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javamemory-managementgarbage-collectionheap-memory

Impact of heap parameters on GC/performance?

Most of the place on net , I get below info about heap parameters

-Xms<size> set initial Java heap size -Xmx<size> set maximum Java heap size

Here is mine understanding/question when I mention -Xms 512M -Xmx 2048M parameters ,

-Xms :- My understanding is if my java process is actually needed only 200M , with mention of -Xms 512M , java process will still be assigned only 200M(actual memory required) instead of 500M . But if I already know that my application is going to take this 512M memory on start up then specifying less than will have impact on performance as anyways heap block need to resized which is costly operation.

Per discussion with my colleague, By default GC will trigger on 60% of Xms value. Is that correct ? If yes is it minor GC or full GC that is dependant on Xms value ?

Update on Xms:- This seems to be true after reading JVM heap parameters but again is value 60% by default and is it minor or full GC that is dependent on Xms value?

-Xmx:- My understanding is with mention of -Xmx 2048M , java process is actually going to reserve 2048M memory for its use from OS itso that another process can not be given its share.If java process needed anyhow more than 2048M memory, then out of memory will be thrown.

Also i believe there is some relation of Full GC trigger on value of -Xmx. Because what I observed is when memory reaches near to 70% of Xmx, Full GC happens in jconsole. Is that correct?

Configuration :- I am using linux box(64 bit JVM 8). Default GC i.e Parallel GC

Solution

  • GC is not triggered based on just Xms or Xmx value.

    Heap = New + Old generations
    The heap size (which is initially set to Xms) is split into 2 generations - New (aka Young) and Old (aka Tenured). New generation is by default 1/3rd of the total heap size while Old generation is 2/3rd of the heap size. This can be adjusted by using JVM parameter called NewRatio. Its default value is 2.

    Young Generation is further divided in Eden and 2 Survivor spaces. The default ratio of these 3 spaces are: 3/4th, 1/8th, 1/8th.

    Side note: This is about Parallel GC collectors. For G1 - new GC algorithm divides the heap space differently.

    Minor GC All new objects are allocated in Eden space (except massive ones which are directly stored in Old generation). When Eden space becomes full Minor GC is triggered. Objects which survive multiple minor GCs are promoted to Old Generation (default is 15 cycles which can be changed using JVM parameter: MaxTenuringThreshold).

    Major GC Unlike concurrent collector, where Major GC is triggered based on used-space (by default 70%), parallel collectors calculate threshold based on 3 goals mentioned below.

    Parallel Collector Goals

    • Max GC pause time - Maximum time spent in doing GC
    • Throughput - Percentage of time spent in GC vs Application. Default (1%)
    • Footprint - Maximum heap size (Xmx)

    Thus by default, Parallel Collector tries to spend maximum 1% of total application running time in Garbage Collection.

    More details here

    Xms to Xmx
    During startup JVM creates heap of size Xms but reserves the extra space (Xmx) to be able to grow later. That reserved space is called Virtual Space. Do note that it just reserves the space and does not commit.

    2 parameters decide when heap size grows (or shrinks) between Xms and Xmx.

    • MinHeapFreeRatio (default: 40%): Once the free heap space dips below 40%, a Full GC is triggered, and the heap size grows by 20%. Thus, heap size can keep growing incrementally until it reaches Xmx.
    • MaxHeapFreeRatio (default: 70%): On the flip side, heap free space crosses 70%, then Heap size is reduced by 5% incrementally during each GC until it reaches Xms.

    These parameters can be set during startup. Read more about it here and here.

    PS: JVM GC is fascinating topic and I would recommend reading this excellent article to understand in-depth. All the JVM tuning parameters can be found here.