Garbage First Collector
44
©2017 CodeKaram
Old
E.g.: After a mixed collection -
O
l
d
Sur
vivo
r
Old
The Garbage First Collector
45
Tuning Considerations
with G1 GC
46
Taming Mixed GCs
47
©2017 CodeKaram
Say,
The young collections are meeting your SLAs,
but…
The mixed collections are far too frequent and
too ...
©2017 CodeKaram49
©2017 CodeKaram
If meeting the latency SLA is your only concern:
Divide the expensive collection further into
smaller inex...
©2017 CodeKaram
Adjusting Each Mixed
Collection
51
Minimum number of old regions to
be included in the mixed collection
se...
©2017 CodeKaram
Adjusting Each Mixed
Collection
52
-XX:G1MixedGCCountTarget=<n>
-XX:G1OldCSetRegionThresholdPercent=<p>
©2017 CodeKaram
If the GC overhead is too high and you have heap
to spare (after considering your humongous
objects requir...
©2017 CodeKaram
Eliminating Expensive Old
Regions From Mixed Collections
• You could eliminate based on the liveness per
o...
©2017 CodeKaram
Eliminating Expensive Old
Regions From Mixed Collections
• -XX:G1MixedGCLiveThresholdPercent = <p>
• -XX:G...
Components of a G1
GC Pause
56
©2017 CodeKaram57
©2017 CodeKaram
Ideally, you will see that most of your pause
time is spent in copying live objects…
58
Major Contributor?
©2017 CodeKaram59
©2017 CodeKaram
but, what if that’s not the case?
60
Major Contributor?
61
©2017 CodeKaram
, 0.4066560 secs]
[Parallel Time: 354.9 ms, GC Workers: 48]
[GC Worker Start (ms): Min: 4500060.2, Avg: 45...
©2017 CodeKaram
[Code Root Fixup: 0.1 ms]
[Code Root Migration: 0.2 ms]
[Clear CT: 1.4 ms]
[Other: 50.1 ms]
[Choose CSet: ...
©2017 CodeKaram
[Code Root Fixup: 0.1 ms]
[Code Root Migration: 0.2 ms]
[Clear CT: 1.4 ms]
[Other: 50.1 ms]
[Choose CSet: ...
©2017 CodeKaram
, 0.4066560 secs]
[Parallel Time: 354.9 ms, GC Workers: 48]
[GC Worker Start (ms): Min: 4500060.2, Avg: 45...
66
Plot Showing Max Times
67
Plot Showing Avg Times
Humongous Objects
Requirements
68
©2017 CodeKaram
Ideally, humongous objects are few in
number and are short lived.
A lot of long-lived humongous objects ca...
©2017 CodeKaram70
Object NOT Humongous
Object Humongous
Object Humongous ->
Needs Contiguous Regions
Humongous Objects
©2017 CodeKaram
Humongous Objects:
• Are allocated out of the old generation
• Are not moved
*Note: Since JDK8 update 40, ...
©2017 CodeKaram72
Object NOT Humongous
Object Humongous
Object Humongous ->
Needs Contiguous Regions
Wasted
Space!
Humongo...
©2017 CodeKaram
Humongous objects can pose the following issues:
Wasted space
Evacuation failures due to not having enough...
Fragmentation In The
G1 Collector
74
©2017 CodeKaram
• G1 GC is designed to “absorb” some
fragmentation.
• Default is 5% of the total Java heap
• Tradeoff so t...
©2017 CodeKaram
G1 Mixed GC (Region)
Liveness Threshold
76
• G1 GC’s old regions are also designed to
“absorb” some fragme...
©2017 CodeKaram
Humongous Objects
77
• Wasted space!
• External fragmentation!
©2017 CodeKaram
Humongous Objects
78
• Wasted space!
• External fragmentation!
©2017 CodeKaram
Fragmentation Can Lead To
Evacuation Failures!
79
Promotion/Evacuation
Failures In The G1
Collector
80
©2017 CodeKaram
Evacuation Failures
81
276.731: [GC pause (G1 Evacuation Pause) (young) (to-space exhausted),
0.8272932 se...
