Garbage collector
… an introduction
Peter Varsanyi
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Agenda
Basics
Memory management
Garbage collector
Garbage collector types
Java garbage collectors
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Garbage Collector
BASICS
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GARBAGE COLLECTOR
Basics
What is it?
• Find data objects in the program, that
cannot be accessed in the future
• Reclaim resources used by these objects
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Basics
Advantages?
• Dangling pointer bugs
• Double free bugs
• Certain type of memory leaks
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Basics
Disadvantages
• Extra computing resources
• Unpredictable collection time
• More memory-related work than useful
work
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Brief history
Automatic garbage collector
• First automatic garbage collection:
LISP(1958)
• Several other languages implemented it:
– BASIC(1964)
– Logo(1967)
– Java 1.0(1996)
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Garbage collector
MEMORY MANAGEMENT
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Memory management
OS level process
• 32 bit: 4GB address space
• 64 bit: 16EB address space
• Kernel space
• User space
0 GB
0x0
2 GB
0x40000000
0x80000000
4 GB
0xC0000000
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0xFFFFFFFF
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Memory management
OS level process
• OS + C runtime
• JVM/Native heap
• Java Heap
(-Xmx2GB)
0 GB
2 GB
OS + C runtime JVM NativeHeap
0x0
0x40000000
0x80000000
4 GB
Java heap
0xC0000000
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Memory management
• Conservative collector
– Any bit pattern can be a pointer
– No compiler cooperation required
• Precise collector
– Require compiler cooperation
– Can move objects
• Commercial JVMs use precise collector
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Memory management
Reference types
• Strong
• Soft
• Weak
• Phantom
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Memory management
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Memory management
Object Lifecycle
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Garbage collector
TYPES
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Garbage collector
Reference counting collector
• An early garbage collection strategy
• Reference count is maintained in the object
• Overhead to increment/decrement
reference count
• Cannot clean cyclic references
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Garbage collector types
Tracing collector
• Trace out the heap starting from roots
• Two phases:
– Mark live objects
– Clean dead objects
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Garbage collection
Mark
• Find objects in the heap
• “Paint” them
• Non-marked objects are dead
• Work is linear to “live set”
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Garbage collection
Sweep
• Scans heap for dead objects
• Work is linear to heap size
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Garbage collection
Compact
• Avoid fragmentation
• Relocates objects
• Remap: fix all references to live objects
• End of heap is a large contagious free area
• Work is linear to “live set”
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Garbage collection
Copy
• Moves all objects
• Single pass operation
• Split memory into 2 region
• Work is linear to “live set”
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Java Garbage collectors
Mark/Sweep (1.0)
• Easy to implement
• Reclaim cyclic structures
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Java Garbage collectors
• Generational collection (1.2)
• Young/Eden generation
• Tenured generation
• Perm generation
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Java Garbage collectors
• Generational collection
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Java Garbage collectors
Separate collectors
• Minor GC
– When eden is full
– Sweeps eden + current survivor
• Major GC
– When old is full
– Perm + tenured collected
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Java garbage collectors
Tenured
S1
S2
Eden
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Java Garbage collectors
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Java Garbage collectors
Remembered set
• Track references into young gen
• Card marking
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Java Garbage collectors
Mark/Compact(1.3)
• Combines copy & mark/sweep
• Mark live objects, move these
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Java Garbage collectors
Concepts
• Parallel
• Uses multiple CPU to perform collection at the same
time
• Concurrent
• Performs GC concurrently with application’s own
execution
• Safe point
• Point in thread execution, where collector can identify all
references in thread’s execution stack
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Java Garbage collectors
Serial GC
• Default with 1 CPU
• Young collector: Copy
• Old collector: Mark/Sweep/Compact
• Faster allocation
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Java Garbage collectors
Parallel GC
• Multiple threads to collect young
• More than 2 CPU pause time will be
reduced
• Old gen collected with parallel mark/sweep
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Java Garbage collectors
CMS(Concurrent Mark Sweep)
• Multiple threads to collect young
• Tenured generation: mark/sweep
• Does not compact or copy
• Low pause time
• Occupancy fraction
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Java garbage collectors
CMS phases
1. Initial mark(STW)
2. Concurrent mark
3. Concurrent preclean
4. Remark(STW)
5. Concurrent sweep
6. Concurrent reset
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Java garbage collectors
CMS advantages/disadvantages
• Advantages:
– Low latency
• Disadvantages
– Cannot work when old is full
– No compaction
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Java Garbage collectors
G1 to the rescue
• Most empty region first
• Estimate region clean time
• User defined target time
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Java Garbage collectors
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Java Garbage collectors
G1 to the rescue
• Heap partitioned into equal size of chunks
• More predictable GC pauses
• High throughput
• Compacting collector
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Java Garbage collectors
G1 phases
1. Initial mark(STW)
2. Root region scanning
3. Concurrent marking
4. Remark(STW)(SATB)
5. Clean up
6. Copy(STW)
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Myths
• Creating object has high cost
• Do not create lots of small objects
• No more memory leak(MAGIC!)
• Increase the heap, what could happen?
• Null everything you don’t use
• System.gc() will collect everything
immediately
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Tools
• Jstat(Part of JDK)
• Java VisualVM(Oracle JDK)
– Visual GC plugin
• -XX:+PrintGC
• -XX:+PrintGCDetails
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Useful links
• http://cscircles.cemc.uwaterloo.ca/java_visu
alize/
• http://www.oracle.com/webfolder/technetwo
rk/tutorials/obe/java/gc01/index.html
• http://www.cubrid.org/blog/devplatform/understanding-java-garbagecollection/
• http://www.slideshare.net/cnbailey/memoryefficient-java
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Q/A
• Questions?
• No I will not clean your room.
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