Garbage Collection in Java

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Garbage Collection in Java

• Toronto Java Users Group
• September 3, 2002
• Ethan Henry
• Sitraka

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What is a JVM?

• The Java Virtual Machine has 2 primary jobs
• Execute Code
• Manage Memory
• It also does stuff like managing locks
• Most other functions are really not part of the
  JVM, but the library, like file network I/O

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Memory Management

• Allocate Memory from OS
• Manage Java Allocations
• including heap compaction
• Remove Garbage Objects
• Java Virtual Machine Specification 2.4.6

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What is Garbage Anyhow?

• An object is created in the heap and is
  garbage-collected after there are no more
  references to it. Objects cannot be reclaimed or
  freed by explicit language directives.
• Objects become garbage when there are no more
  references to the object
• This is not to suggest that garbage collectors
  have to be implemented using reference counting
  however...

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Reachability

• Objects become garbage when theyre no longer
  reachable from the root set
• The root set consists of
• static reference fields in classes
• local references
• Exact versus Non-Exact Garbage Collectors

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Reachable Objects

• Elements within the root set directly refer to
  objects within the heap of the JVM
• Reference variables within those objects refer to
  further objects within the Heap (indirectly
  reachable from the Root Set)

Heap
Root Set
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How is it implemented?

• Sun Classic
• Sun HotSpot
• IBM

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Sun Classic

• Mark, Sweep Compact
• Mark identify garbage
• Sweep Find garbage on heap, de-allocate it
• Compact collect all empty memory together
• Eligibility for garbage collection is determined
  by walking across memory, determining
  reachability and then compacting the heap
• Compaction is just copying the live objects so
  that theyre adjacent in memory
• theres one large, contiguous block of free memory

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The Pause

• The main problem with classic mark, sweep and
  compact is that all other threads have to be
  suspended while the garbage collector runs
• Pause time is proportional to the number of
  objects on the heap
• not the amount of garbage

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Sun HotSpot

• Sun improved memory management in the Java 2 VMs
  (JDK 1.2 and on) by switching to a generational
  garbage collection scheme
• The heap is separated into two regions
• New Objects
• Old Objects

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New Object Region

• The idea is to use a very fast allocation
  mechanism and hope that objects all become
  garbage before you have to garbage collect
• The New Object Regions is subdivided into three
  smaller regions
• Eden, where objects are allocated
• 2 Survivor semi-spaces From and To

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New Object Region

• The Eden area is set up like a stack - an object
  allocation is implemented as a pointer increment
• When the Eden area is full, the GC does a
  reachability test and then copies all the live
  objects from Eden to the To region
• The labels on the regions are swapped
• To becomes From - now the From area has
  objects

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New Object Region

• The next time Eden fills objects are copied from
  both the From region and Eden to the To area
• Theres a Tenuring Threshold that determines
  how many times an object can be copied between
  survivor spaces before its moved to the Old
  Object region
• Note that one side-effect is that one survivor
  space is always empty

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Old Object Region

• The old object region is for objects that will
  have a long lifetime
• The hope is that because most garbage is
  generated by short-lived objects that you wont
  need to GC the old object region very often

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Generational Garbage Collection
New Object Region
Old Object Region
Eden
SS1
SS2
Old
FirstGC
Eden
SS1
SS2
Old
Eden
SS1
SS2
Old
SecondGC
Eden
SS1
SS2
Old
Eden
SS1
SS2
Old
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GC Output

• Running the JVM with -verbosegc will show
  information like this
• GC 1667K-gt1295K(1984K), 0.0101756 secs
• GC 1807K-gt1434K(1984K), 0.0223998 secs
• GC 1946K-gt1574K(2112K), 0.0116185 secs
• Full GC 1574K-gt1574K(2112K), 0.0830561 secs
• GC 3454K-gt2081K(4672K), 0.0495951 secs
• GC 4001K-gt2599K(4672K), 0.0274256 secs
• GC 4519K-gt3101K(5056K), 0.0308995 secs
• Full GC 3101K-gt3101K(5056K), 0.1452472 secs
• GC 7039K-gt4131K(9452K), 0.0777414 secs
• GC 8227K-gt5174K(9452K), 0.0627538 secs
• GC 9270K-gt6209K(10348K), 0.1125570 secs

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Incremental Garbage Collection

• -Xincgc
• Sun also has an incremental collector that breaks
  that old-object region into smaller chunks and
  GCs them individually
• Pause times are smaller but overall throughput is
  decreased
• Inc GC 3566K-gt3950K(5120K), 0.0309922 secs
• GC 4078K-gt3594K(5184K), 0.0264542 secs
• Inc GC 3594K-gt3978K(5120K), 0.0272683 secs
• GC 4106K-gt3627K(5120K), 0.0272381 secs
• Inc GC 3627K-gt4011K(5056K), 0.0285464 secs
• GC 4139K-gt3666K(5184K), 0.0281388 secs

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Concurrent Garbage Collection

• -Xconcgc
• Concurrent GC allows other threads to keep
  running in parallel with the GC
• Available in JDK 1.4.1
• GC 1463K-gt1093K(2560K), 0.0089573 secs
• GC 1093K(2560K), 0.0053470 secs
• GC 1094K(2560K), 0.0092867 secs
• GC 1604K-gt1228K(2560K), 0.0104823 secs
• GC 1228K(2560K), 0.0062662 secs
• GC 1234K(2560K), 0.0097820 secs
• GC 1740K-gt1373K(2560K), 0.0115875 secs

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Parallel GC

• Future JVMs will include the ability to run GC on
  separate processors on a multi-processor machine
• This is already available in BEAs JRockit JVM
• http//www.jrockit.com