©2017 CodeKaram
• When there are no more regions available for
survivors or tenured objects, G1 GC encounters
an evacuatio...
©2017 CodeKaram
A heavily tuned JVM command line
may be restricting the G1 GC
ergonomics and adaptability.
Start with just...
©2017 CodeKaram
Your live data set + long live
transient data may be too large for
the old generation
Check LDS+ and incre...
©2017 CodeKaram
Initiating Heap Occupancy Threshold
could be the issue.
Check IHOP and make sure it accommodates
the LDS+....
©2017 CodeKaram
Marking Cycle could be taking too
long to complete?
Increase concurrent marking threads
Reduce IHOP
86
Avo...
©2017 CodeKaram
to-space survivors are the problem?
Increase the G1ReservePercent, if to-space
survivors are triggering th...
88
89
90
©2017 CodeKaram
• 487.817: [G1Ergonomics (Heap Sizing) attempt heap
expansion, reason: region allocation request failed, a...
©2017 CodeKaram
• 487.817: [G1Ergonomics (Heap Sizing) attempt heap
expansion, reason: region allocation request failed, a...
93
Allocation and
Promotion Rates
94
©2017 CodeKaram
Plot Allocation & Promotion
Rates
95
©2017 CodeKaram96
Young Occupancy before GC Young Gen Size Old Gen Occupancy after GC
Heap Occupancy before GC Heap Occupa...
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Garbage First Garbage Collector: Where the Rubber Meets the Road!

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Garbage First Garbage Collector: Where the Rubber Meets the Road!

  1. 1. Garbage First Garbage Collector: Where The Rubber Meets The Road! -By Monica Beckwith Code Karam LLC @mon_beck; monica@codekaram.com 1
  2. 2. ©2017 CodeKaram About me • Java performance engineer • President @Code Karam LLC • Partner @Extreme Performance Experts • I have worked with Oracle, Sun, AMD … • I used to work in the capacity of G1 GC performance lead @Oracle. 2
  3. 3. ©2017 CodeKaram Agenda • Background on G1 GC • Heap regions - Regular and Humongous • Additional data structures • G1 GC phases 3
  4. 4. ©2017 CodeKaram Agenda • Tuning considerations with G1 GC • Taming mixed GCs • Components of a G1 GC pause • Humongous object requirements • Fragmentation • Evacuation failures • Allocation rate and promotion rate 4
  5. 5. G1 GC Background 5
  6. 6. Regionalized Heap 6
  7. 7. ©2017 CodeKaram Traditional Java Heap 7 Contiguous Java Heap Eden S0 S1 Old Generation
  8. 8. ©2017 CodeKaram Garbage First GC - Heap Regions 8 Contiguous Java Heap Free Region Non-Free Region
  9. 9. ©2017 CodeKaram G1 GC Heap Regions • Young Regions - Regions that contain objects in the Eden and Survivor Spaces • Old Regions - Regions that contain objects in the Old generation. • Humongous Regions - Regions that contain Humongous Objects. 9
  10. 10. ©2017 CodeKaram Eden Old Old Eden Old Survivor Humongous 10 Garbage First GC - Heap Regions
  11. 11. Humongous Objects 11
  12. 12. ©2017 CodeKaram Humongous Objects 12 An old generation region A young generation region
  13. 13. ©2017 CodeKaram Humongous Objects 13 Object < 50% of G1 region size Object >= 50% of G1 region size Object > G1 region size ??
