I would like to understand why the GC gets triggered even though I have plenty of heap left unused.. I have allocated 1.7 GB of RAM. I still see 10% of GC CPU usage often.
I use this - -XX:+UseG1GC with Java 17
JVMs will always have some gc threads running (unless you use Epsilon GC which perform no gc, I do not recommend using this unless you know why you need it), because the JVM manages memory for you.
Heap in G1 is divided two spaces: young and old. All objects are created in young space. When the young space fills (it always do eventually, unless you are developing zero garbage), it will trigger some gc cleaning unreferenced objects from the young and promoting some objects which are still referenced to old.
Those spikes in the right screenshot will correspond to young collection events (where unreferenced objects get cleaned). Young space is always much more small than the old space. So it fills frequently. That is why you see those spikes regarding there is much more memory free.
DISCLAIMER This is a really very high level explanation of memory management in the JVM. Some important concepts have been not mentioned.
You can read more about g1 gc collector here
Also take a look at jstat tool which will help you understand what is happening in your heap.
We migrated our application from CMS to G1GC and post that the heap monitoring system is triggering high heap usage alerts. Max heap usage is over 80% of total heap and is almost double of what we used to get with CMS. The memory graph is in zig-zag pattern with high and low usage.
Going through the GC log we noticed that in-order to reduce the frequency of GC cycle, G1GC is allocating around 50% of heap to Eden space. Majority of objects in Eden are temporary objects which get cleared on next GC. Since G1GC delays the GC cycle, major portion of reported high heap consists of temporary objects.
The monitoring system basically captures the Memory_HeapMemoryUsage MBean to get the memory snapshot. But with the way G1GC works, this data is misleading.
We can change the way monitoring system captures the data but then it will become G1GC specific?
Is there any better approach which is independent of GC implementation?
(Committed and Max lines are the same)
I am looking at the memory usage for a Java application in newrelic. Here are several questions:
# 1
The committed PS Survivor Space Heap varied in past few days. But should it be a constant since it is configured by JVM?
# 2
From what I am understanding, the heap memory should decrease when there is a garbage collection. The memory of Eden could decrease when a major gc or a minor gc happens, while the memory of Old could decrease when a major gc happens.
But if you look at Old memory usage, some time between June 6th and 7th, the memory went up and then later it went down. This should represent that a major gc happend, right? However, there was still lots of unused memory left. It didn't seem it almost reach the limit. Then how did the major gc be triggered? Same for Eden memory usage, it never reached the limit but it still decreased.
The application fetches a file from other places. This file could be large and be processed in memory. Could this explain the issue above?
You need to provide more information about your configuration to answer this definitively, I will assume you are using the Hotspot JVM from Oracle and that you are using the G1 collector. Posting the flags you start the JVM with would also be useful.
The key term here is 'committed'. This is memory reserved by the JVM, but not necessarily in use (or even mapped to physical pages, it's just a range of virtual memory that can be used by the JVM). There's a good description of this in the MemoryUsage class of the java.lang.management package (check the API docs). It says, "committed represents the amount of memory (in bytes) that is guaranteed to be available for use by the Java virtual machine. The amount of committed memory may change over time (increase or decrease). The Java virtual machine may release memory to the system..." This is why you see it change.
Assuming you are using G1 then the collector performs incremental compaction. You are correct that if the collector could not keep up with allocation in the old gen and it was getting low on space it would perform a full compacting collection. This is not happening here as the last graph shows you are using nowhere near the allocated heap space. However, to avoid this G1 will collect and compact concurrently with your application. This is why you see usage go up (as you application instantiates more objects) and then go down (as the G1 collector reclaims space from no longer required objects). For a more detailed explanation of how G1 works there is a good read in the documentation, https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/g1_gc.html.
Below are my JVM settings:
JAVA_OPTS=-server -Xms2G -Xmx2G -XX:MaxPermSize=512M -Dsun.rmi.dgc.client.gcInterval=1200000 -Dsun.rmi.dgc.server.gcInterval=1200000 -XX:+UseParallelOldGC -XX:ParallelGCThreads=2 -XX:+UseCompressedOops -Djava.net.preferIPv4Stack=true -Djboss.modules.system.pkgs=org.jbos88,server=y,suspend=n
Problem:
Total Heap Memory: 2GB
Old Gen: 1.4GB (2/3 of Heap)
New Gen: 600MB(1/3 of Heap)
The Old Gen grows in memory beyond 70% 0f its allocated size and never is subjected to GC even at 100% i.e 1.4GB.
One can see the graph below it peaks and never is GC, the drop in memory is when it was forced to GC from the JConsole.
This problem is eventually bringing the web server down.
Anything that i am missing or wrongly setting the JVM?
Thanks for the help in advance.
Updating my Question:
After heap analysis it appears like Stateful session bean is the prime suspect:
We have stateful session beans that hold the persistence logic assisted by Hibernate.
The GC will be called eventually, the old gen is almost never called (because it is extremely slow).
The GC does run but it will only run on the new gen and survivor gen at first, it has a completely different algorithm for cleaning the old gen which is slower then new/survivor gens.
Those numbers are really high, the oldgen should never reach sum a high number compared to the newgen.
My guess is that you have a memory leak.
I can only guess your program is dealing with big files, you are probably saving references to them for too long.
Even still having the main problem (memory leak) resolved, if you still want the old gen to be cleared in frequent small sized pauses, you may try setting
-XX:MaxGCPauseMillis=(time in millis)
and this is only applicable with Parallel Collector and when Adaptive Sizing Policy is on. By default Adaptive Sizing Policy is on, but, if you want to explicitly mention this you can use.
