My code is:
class Test{
Test c=new Text();
System.out.println(c.size());
System.gc();
}
Can programmer use System.gc() for garbage collection in java? Is it preferrable? JVM performs automatically, then why should programmer to call System.gc()?
System.gc() sends a request to the GC to perform a collection cycle. This request may be served or it may be ignored, therefore neither result should be relied on.
A garbage collection cycle will happen automatically (without any action on your part), usually when the generation responsible for allocation of new objects is full or an allocation request cannot be satisfied at all.
In most cases, you should not need to call System.gc() at all in your code. System.gc() should be used in a few cases in which conditions similar to the following apply:
You know that a large amount of memory has just become unreachable.
It is essential that this amount of memory be freed quickly.
Or your program is about to enter a time-critical state where a GC cycle should happen as late as possible (or not at all) and so it helps to perform a GC cycle before you enter that state.
You have at least a rough idea of how the GC of the target environment works.
You have verified that the strategy of that GC is not optimal for your scenario at that point.
You can call it. There will be no harm in that. But there is no gaurentee that the memory of object you are expecting immediately gets free or not.
More over JVM runs GC asynchronously and we need not to drive it. JVM intelligent enough to free memory.
Just for knowing purpose it is OK, If you are really thinking about to clear memory due to XYZ reason, definitely a design flaw is there in your programm structure.
Even if you use System.gc() there is no guarantee that memory will be freed
from the oracle site
Calling the gc method suggests that the Java Virtual Machine expend effort toward recycling unused objects in order to make the memory they currently occupy available for quick reuse. When control returns from the method call, the Java Virtual Machine has made a best effort to reclaim space from all discarded objects.
You can call it, but is not guarantee that memory will be freed. Furthermore, if the memory was released, this could have negative consequences for the execution of your program. I will try to explain it to you, but I noticed you that my english is not very good XD
Java heap memory is divided in three zones based on objects generations. Oversimplifying: young, adult and old. When occur an invocation to GC, first it do is to check "young zone" for unused objects and liberate them. If GC doesn't free enought memory in the "young zone", it examine "adult zone". If GC doesn't free enought memory in the "adult zone", it examine the "old zone". Each generation is more costly for examine by GC than the last.
Well, objects are initialy created in young zone, if GC perform an execution in the young zone and the object is still used, that object pass to adult zone. Idem for adult -> old zones. If you invoke an execution of GC, it can think that an young object is candidate for adult object, and move it to adult zone. This causes your adult zone grows in an unnecesary way. Later, when GC have to examine adult zone, the operation is more costly for him, and your program performance can go down.
Related
I am trying to understand Java's garbage collection more deeply.
In HotSpot JVM generational collection, in the heap, there are three areas (Young generation, Old generation and permanent generation). Also, there are two kinds of algorithms:
1) Mark Sweep Compact.
2) Concurrent Mark and Sweep.
Is that true whether GC needs "Stop-the-world" depends on the algorithm it uses rather than which generation it operates on? In another word, if I use 1) as GC algorithm on all three areas, STW will always happen ?
Also, I understand the difference is the second GC algorithm doesn't require Compaction which will result in fragmentation eventually. So the second question comes to why the compaction needs a STW pause?
Key reason why compaction leads to STW pause is as follows, JVM needs to move object and update references to it. now if you move object before updating the references and application that is running access it from old reference than there is trouble. if you update reference first and than try to move object the updated reference is wrong till object is moved and any access while object has not moved will cause issue.
For both CMS and Parallel collecter the young generation collection algorithm is similar and it is stop the world ie application is stopped when collection is happening
Stuff JVM is doing is, marking all objects reachable from root set, moving the objects from Eden to survivor space and moving objects that have survived collections beyond tenuring threshold to the old generation. Of course JVM has to update all the references to the objects that have moved.
For old generation parallel collector is doing all marking, compaction and reference updates in a single stop the world(STW) phase, this leads to pauses in seconds for heaps in GBs. This was painful for the applications that have strict response time requirements. Till date Paralle collector is still the best collectors(among Oracle Java) for throughput or batch processing. In fact we have seen for same scenario even if time spent in pauses is more in parallel collector than CMS still we get a higher throughput, this I think has to do with better spatial locality due to compaction.
CMS solved the problem of high pauses in major collection by doing the Marking concurrently. There are 2 STW parts, Initial marking (getting the references from root set) and Remark Pause (a small STW pause at the end of marking to deal with changes in the object graph while marking and application was working concurrently). Both these pauses are in range of 100 -200 milliseconds for few GB of heap sizes and reasonable number of application threads(remember more active threads more roots)
G1GC is planned to be a replacement of CMS and accept goals for pauses. takes care of fragmentation by incrementally compacting the heap.Though the work is incremental so you can get smaller pauses but that may come at the cost of more frequent pauses
None of the above can compact heap(CMS does not compact at all) while application is running. AZUL GPGC garbage collection can even compact without stopping the application and also handle reference update. So if you want to go deep into how GCs work it will be worth reading the algorithm for GPGC. AZUL markets it as a pause-less collector.
All freely available GCs in openjdk have some stop the world events. And not just the GCs, other things such as deoptimizations can trigger safepoints too.
But not all pauses are equal. CMS and G1 do not need to scale their pause times with the live data set in the old generation because they only scan a subset of the objects during the pauses and do a major fraction of their work concurrently, unlike the Serial and Throughput collectors.
ZGC (available since OpenJDK11) and Shenandoah (since 12) are collectors that further decouple pause times from the live data set size and scale their pauses with only the root set size instead.
