Thursday, September 8, 2011

HotSpot JVM garbage collection options cheat sheet (v2)

In this article I have collected a list of options related to GC tuning in JVM. This is not a comprehensive list, I have only collected options which I use in practice (or at least understand why I may want to use them).
Compared to previous version a few useful diagnostic options was added. Additionally section for G1 specific options was introduced.

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HotSpot GC collectors

HotSpot JVM may use one of 6 combinations of garbage collection algorithms listed below.
Young collector
Old collector
JVM option
Serial (DefNew)
Serial Mark-Sweep-Compact
Parallel scavenge (PSYoungGen)
Serial Mark-Sweep-Compact (PSOldGen)
Parallel scavenge (PSYoungGen)
Parallel Mark-Sweep-Compact (ParOldGen)
Serial (DefNew)
Concurrent Mark Sweep
Parallel (ParNew)
Concurrent Mark Sweep

GC logging options

Please note that many of logging options could be modified on running JVM using JMX (e.g. via JConsole).
JVM option
General options
-verbose:gc or -XX:+PrintGC
Print basic GC info
Print more elaborated GC info
Print timestamps for each GC event (seconds count from start of JVM)
Print date stamps at garbage collection events (e.g. 2011-09-08T14:20:29.557+0400: [GC... )
Print timestamps for individual GC worker thread tasks (very verbose)
Redirects GC output to a file instead of console
Print detailed demography of young space after each collection
Print TLAB allocation statistics
Print times for weak/soft/JNI/etc reference processing during STW pause
Reports if GC is waiting for native code to unpin object in memory
Print pause summary after each stop-the-world pause
Print time for each concurrent phase of GC
Prints class histogram after full GC
Prints class histogram before full GC
Creates heap dump file after full GC
Creates heap dump file before full GC
Creates heap dump in out-of-memory condition
Specifies path to save heap dumps
CMS specific options
Print additional CMS statistics if n >= 1
Print CMS initiation details
Print additional info concerning free lists
Print additional info concerning free lists
Print additional diagnostic information following promotion failure
Dump useful information about the state of the CMS old generation upon a promotion failure.
In a CMS dump enabled by option above, include more detailed information about the free chunks.
In a CMS dump enabled by option above, include more detailed information about the allocated objects.

JVM sizing options

JVM option
-Xms2048m -Xmx2g or
Initial and max size of heap space (young space + tenured space). Permanent space does not count to this size.
Initial and max size of young space.
Alternative way to specify young space size. Sets ration of young vs tenured space (e.g. -XX:NewRatio=2 means that young space will be 2 time smaller than tenuted space).
Sets size of single survivor space as a portion of Eden space size (e.g. -XX:NewSize=64m -XX:SurvivorRatio=6 means that each survivor space will be 8m and eden will be 48m).
Initial and max size of permanent space.
-Xss256k (size in bytes) or  
-XX:ThreadStackSize=256 (size in Kbytes)
Sets size of stack area dedicated to each thread. Thread stacks do not count to heap size.
Maximum size of off-heap memory available for JVM

Young collection tuning

JVM option
Initial value for tenuring threshold (number of collections before object will be promoted to tenured space).
Max value for tenuring threshold.
Max object size allowed to be allocated in young space (large objects will be allocated directly in old space). Thread local allocation bypasses this check so if TLAB is large enough object exciding size threshold still may be allocated in young.
Promote all objects surviving young collection immediately to tenured space (equivalent of -XX:MaxTenuringThreshold=0)
Objects from young space will never get promoted to tenured space while survivor space is large enough to keep them.
Thread local allocation blocks
Use thread local allocation blocks in young space. Enabled by default.
Allow JVM to adaptively resize TLAB for threads.
Initial size of TLAB for thread
Minimal allowed size of TLAB

CMS tuning options

JVM option
Controlling initial mark phase
Only use occupancy as a criterion for starting a CMS collection.
Percentage CMS generation occupancy to start a CMS collection cycle. A negative value means that CMSTriggerRatio is used.
Percentage CMS generation occupancy at which to initiate CMS collection for bootstrapping collection stats.
Percentage of MinHeapFreeRatio in CMS generation that is allocated before a CMS collection cycle commences.
Percentage of MinHeapFreeRatio in the CMS perm generation that is allocated before a CMS collection cycle commences, that also collects the perm generation.
Once CMS collection is triggered, it will wait for next young collection to perform initial mark right after. This parameter specifies how long CMS can wait for young collection.
Controlling remark phase
Whether parallel remark enabled (only if ParNewGC), enabled by default
Whether parallel remark of survivor space enabled (effective only with option above), enabled by default
Force young collection before remark phase.
If Eden used is below this value, don't try to schedule remark
The Eden occupancy % at which to try and schedule remark pause
Start sampling Eden top at least before young generation occupancy reaches 1/ of the size at which we plan to schedule remark
Choose variant (1,2) of verification following remark
Parallel execution
Use parallel algorithm for young space collection.
Use multiple threads for concurrent phases.
Number of parallel threads used for concurrent phase.
Number of parallel threads used for stop-the-world phases.
CMS incremental mode
Enable incremental CMS mode. Incremental mode is meant for severs with small number of CPU. Do not use it on modern hardware.
Miscellaneous options
If not enabled, CMS will not clean permanent space. You should always enable it in multiple class loader environments such as JEE or OSGi.
Let System.gc() trigger concurrent collection instead of full GC.
Same as above but also triggers permanent space collection.

