G1 TUNING Shubham Modi( ) Ujjwal Kumar Singh(10772) Vaibhav(10780)

Motivation  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.  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.  This affects the performance of the application and will decrease the responsiveness and throughput.

G1 Garbage Collector  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.  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?  Heap is split into many fixed size region ranging from 1 to 32 Mb [6]

 This is a STW pause. Eden and Survivor size is calculated for the next GC. GC Pause time is taken into consideration.  Regions are mapped into Eden, Survivor and Old generation [6]

 Live objects have been evacuated to survivor regions or to old generation regions  Liveness of regions is calculated and empty regions are marked X and are deleted in remark phase [6]

 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.  After it we get compacted dark blue and dark green regions [6]

Why G1?  Provide predictable GC pause duration  Decrease frequency of Stop The World(Full GC Cycle)  Compact free space without lengthy GC induced pause times  Do not require much larger Java heap space  Do not sacrifices throughput of application  Compaction mostly eliminating the fragmentation issues

Objectives  Improves responsiveness  Increase throughput

Problem  When we try to create 10 million children of root, it gives OutofHeap Error  We created first tree with root and 5 million children occupying ~ 55% of heap space  Then we made child1 as root of tree  Therefore all other nodes becomes unreachable  Then we created another tree with a root and its 5 million children  Hence Garbage collector is called to free the space causing Stop The World event to occur root Child 5million child3child2 child1 ………

Factors affecting G1 Performance  GC Pause Time: Set the target for the maximum GC pause time(soft goal)  Heap Occupancy: Percentage of the entire heap occupancy to start the concurrent GC cycle(Default 45)  Old Generation size: New to Old Generation size ratio(Default 2)  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 sec which is normally 0.1 sec  Grey region shows concurrent GC pauses of an application  Dark grey region shows Stop The World event 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

Tools used  Eclipse for generating Tree  GCViewer to get plots and other details of G1 performance like throughput etc.

References  [ 1 ] Domani, Tamar, Elliot K. Kolodner, and Erez Petrank. "A generational on- the-y garbage collector for Java." ACM SIGPLAN Notices. Vol. 35. No. 5. ACM,  [ 2 ] collection  [ 3 ]  [ 4 ]  [ 5 ]  [ 6 ] ed/index.html  [ 7 ] github.com/chewiebug/GCViewer/blob/master/README