-XX:+UseAdaptiveSizePolicy
Or else you can switch to CMS collector where you can use
-XX:CMSInitiatingOccupancyFraction=(% value)
-XX:+UseCMSInitiatingOccupancyOnly
Which is a more reliable way of collecting the old gen when it has reached a certain fraction of the the old gen.
The stateful session beans were making the JVM run out of memory.
Explicitly handling them using #Remove annotation resolved this issue.
I have a Java app which shows different GC behaviors in different environments. In one environment, the heap usage graph is a slow sawtooth with major GCs every 10 hours or so, only when the heap is >90% full. In another environment, the JVM does major GCs every hour on the dot (the heap is normally between 10% and 30% at these times).
My question is, what are the factors which cause the JVM to decide to do a major GC?
Obviously it collects when the heap is nearly full, but there is some other cause at play which I am guessing is related to an hourly scheduled task within my app (although there is no spike in memory usage at this time).
I assume GC behaviour depends heavily on the JVM; I am using:
Java HotSpot(TM) 64-Bit Server VM 1.7.0_21 Oracle Corporation
No specific GC options, so using the default settings for 64-bit server (PS MarkSweep and PS Scavenge)
Other info:
This is a web app running in Tomcat 6.
Perm gen hovers around 10% in both environments.
The environment with the sawtooth behaviour has 7Gb max heap, the other has 14Gb.
Please, no guesswork. The JVM must have rules for deciding when to perform a major GC, and these rules must be encoded deep in the source somewhere. If anyone knows what they are, or where they are documented, please share!
I have found four conditions that can cause a major GC (given my JVM config):
The old gen area is full (even if it can be grown, a major GC will still be run first)
The perm gen area is full (even if it can be grown, a major GC will still be run first)
Someone is manually calling System.gc(): a bad library or something related to RMI (see links 1, 2 and 3)
The young gen areas are all full and nothing is ready to be moved into old gen (see 1)
As others have commented, cases 1 and 2 can be improved by allocating plenty of heap and permgen, and setting -Xms and -Xmx to the same value (along with the perm equivalents) to avoid dynamic heap resizing.
Case 3 can be avoided using the -XX:+DisableExplicitGC flag.
Case 4 requires more involved tuning, e.g., -XX:NewRatio=N (see Oracle's tuning guide).
Garbage collection is a pretty complicated topic, and while you could learn all the details about this, I think what’s happening in your case is pretty simple.
Sun’s Garbage Collection Tuning guide, under the “Explicit Garbage Collection” heading, warns:
applications can interact with garbage collection … by invoking full garbage collections explicitly … This can force a major collection to be done when it may not be necessary … One of the most commonly encountered uses of explicit garbage collection occurs with RMI … RMI forces full collections periodically
That guide says that the default time between garbage collections is one minute, but the sun.rmi Properties reference, under sun.rmi.dgc.server.gcInterval says:
The default value is 3600000 milliseconds (one hour).
If you’re seeing major collections every hour in one application but not another, it’s probably because the application is using RMI, possibly only internally, and you haven’t added -XX:+DisableExplicitGC to the startup flags.
Disable explicit GC, or test this hypothesis by setting -Dsun.rmi.dgc.server.gcInterval=7200000 and observing if GCs happen every two hours instead.
It depends on your configurations, since HotSpot configures itself differently in different Java environments. For example, in a server with more than 2GB and two processors some JVMs will be configured in '-server' mode instead of the default '-client' mode, which configure the sizes of the memory spaces (generations) differently, and that has an impact as to when garbage collection will occur.
A full GC can occur automatically, but also if you call the garbage collector in your code (ex: using System.gc()). Automatically, it depends on how the minor collections are behaving.
There are at least two algorithms being used. If you are using defaults, a copying algorithm is used for minor collections, and a mark-sweep algorithm for major collections.
A copying algorithm consists of copying used memory from one block to another, and then clearing the space containing the blocks with no references to them. The copying algorithm in the JVM uses uses a large area for objects that are created for the first time (called Eden), and two smaller ones (called survivors). Surviving objects are copied once from Eden and several times from the survivor spaces during each minor collection until they become tenured and are copied to another space (called tenured space) where they can only be removed in a major collection.
Most of the objects in Eden die quickly, so the first collection copies the surviving objects to the survivor spaces (which are by default much smaller). There are two survivors s1 and s2. Every time the Eden fills, the surviving objects from Eden and s1 are copied to s2, Eden and s1 are cleared. Next time, survivors from Eden and s2 are copied back to s1. They keep on being copied from s1 to s2 to s1 until a certain number of copies is reached, or because a block is too big and doesn't fit, or some other criteria. Then the surviving memory block is copied to the tenured generation.
The tenured objects are not affected by the minor collections. They accumulate until the area gets full (or the garbage collector is called). Then the JVM will run a mark-sweep algorithm in a major collection which will preserve only the surviving objects that still have references.
If you have larger objects that don't fit into the survivors, they might be copied directly to the tenured space, which will fill more quickly and you will get major collections more frequently.
Also, the sizes of the survivor spaces, amount of copies between s1 and s2, Eden size related to the size of s1 and s2, size of the tenured generation, all these may be automatically configured differently in different environments with JVM ergonomics, which may automatically select a -server or -client behavior. You might try to run both JVMs as -server or -client and check if they still behave differently.
Even if this will get down votes... My best guess (you will have to test this) would be that the heap needs to expand and when this happens a full gc will be triggered. Not all memory is allocated at once to JVM.
You can test this by setting -Xms and -Xmx to the same value, for example 7GB each