Miscellaneous GC options

JVM option
JVM will ignore application calls to System.gc()
Soft references time to live in milliseconds per MB of free space in the heap
OutOfMemoryError conditions
Use policy to limit of proportion of time spent in GC before an OutOfMemory error is thrown
Limit of proportion of time spent in GC before an OutOfMemory error is thrown (used with GCHeapFreeLimit)
Minimum percentage of free space after a full GC before an OutOfMemoryError is thrown (used with GCTimeLimit)

G1 (garbage first) GC options

G1 is still evolving, so table below have known supported JVM version column
JVM option
Size of heap region for G1 collector. Region size could be from 1MiB to 32MiB. Default is initial heap size / 2048.
Confidence coefficient for G1 pause prediction heuristics
It determines the minimum reserve we should have in the heap to minimize the probability of promotion failure
GC time per MMU time slice in milliseconds (for G1 only)
Time slice for MMU specification in milliseconds (for G1 only). Should be greater than parameter above.


  1. Very good list, Main problem with JVM options is that they are lot and you can't remember all of those so having some of them is good idea. I have also shared Some important memory and gc JVM options you may find worth reading.

  2. Thanks for the list! We started to test with G1 and couldn't find a way (as we have CMS) to control when it starts.
    Meaning, we don't want it to wait until it uses 80% of the memory (as we seen happen) since we only really use 50% of the memory.
    Do you know if there's a parameter similar to the "-XX:CMSInitiatingOccupancyFraction=60 -XX:+UseCMSInitiatingOccupancyOnly"


  3. Hi Guy,
    I'm not aware of any such optionsfor G1.
    You probably should try to ask on mailing list.


  4. Very helpful list. Would be nice if most current default values could be included where they are known.

  5. Hello Alexey,

    I wonder if there isn't à little mistake in the array of the Hotspot collectors options. For Serial (DefNew) and Concurrent Mark Sweep, you have indicated the following options to activate this mode :
    -XX:+UseConcMarkSweepGC and -XX:-UseParNewGC
    Is UseParNewGC correct ? IMHO, this is rather the option for the ParNew mode, no ?

    1. Notice difference between -XX:- and -XX:+ options. -XX:-UseParNewGC forces ParNew to be disabled, so DefNew will be used.

    2. My apologies ! Indeed, I didn't notice the little, but very important difference (+ is not - of course !). Your options are OK.
      Best regards,

  6. Hi, there is a small mistake in the list regarding -XX:ThreadStackSize=256k should be changed to XX:ThreadStackSize=256 (without the 'k')

    Otherwise the JVM fails to start. Not sure whether -Xss is ok with the 'k'

    1. Indeed -Xss interpreted as value in byte, but -XX:ThreadStackSize=256k as value in Kbytes (256k is translated to 256m).
      Thanks for pointing this out

  7. Hi, I am trying to use the option -XX:ConcGCThreads=2 for CMS but my JVM did not recognize. I am using the below version.

    java version "1.5.0_11"
    Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_11-b03)
    Java HotSpot(TM) Server VM (build 1.5.0_11-b03, mixed mode)

    pls advise if we have alternate option which can perform the same action.



    1. 1.5 is pretty old now, I doubt it supports this feature

  8. Since 7U2 HS supports rotating GC logfiles:

    java -Xloggc:app.vgc -XX:+PrintGCDetails
    -XX:GCLogFileSize=10M -XX:NumberOfGCLogFiles=10
    -XX:+PrintGCDateStamps -XX:-PrintGCTimeStamps

    See also my german blog:

  9. Hello Alexey,

    I'd like to thank you for the good articles (not only this one).

    Could you please also advice something about the following post:

    1. Hi Maxim,

      GC tuning strategy really depends on numbers (capacity, number of evictions per second, memory you a willing to waste).

      Question you have posted in stackoverflow is too generic to be answered properly.

      Generic guidelines for tuning CMS could be found on this blog

  10. Hi Alexey,

    I am getting "permgen out of memory". In Jave 5 this was fixed with the options "XX:+CMSClassUnloadingEnabled XX:+CMSPermGenSweepingEnabled".

    Do you know if XX:+UseG1GC will perform the same function in java 6 (release 43)?