  14. 14. ©2017 CodeKaram14 Object < 50% of G1 region size Humongous Objects
  15. 15. ©2017 CodeKaram15 Object >= 50% of G1 region size Humongous Objects
  16. 16. ©2017 CodeKaram16 Object > G1 region size Humongous Objects
  17. 17. ©2017 CodeKaram17 Object NOT Humongous Object Humongous Object Humongous -> Needs Contiguous Regions Humongous Objects
  18. 18. G1 GC Maintenance 18
  19. 19. Collection Set 19
  20. 20. ©2017 CodeKaram Collection Set • A young collection set (CSet) will incorporate all the young regions • A mixed collection set will incorporate all the young regions and a few candidate old regions based on the “most garbage first” principle. 20
  21. 21. ©2017 CodeKaram Eden Old Old Eden Old Surv ivor CSet during a young collection - 21 Collection Set
  22. 22. ©2017 CodeKaram Old Old Old CSet during a mixed collection - Sur viv or Ol d Eden Eden 22 Collection Set
  23. 23. Remembered Sets 23
  24. 24. ©2017 CodeKaram Remembered Sets • Additional data structures to help with maintenance • Add a slight footprint overhead (~5%) 24
  25. 25. ©2017 CodeKaram • Maintain and track incoming references into its region • old-to-young references • old-to-old references • Remembered sets have varying granularity based on the “popularity” of objects or regions. 25 Remembered Sets
  26. 26. ©2017 CodeKaram26 Figure 2.3 Remembered sets with incoming object references** <insert G1-Deep-Dive-Figure-3.tif> empty region young region old region incoming reference into a region incoming reference into an RSet’s owning region RSet for Region y RSet for Region z Region z Region x Region y RSet for Region x <insert G1-Deep-Dive-Figure-3.tif> Figure G1-Deep-Dive-Figure-3: RSet example for one young and two old regions. G1 GC has its way of handling such demands of popularity and it does so by changing the density of RSets. The density of RSets follow three levels of granularity namely; sparse, fine and coarse. For a popular region, the RSet would probably get coarsened to accommodate the pointers from various other regions. This will be reflected in the RSet scanning time for those regions. LEGENDS empty region young region old region incoming reference into a region incoming reference into an RSet’s owning region RSet for Region y RSet for Region z Region z Region x Region y RSet for Region x
  27. 27. G1 GC Phases 27
  28. 28. ©2017 CodeKaram G1 GC - Pause Histogram 28 Pausetimeinmilliseconds 0 30 60 90 120 Timestamps 3415 3416.3 3417.2 3418.4 3419 3422 3423.4 3432.2 3433.2 3436.8 3437.6 3438.9 3440 Young Collection Initial Mark Remark Cleanup Mixed Collection Occupancy == Initiating Heap Occupancy Percent
  29. 29. A Young Collection 29
  30. 30. ©2017 CodeKaram The Garbage First Collector Eden Old Old Eden Old Surv ivor E.g.: Current heap configuration - 30
  31. 31. ©2017 CodeKaram The Garbage First Collector Eden Old Old Eden Old Surv ivor E.g.: During a young collection - 31
  32. 32. ©2017 CodeKaram The Garbage First Collector Old Old Old E.g.: After a young collection - Sur viv or Ol d 32
  33. 33. ©2017 CodeKaram Old Old Old Sur viv or Ol d Eden Eden Old The Garbage First Collector 33 E.g.: Current heap configuration -
  34. 34. Marking Initiation 34
  35. 35. ©2017 CodeKaram Initiating Heap Occupancy Percent 35 • Threshold to start the concurrent marking cycle to identify candidate old regions. • When old generation occupancy crosses this adaptive threshold. • Based on the total heap size.
