Introduction to G1 collector

1. An introduction to G1 Collector
for busy developers
Sanjoy Kumar Roy
sanjoykr78@gmail.com
09-05-2014

2. Hotspot Architecture
Method
Area
Heap
Java
Threads
Program
Counter
Registers
Native
Internal
Threads
JIT
Complier
Garbage
Collector
Execution
Engine
Native
Method
Interface
Native
Method
Libraries
Runtime Data Areas

3. Hotspot Architecture
Method
Area
Heap
Java
Threads
Program
Counter
Registers
Native
Internal
Threads
JIT
Complier
Garbage
Collector
Execution
Engine
Native
Method
Interface
Native
Method
Libraries
Runtime Data Areas
Key components related to performance

4. Performance Basics
Two main goals
Responsiveness Throughput
Large pause
time

5. Heap in old garbage collectors
Eden S1 S2 Tenured Permanent
Young Generation Old Generation
Permanent
Generation
Survivor Space
In all older garbage collectors (Serial, Parallel, CMS)
heap is divided into three sections:
All memory objects end up in one of these three sections.

6. Heap in G1Collector
G1 Collector takes a different approach:
Heap is partitioned into a set of equal-sized heap
regions, each a contiguous range of virtual memory.
Certain region sets are assigned the same roles (eden
/ survior / old) as in the older collections, but there is
not a fixed size for them. This provides greater
flexibility in memory usage.

7. Heap in G1Collector (cont.)
O
O
E
O
S
E
SO
O Heap is one memory are
split into many fixed
sized regions.
Region size is chosen
by the JVM at start-up.
The JVM generally
targets around 2048
regions varying in size
from 1 to 32Mb.
H
E
E
S
S
E
E
H
E Eden
S Survior
O Old
H Humongous
Empty

8. G1 Heap Allocation
These regions are mapped into logical representations of
eden, survivor, and old generation spaces
Each region is assigned to one role (eden/survior/old)
Live objects are evacuated (copied / moved) from one region
to another
Regions are designed to be collected in parallel with or
without stopping all other application threads.
There is fourth type of object known as Humongous regions.
These regions are designed to hold objects that are 50% the
size of a standard region or larger. They are stored as a set of
contiguous regions.
The last type of regions would be the unused areas of the
heap.

9. Garbage First (G1) Garbage Collector
G1 is targeted for multi-processor machines
with large memories.
It meets garbage collection (GC) pause time
goals with a high probability, while achieving
high throughput.
Fully supported JDK 7 update 4 and later
releases.

10. Garbage collection in G1
When performing garbage collections, G1 operates
in a manner similar to the CMS collector.
G1 performs a concurrent global marking phase to
determine the liveness of objects throughout the
heap.
After the mark phase completes, G1 knows which
regions are mostly empty. It collects in these
regions first, which usually yields a large amount
of free space. This is why this method of garbage
collection is called GARBAGE-FIRST.

11. G1 does an estimate of how many regions can be
collected within the user specified target time.
Thus the collector has a reasonably accurate
model of the cost of collecting the regions.
It uses this pause prediction model to determine
which and how many regions to collect while
staying within specified the pause time target.
Pause Prediction Model

12. Evacuation Process in G1
In G1, garbage collected from regions using
evacuation. G1 copies objects from one or more
regions of the heap to a single region on the heap,
and in the process both compacts and frees up
memory.
This evacuation is performed in parallel on multi-
processors, to decrease pause times and increase
throughput. Thus, with each garbage collection, G1
continuously works to reduce fragmentation, working
within user defined pause times.

13. G1 has both
 Concurrent (runs along with application threads:
refinement, marking, cleanup)
 Parallel (multi-threaded, e.g. Stop the world) phases
Full garbage collections are still single threaded.
G1 is not a real-time collector. It meets the set pause
time target with high probability but not absolute
certainty.
Evacuation Process in G1 (cont.)

14. G1 Footprint
In G1, JVM process size will likely to be
larger. This is largely related to “accounting”
data structures:
a) Remembered Sets (RSets)
b) Collection Sets (CSets)

15. Remembered Sets (RSets)
Used to track object references into a given region
There is one RSet per region in the heap
The RSet enables the parallel and independent
collection of a region
The overall footprint impact of RSets is less than 5%

16. Remembered Sets (RSets)

17. Collection Sets (CSets)
CSets are the set of regions that will be collected
in a GC
All the live data in a CSet is evacuated (copied or
moved) during a GC
Sets of regions can be eden, survivor, and/or old
generation
CSets have a less than 1% impact on the size of
the JVM.

18. G1 Collector is designed for applications that:
Can operate concurrently with applications’ threads like
the CMS collector.
More than 50% of Java Heap is occupied with live data.
Has undesired long garbage collection or compaction
pauses (longer than 0.5 to 1 second)
Need more predictable GC pause durations.
Do not want to sacrifice a lot of throughput performance.
Require large heaps with limited GC latency (heap sizes
>= 6 GB & stable and predictable pause time < 0.5 sec)

19. Thank you.