Transcript Java_Garbage_Collector

G1 TUNING
Shubham Modi(10327729)
Ujjwal Kumar Singh(10772)
Vaibhav(10780)
Motivation
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There is memory sensitive application which is running and now a new object
has to be created. Suppose that the memory heap has insufficient space for
the newly created objects. Such conditions would trigger a full garbage
collection cycle, which will perform compaction of the heap.
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With CMS GC, the full collection is serial and STW, hence your application
threads are stopped for the entire duration while the heap space is reclaimed
and then compacted.
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This affects the performance of the application and will decrease the
responsiveness and throughput.
G1 Garbage Collector
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G1 is a compacting collector i.e. it compacts sufficiently to completely avoid
the use of fine-grained free lists for allocation, and instead relies on regions.
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The heap is partitioned into a set of equal-sized heap regions, each a
contiguous range of virtual memory providing greater flexibility in memory
usage.
[6]
How G1 works?
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Heap is split into many fixed size region ranging from 1 to 32 Mb
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Regions are mapped into Eden,
Survivor and Old generation
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 This is a STW pause. Eden and
Survivor size is calculated for the
next GC. GC Pause time is taken into
consideration.
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Live objects have been evacuated to
survivor regions or to old generation
regions
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 Liveness of regions is calculated
and empty regions are marked X
and are deleted in remark phase
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After calculating liveness of regions
G1 selects the region with least
liveness and collects it. Both young
and old generation are collected at
the same time.
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After it we get compacted dark blue
and dark green regions
Why G1?
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Provide predictable GC pause duration
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Decrease frequency of Stop The World(Full GC Cycle)
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Compact free space without lengthy GC induced pause times
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Do not require much larger Java heap space
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Do not sacrifices throughput of application
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Compaction mostly eliminating the fragmentation issues
Objectives
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Improves responsiveness
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Increase throughput
Problem
root
child1
child2
child3
………
Child
5million
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When we try to create 10 million children of root, it gives OutofHeap Error
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We created first tree with root and 5 million children occupying ~ 55% of heap
space
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Then we made child1 as root of tree
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Therefore all other nodes becomes unreachable
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Then we created another tree with a root and its 5 million children
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Hence Garbage collector is called to free the space causing Stop The World
event to occur
Factors affecting G1 Performance
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GC Pause Time: Set the target for the maximum GC pause time(soft goal)
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Heap Occupancy: Percentage of the entire heap occupancy to start the
concurrent GC cycle(Default 45)
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Old Generation size: New to Old Generation size ratio(Default 2)
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Heap region size: Size of the single region of uniformly divided space(Min 1
and Max 32Mb)
Results
Here we achieved high
responsiveness with average
GC pause time as 0.010 sec
which is normally 0.1 sec
Here we achieved high
throughput 35% which is
normally 25-27% in other cases.
As shown in this case we
skipped Stop The World event
 Grey region shows concurrent GC pauses of an application
 Dark grey region shows Stop The World event
Tools used
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Eclipse for generating Tree
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GCViewer to get plots and other details of G1 performance like throughput
etc.
References
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[ 1 ] Domani, Tamar, Elliot K. Kolodner, and Erez Petrank. "A generational onthe-y garbage collector for Java." ACM SIGPLAN Notices. Vol. 35. No. 5. ACM,
2000.
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[ 2 ] www.cubrid.org/blog/dev-platform/understanding-java-garbagecollection
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[ 3 ] www.ibm.com/developerworks/library/j-leaks
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[ 4 ] www.cubrid.org/blog/textyle/428187
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[ 5 ] www.infoq.com/articles/JavaGarbageCollectionDistilled
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[6]
www.oracle.com/webfolder/technetwork/tutorials/obe/java/G1GettingStart
ed/index.html
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[ 7 ] github.com/chewiebug/GCViewer/blob/master/README