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IBM

• Improved single-heap mark, sweep compact
• Uses parallel marking in JDK 1.3.0 and concurrent
  marking in JDK 1.3.1
• IBMs JVM provides a much more detailed breakdown
  of whats happening during GC

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IBM GC

• After the IBM JVM GC has identified garbage it
  tries to free memory for the new allocation by
• compacting allocated memory
• finalizing pending dead objects
• removing weakly reachable objects
• increasing the heap space
• for more on weak refs, see http//developer.java.s
  un.com/developer/technicalArticles/ALT/RefObj/

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IBM GC

• ltAF9 Allocation Failure. need 2064 bytes, 4096
  ms since last AFgt
• ltAF9 managing allocation failure, action1
  (0/16086056) (34768/34768)gt
• ltGC Tue Sep 03 180742 2002
• ltGC(9) freed 2345744 bytes in 234 ms, 14 free
  (2380512/16120824)gt
• ltGC(9) mark 226 ms, sweep 8 ms, compact 0
  msgt
• ltGC(9) refs soft 0 (age gt 1), weak 0, final
  0, phantom 0gt
• ltAF9 managing allocation failure, action3
  (2380512/16120824)gt
• ltGC(9) need to expand mark bits for
  19659768-byte heapgt
• ltGC(9) expanded mark bits by 53248 to 307200
  bytesgt
• ltGC(9) need to expand alloc bits for
  19659768-byte heapgt
• ltGC(9) expanded alloc bits by 53248 to 307200
  bytesgt
• ltGC(9) expanded heap by 3538944 to 19659768
  bytes, 30 freegt
• ltAF9 completed in 2974 msgt

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IBM GC

• ltAF20 Allocation Failure. need 24 bytes, 29793
  ms since last AFgt
• ltAF20 managing allocation failure, action1
  (0/37167760) (1956200/1956200)gt
• ltGC Tue Sep 03 180936 2002
• ltGC(20) freed 20688256 bytes in 467 ms, 57 free
  (22644456/39123960)gt
• ltGC(20) mark 453 ms, sweep 14 ms, compact 0
  msgt
• ltGC(20) refs soft 0 (age gt 6), weak 0, final
  41, phantom 0gt
• ltGC(20) stop threads time 405, start threads
  time 56gt
• ltAF20 completed in 961 msgt

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How To Tune GC

• Not a simple topic
• There are no universal magic values - every app
  is different
• Things to tune
• Memory Size
• overall size, individual region sizes
• GC parameters
• Minimum/maximum of free heap,
• Type of GC
• single heap, generational, incremental,
  concurrent, parallel

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Tuning Parameters - Both

• -ms, -Xms
• sets the initial heap size
• -mx, -Xmx
• sets the maximum heap size
• -Xss
• sets the size of the per-thread stacks
• -Xminf 0-1, -XXMinHeapFreeRatio 0-100
• sets the percentage of minimum free heap space -
  controls heap expansion rate
• -Xmaxf 0-1, -XXMaxHeapFreeRatio 0-100
• sets the percentage of maximum free heap space -
  controls when the VM will return unused heap
  memory to the OS

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Tuning Parameters - Sun

• -XXNewRatio
• sets the ratio of the old and new generations in
  the heap. A NewRatio of 5 sets the ratio of new
  to old at 15, making the new generation occupy
  1/6th of the overall heap
• defaults client 8, server 2
• -XXNewSize, -XXMaxNewSize 1.3
• -Xmn 1.4
• sets the minimum and maximum sizes of the new
  object area, overriding the default calculated by
  the NewRatio
• -XXSurvivorRatio
• sets the ratio of the survivor space to the eden
  in the new object area. A SurvivorRatio of 6 sets
  the ratio of the three spaces to 116, making
  each survivor space 1/8th of the new object
  region
• default 25

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Tuning Parameters - Sun

• -XXAggressiveHeap
• This option instructs the JVM to push memory use
  to the limit the overall heap is around 3850MB,
  the memory management policy defers collection as
  long as possible, and (in some VMs) some GC
  activity is done in parallel. Because this option
  sets heap size, do not use in conjunction with
  the -Xms or -Xmx options

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Tuning Parameters - IBM

• -Xgcpolicyltoptthruput optavgpausegt
• Setting gcpolicy to optthruput disables
  concurrent mark. Users who do not have pause time
  problems (as seen by erratic application response
  times) should get the best throughput with this
  option. Optthruput is the default setting.
• Setting gcpolicy to optavgpause enables
  concurrent mark with its default values. Users
  who are having problems with erratic application
  response times caused by normal garbage
  collections can alleviate those problems at the
  cost of some throughput when running with the
  optavgpause option.

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• http//java.sun.com/docs/hotspot/gc
• http//java.sun.com/docs/performance
• http//www-106.ibm.com/developerworks/library/j-jt
  c/index.html

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Loitering Objects

• What wont the garbage collector clean up?

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Loiterers
Allocated
Reachable
Live
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Finding Loiterers

• How do you identify these loitering objects?
• You need to
• identify specific use cases for your application
  that are important or that you suspect have
  problems
• get a tool that lets you inspect whats on the
  heap before and after each use case
• look at the difference - are those the objects
  you expect to see created?
• are any unnecessary objects still being
  referenced?

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Tools

• See some reviews on http//www.javaperformancetuni
  ng.com/tools/index.shtml

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Fixing Loiterers

• Track down what objects are loitering
• Track the instances to their allocation point
• Decide when they should become unreachable and
  make sure it actually happens

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End