  36. 36. ©2017 CodeKaram Initiating Heap Occupancy Percent - Helpful Options 36 • -XX:InitiatingHeapOccupancyPercent=<p> • -XX:ConcGCThreads=<n>
  37. 37. The Concurrent Marking Stages 37
  38. 38. ©2017 CodeKaram Stages of Concurrent Marking 38 • Initial-mark • Root region scan • Concurrent mark • Remark / Final mark • Cleanup
  39. 39. ©2017 CodeKaram Class Unloading with Concurrent Mark • With JDK 8 update 40, you have ClassUnloadingWithConcurrentMark, and unreachable classes can be unloaded during Remark. 39
  40. 40. ©2017 CodeKaram Old Old Old E.g.: Reclamation of a garbage-filled region during the cleanup phase - Sur viv or Ol d Eden Eden Old The Garbage First Collector 40
  41. 41. ©2017 CodeKaram Old Old Old E.g.: Reclamation of a garbage-filled region during the cleanup phase - Sur viv or Ol d Eden Eden The Garbage First Collector 41
  42. 42. Incremental Compaction aka Mixed Collection 42
  43. 43. ©2017 CodeKaram Old Old Old E.g.: Current heap configuration - Sur viv or Ol d Eden Eden The Garbage First Collector 43
  44. 44. ©2017 CodeKaram Old Old Old E.g.: During a mixed collection - Sur viv or Ol d Eden Eden The Garbage First Collector 44
  45. 45. ©2017 CodeKaram Old E.g.: After a mixed collection - O l d Sur vivo r Old The Garbage First Collector 45
  46. 46. Tuning Considerations with G1 GC 46
  47. 47. Taming Mixed GCs 47
  48. 48. ©2017 CodeKaram Say, The young collections are meeting your SLAs, but… The mixed collections are far too frequent and too many OR The mixed collections are blowing your SLAs 48 Why Tame Mixed Collections?
  49. 49. ©2017 CodeKaram49
  50. 50. ©2017 CodeKaram If meeting the latency SLA is your only concern: Divide the expensive collection further into smaller inexpensive collections 50 What Can We Do?
  51. 51. ©2017 CodeKaram Adjusting Each Mixed Collection 51 Minimum number of old regions to be included in the mixed collection set. Maximum number of old regions to be included in the mixed collection set.
  52. 52. ©2017 CodeKaram Adjusting Each Mixed Collection 52 -XX:G1MixedGCCountTarget=<n> -XX:G1OldCSetRegionThresholdPercent=<p>
  53. 53. ©2017 CodeKaram If the GC overhead is too high and you have heap to spare (after considering your humongous objects requirements): Remove the expensive regions from your collection set 53 What Can We Do?
  54. 54. ©2017 CodeKaram Eliminating Expensive Old Regions From Mixed Collections • You could eliminate based on the liveness per old region • You could also eliminate expensive old regions towards the end of the sorted array 54
  55. 55. ©2017 CodeKaram Eliminating Expensive Old Regions From Mixed Collections • -XX:G1MixedGCLiveThresholdPercent = <p> • -XX:G1HeapWastePercent = <p> 55
  56. 56. Components of a G1 GC Pause 56
  57. 57. ©2017 CodeKaram57
  58. 58. ©2017 CodeKaram Ideally, you will see that most of your pause time is spent in copying live objects… 58 Major Contributor?
  59. 59. ©2017 CodeKaram59
  60. 60. ©2017 CodeKaram but, what if that’s not the case? 60 Major Contributor?
  61. 61. 61