Additionally other GC implementations exist which avoid global pauses - they may still experience per-thread pauses - or make the pause durations O(1), i.e. independent of live data set size. A commonly cited example is azul's C4 collector.
So the second question comes to why the compaction needs a STW pause?
Compacting means moving objects. Moving objects means pointers need to be updated. This is very difficult or costly to achieve safely when applications threads are still running.
Concurrent algorithms generally pay some cost in throughput and complexity in exchange for their lower pause times. Not doing compactation makes CMS relatively(!) simple for a concurrent collector.
Here is a link that gives some good information about the different collectors in java 8: https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/collectors.html#sthref27
All strategies will stop-the-world. But, your performance requirements can drive you to choose differing GC strategies to improve performance or response times.
Stop-the-world will occur no matter which GC algorithm you choose. Stop-the-world means that the JVM is stopping the application from running to execute a GC. When stop-the-world occurs, every thread except for the threads needed for the GC will stop their tasks.
I am testing the usage of Heap size in a java application running in JDK 1.6. I use the tool VisualVM to monitor the heap usage. I found the Maximum heap size usage of around 500 MB for a few mins. I used the option "Perform GC" which calls System.gc(). The first time i used it, the Maximum heap is reduced to 410MB, then once again I used it to get 130MB and the next time to 85MB. I made all the four calls next to next without any interval. Why does the call System.gc() does not collect all the Heap to 85MB at first time. Is there any other reason behind this. Or I should try with any other methods?
The System.gc() will return when all objects have been scanned once.
An object should be finalized() AFTER it has been collected. Most objects don't implement this method but for the ones which do, they are added to a queue to be cleaned up later. This means those objects cannot be cleaned up yet (not the queue nodes which hold them) i.e. the act of triggering a GC can increase memory consumption temporarily.
Additionally there are SoftReferences to objects which may or may not be cleaned up by a GC. The assumption is these should only be cleaned up if not much else was cleaned up.
In short, not all objects can be cleaned up in one cycle.
System.gc() requests the JVM to start garbage collection. If you are expecting that GC is invoked as soon as System.gc() then it is a wrong notion. Calling it multiple times will not help. It is not possible to map System.gc() with the actual garbage collection. Also no matter how many times you call System.gc(), JVM will do the GC only when it is ready to do so. What may be happening is that heap size is getting reduced even with the first System.gc() but not exactly as soon as you call it. Garbage collection related to your first System.gc() may be finishing in background and in parallel your code is reaching third System.gc() statement.
If you are pretty sure that only adding multiple System.gc() helps you reducing the heap size. Then you need to check what all objects are getting created in JVM in between first and last System.gc(). There may be other threads creating the objects.
One possible reason might be the use of java.lang.ref.Reference types. If the GC is going to break a "Reference" this will happen after the GC proper has completed. Any objects that become unreachable as a result are left for the next GC cycle to deal with.
Finalization works the same way. If an object requires finalization, it and all of the objects reachable from it (only) are likely to only be collectable in the next GC cycle.
Then there is the issue that the GC's algorithm for shrinking the heap is non-aggressive. According to the Java HotSpot VM Options page, the GC only shrinks the heap if more than 70% is free after garbage collection. However, it is not entirely clear if this refers to a full GC or not. So you could get the GC doing a partial GC and shrinking, and then a full GC and shrinking some more.
(Some people infer from the wording of the System.gc() javadocs that it will perform a full GC. However, I suspect that this is actually version / GC dependent.)
But to be honest this should all be moot. Trying to coerce an application into giving back as much memory is possible is pointless. The chances are that you are forcing it to throw away cached data. When the application gets active again it will start reloading its caches.
This question already has answers here:
How to force garbage collection in Java?
(25 answers)
Closed 8 years ago.
I have a complex java application running on a large dataset. The application performs reasonably fast but as time goes it seems to eat lots of memory and slow down. Is there a way to run the JVM garbage collector without re-starting the application?
No, You cant force garbage collection.
Even using
System.gc();
You can just make a request for garbage collection but it depends on JVM to do it or not.
Also Garbage collector are smart enough to collect unused memory when required so instead of forcing garbage collection you should check if you are handling objects in a wrong way.
If you are handling objects in a wrong way (like keeping reference to unnecessary objects) there is hardly anything JVM can do to free the memory.
From Doc
Calling the gc method suggests that the Java Virtual Machine expend
effort toward recycling unused objects in order to make the memory
they currently occupy available for quick reuse. When control returns
from the method call, the Java Virtual Machine has made a best effort
to reclaim space from all discarded objects.
Open Bug regarding System.gc() documentation
The documentation for System.gc() is extremely misleading and fails to
make reference to the recommended practise of never calling
System.gc().
The choice of language leaves it unclear what the behaviour would be
when System.gc() is called and what external factors will influence
the behaviour.
Few useful link to visit when you think you should force JVM to free up some memory
1. How does garbage collection work
2. When does System.gc() do anything
3. Why is it bad practice to call System.gc()?
All says
1. You dont have control over GC in Java even System.gc() dont guarantee it.
2. Also its bad practise as forcing it may have adverse effect on performance.
3. Revisit your design and let JVM do his work :)
you should not relay on System.gc() - if you feel like you need to force GC to run it usually means that there is something wrong with your code/design. GC will run and clear your unused objects if they are ready to be created - please verify your design and think more about memory management, look as well for loops in object references.
The
System.gc()
call in java, suggest to the vm to run garbage collection. