In jvisual vm i see three attributes under Monitor>Heap, i see 3 attributes depicting memory details all with differnt figures
Size : ?
Used :- I believe this is the actual memory used
Max :- I believe this is the max heap size allocated to java process (specified with Xmx)
I am not sure what size actually depicts?
The three attributes can be defined as next:
Size: The actual total reserved heap size
Used: The actual used heap size.
Max: The max size of the Java heap (young generation + tenured generation)
Indeed when you launch your JVM, the initial heap size (can be defined with -Xms) will be the initial total reserved heap size, then according to how your application behaves, it could need to increase the total reserved size until it reaches the max size and if it is still not enough you could get OOME.
Size depicts the heap block size assigned to java process. Try with -Xms 512m or 1024m then your size to start with will be 512m but used memory may be much lower. As soon as used memory grows , heap resizing occurs proactively so that memory can be allocated to live objects.
Its like you have Gas tank of 30 litre max capacity . But you know for now you may just need 20 litres for the trip but actually used in trip is 5 litres
Heap size is actual size of heap your running application has.
Used heap is used portion of heap size.
Max heap size is the maximum value the application's heap size can have (can be defined by the arg option -Xmx).
When monitor memory usage of a java application, you see that heap size may vary during running of the application. It can not be greater than max heap size. For a sample profiling (monitoring of an application), see below image:
Related
I am profiling a Java process with VisualVM and I found out that while my used heap remains constantly below 100 MB, the heap size keeps increasing to a point at which it is 10 times bigger than the used heap!
Reading from the docs:
By default, the virtual machine grows or shrinks the heap at each
collection to try to keep the proportion of free space to live objects
at each collection within a specific range. This target range is set
as a percentage by the parameters -XX:MinHeapFreeRatio= and
-XX:MaxHeapFreeRatio=, and the total size is bounded below by -Xms and above by -Xmx.
So, consindering that MinHeapFreeRatio and MaxHeapFreeRatio are set to 40% and 70% respectively, why is this happening?
I run my Java Application through tomcat, and set the -Xms1024m, however I found the size of the Java heap just 200~300m after start the application, I think the Xms means the minimum heap size, why the application doesn't reach to the minimum Heap size 1024m immediately after the application startup?
Edit, BTW, the JVM is hotspot 7.0.
It seems the GC does it in the
method HeapRegion::setup_heap_region_size(uintx min_heap_size) from the c++ file \openjdk-7-fcs-src-b147-27_jun_2011\openjdk\hotspot\src\share\vm\gc_implementation\g1\heapRegion.cpp , and method parse_each_vm_init_arg from file \openjdk-7-fcs-src-b147- 27_jun_2011\openjdk\hotspot\src\share\vm\runtime\arguments.cpp , someone who familiar with JVM GC source code can help to do some analysis for it.
It only shows you the space used. The heap size determines the capacity of each region. Note: the actual size available is less as you have two survivors spaces and only one is active in normal operation. e.g. say you have a survivor spaces of 50 MB each, a 1 GB heap will say only 950 MB is available.
When you set the minimum heap size, this doesn't mean that space has been used yet. However you might not get a GC until this heap size is reached (or when the Eden which is a portion of the heap is full)
Say you have a 1 GB heap and the Eden space is 100 MB to start with. You will get a GC once the Eden fills up even though little tenured space is used. Eventually the Eden grows and the tenured space starts to fill and once it is full, it might grow the heap size to greater than the minimum heap size you gave it.
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
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.
While reading some notes on performance tuning, I did notice a recommendations while setting memory size:
Java application should size both initial and maximum permanent generation size to the same value since growing or contracting the permanent generation space requires a full GC. Similar suggestion is given while setting the heap size, I.e. -Xmx=-Xms.
My question is, then why do we have -Xms setting at all?
Also,
Why GC gets triggered often if I''ve different value for -Xmx and -Xms, and not when I''ve same size for -Xmx and -Xms.
To add more to my second question, If I start with minimum heap size of 64M and Max 512 M, I believe full GC will not get triggered unless memory utilized by my app reaches 512M.
Similarly If I start with 512M for both -Xmx and -Xms, still JVM will trigger full GC when my app memory use reaches this limit. So why it's advised to set both max and min to the same value?
The setting flags were designed before the VM had generational, incremental collection. In that case complete collections were all there were. In more modern collectors full collections are rare. That's good because incremental collections are normally a few milliseconds, so the UI experience doesn't change. Complete collections of big arenas can take several seconds ore more. Changing the arena size - as the document says - is guarenteed to cause a full collection every time.
The guidance isn't perfect 100% of the time. There are few kinds of apps where allowing the arena to grow is reasonable.
-Xms=64m -Xmx=512m does not mean "start up with a heap between 64 and 512 MB". It instructs the JVM to request 64MB of committed memory and 512MB of reserved memory at startup. The heap will be 64MB to start with, and as it fills up, will expand into the space reserved for it. So, with Xms of 64MB you would see a full collection before the heap filled to 64MB.
If you start your application with a low value for Xms and turn on GC logging (-verbose:gc -Xloggc:FILENAME) the log file will show how the heap and generation sizes change as the application runs.
Minor collections may be more frequent with a lower Xms because the new generation will be smaller (assuming you are using proportional generation sizing rather than explicit) and so will fill more quickly.
One reason to have -Xms < -Xmx is to allow the JVM to not pre-allocate the whole Xmx upfront so that the difference is available (foe a while perhaps) to other applications. Gory details here: http://www.ibm.com/developerworks/library/j-memusage/
I can set the Java Heap Size minimum and maximum by passing the parameters -Xms and -Xmx, respectively.
I understand that the maximum flag is necessary to limit the space, but what's the reason for setting a minimum initial heap size? If the maximum is big enough, the space will increase? Or do i miss something?
Minimun size is set so that the JVM doesn't have to "resize" the heap space if it starts off too small.
Java will start with the minimum heap size and will try to avoid enlarging the heap by garbage collecting.
So if the steady-state memory size of your app is larger than the default heap minimum heap size, you may do a lot of pointless GC on the way to that size. Setting the min size to that size will avoid the pointless GC.
You can set the minimum heap size if your application needs a certain amount to run. If you set the maximum heap size, the jvm isn't supposed to exceed it.