  62. 62. ©2017 CodeKaram , 0.4066560 secs] [Parallel Time: 354.9 ms, GC Workers: 48] [GC Worker Start (ms): Min: 4500060.2, Avg: 4500068.1, Max: 4500085.2, Diff: 24.9] [Ext Root Scanning (ms): Min: 0.0, Avg: 4.8, Max: 31.8, Diff: 31.8, Sum: 231.0] [Update RS (ms): Min: 0.0, Avg: 21.6, Max: 116.3, Diff: 116.3, Sum: 1036.3] [Processed Buffers: Min: 0, Avg: 4.9, Max: 29, Diff: 29, Sum: 234] [Scan RS (ms): Min: 0.0, Avg: 41.5, Max: 62.3, Diff: 62.2, Sum: 1992.1] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.3] [Object Copy (ms): Min: 195.6, Avg: 233.1, Max: 261.1, Diff: 65.5, Sum: 11189.0] [Termination (ms): Min: 21.0, Avg: 45.6, Max: 74.4, Diff: 53.4, Sum: 2188.5] [GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 4.8] [GC Worker Total (ms): Min: 329.6, Avg: 346.7, Max: 354.7, Diff: 25.1, Sum: 16642.0] [GC Worker End (ms): Min: 4500414.7, Avg: 4500414.8, Max: 4500415.0, Diff: 0.2] 62
  63. 63. ©2017 CodeKaram [Code Root Fixup: 0.1 ms] [Code Root Migration: 0.2 ms] [Clear CT: 1.4 ms] [Other: 50.1 ms] [Choose CSet: 0.0 ms] [Ref Proc: 34.6 ms] [Ref Enq: 0.3 ms] [Free CSet: 7.5 ms] 63
  64. 64. ©2017 CodeKaram [Code Root Fixup: 0.1 ms] [Code Root Migration: 0.2 ms] [Clear CT: 1.4 ms] [Other: 50.1 ms] [Choose CSet: 0.0 ms] [Ref Proc: 34.6 ms] [Ref Enq: 0.3 ms] [Free CSet: 7.5 ms] 64
  65. 65. ©2017 CodeKaram , 0.4066560 secs] [Parallel Time: 354.9 ms, GC Workers: 48] [GC Worker Start (ms): Min: 4500060.2, Avg: 4500068.1, Max: 4500085.2, Diff: 24.9] [Ext Root Scanning (ms): Min: 0.0, Avg: 4.8, Max: 31.8, Diff: 31.8, Sum: 231.0] [Update RS (ms): Min: 0.0, Avg: 21.6, Max: 116.3, Diff: 116.3, Sum: 1036.3] [Processed Buffers: Min: 0, Avg: 4.9, Max: 29, Diff: 29, Sum: 234] [Scan RS (ms): Min: 0.0, Avg: 41.5, Max: 62.3, Diff: 62.2, Sum: 1992.1] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.3] [Object Copy (ms): Min: 195.6, Avg: 233.1, Max: 261.1, Diff: 65.5, Sum: 11189.0] [Termination (ms): Min: 21.0, Avg: 45.6, Max: 74.4, Diff: 53.4, Sum: 2188.5] [GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 4.8] [GC Worker Total (ms): Min: 329.6, Avg: 346.7, Max: 354.7, Diff: 25.1, Sum: 16642.0] [GC Worker End (ms): Min: 4500414.7, Avg: 4500414.8, Max: 4500415.0, Diff: 0.2] 65
  66. 66. 66 Plot Showing Max Times
  67. 67. 67 Plot Showing Avg Times
  68. 68. Humongous Objects Requirements 68
  69. 69. ©2017 CodeKaram Ideally, humongous objects are few in number and are short lived. A lot of long-lived humongous objects can cause evacuation failures since humongous regions add to the old generation occupancy. 69 Humongous Objects
  70. 70. ©2017 CodeKaram70 Object NOT Humongous Object Humongous Object Humongous -> Needs Contiguous Regions Humongous Objects
  71. 71. ©2017 CodeKaram Humongous Objects: • Are allocated out of the old generation • Are not moved *Note: Since JDK8 update 40, they can be collected during a young collection 71 So, What Did We Observe?
  72. 72. ©2017 CodeKaram72 Object NOT Humongous Object Humongous Object Humongous -> Needs Contiguous Regions Wasted Space! Humongous Objects
  73. 73. ©2017 CodeKaram Humongous objects can pose the following issues: Wasted space Evacuation failures due to not having enough (to-space) regions 73 So, What Did We Observe?
  74. 74. Fragmentation In The G1 Collector 74
  75. 75. ©2017 CodeKaram • G1 GC is designed to “absorb” some fragmentation. • Default is 5% of the total Java heap • Tradeoff so that expensive regions are left out. G1 Heap Waste Percentage 75
  76. 76. ©2017 CodeKaram G1 Mixed GC (Region) Liveness Threshold 76 • G1 GC’s old regions are also designed to “absorb” some fragmentation. • Default is 85% liveness in a G1 region. • Tradeoff so that expensive regions are left out.
  77. 77. ©2017 CodeKaram Humongous Objects 77 • Wasted space! • External fragmentation!
  78. 78. ©2017 CodeKaram Humongous Objects 78 • Wasted space! • External fragmentation!
  79. 79. ©2017 CodeKaram Fragmentation Can Lead To Evacuation Failures! 79
  80. 80. Promotion/Evacuation Failures In The G1 Collector 80
  81. 81. ©2017 CodeKaram Evacuation Failures 81 276.731: [GC pause (G1 Evacuation Pause) (young) (to-space exhausted), 0.8272932 secs] [Parallel Time: 387.0 ms, GC Workers: 8] <snip> [Code Root Fixup: 0.1 ms] [Code Root Purge: 0.0 ms] [Clear CT: 0.2 ms] [Other: 440.0 ms] [Evacuation Failure: 437.5 ms] [Choose CSet: 0.0 ms] [Ref Proc: 0.1 ms] [Ref Enq: 0.0 ms] [Redirty Cards: 0.9 ms] [Humongous Reclaim: 0.0 ms] [Free CSet: 0.9 ms] [Eden: 831.0M(900.0M)->0.0B(900.0M) Survivors: 0.0B->0.0B Heap: 1020.1M(1024.0M)- >1020.1M(1024.0M)] [Times: user=3.64 sys=0.20, real=0.83 secs] **
  82. 82. ©2017 CodeKaram • When there are no more regions available for survivors or tenured objects, G1 GC encounters an evacuation failure. • An evacuation failure is expensive and the usual pattern is that if you see a couple of evacuation failures; full GC could* soon follow. 82 Evacuation Failures
  83. 83. ©2017 CodeKaram A heavily tuned JVM command line may be restricting the G1 GC ergonomics and adaptability. Start with just your heap sizes and a reasonable pause time goal 83 Avoiding Evacuation Failures
  84. 84. ©2017 CodeKaram Your live data set + long live transient data may be too large for the old generation Check LDS+ and increase heap to accommodate everything in the old generation. 84 Avoiding Evacuation Failures
  85. 85. ©2017 CodeKaram Initiating Heap Occupancy Threshold could be the issue. Check IHOP and make sure it accommodates the LDS+. IHOP threshold too high -> Delayed marking -> Delayed incremental compaction -> Evacuation Failures! 85 Avoiding Evacuation Failures
  86. 86. ©2017 CodeKaram Marking Cycle could be taking too long to complete? Increase concurrent marking threads Reduce IHOP 86 Avoiding Evacuation Failures
  87. 87. ©2017 CodeKaram to-space survivors are the problem? Increase the G1ReservePercent, if to-space survivors are triggering the evacuation failures! 87 Avoiding Evacuation Failures
  88. 88. 88
  89. 89. 89
  90. 90. 90
  91. 91. ©2017 CodeKaram • 487.817: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason: region allocation request failed, allocation request: 524280 bytes] • 487.817: [G1Ergonomics (Heap Sizing) expand the heap, requested expansion amount: 524280 bytes, attempted expansion amount: 1048576 bytes] • 487.817: [G1Ergonomics (Heap Sizing) did not expand the heap, reason: heap already fully expanded] • 487.888: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason: recent GC overhead higher than threshold after GC, recent GC overhead: 28.40 %, threshold: 10.00 %, uncommitted: 0 bytes, calculated expansion amount: 0 bytes (20.00 %)] 91
  92. 92. ©2017 CodeKaram • 487.817: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason: region allocation request failed, allocation request: 524280 bytes] • 487.817: [G1Ergonomics (Heap Sizing) expand the heap, requested expansion amount: 524280 bytes, attempted expansion amount: 1048576 bytes] • 487.817: [G1Ergonomics (Heap Sizing) did not expand the heap, reason: heap already fully expanded] • 487.888: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason: recent GC overhead higher than threshold after GC, recent GC overhead: 28.40 %, threshold: 10.00 %, uncommitted: 0 bytes, calculated expansion amount: 0 bytes (20.00 %)] 92
  93. 93. 93
  94. 94. Allocation and Promotion Rates 94
  95. 95. ©2017 CodeKaram Plot Allocation & Promotion Rates 95
  96. 96. ©2017 CodeKaram96 Young Occupancy before GC Young Gen Size Old Gen Occupancy after GC Heap Occupancy before GC Heap Occupancy after GC Heap Size Timestamps

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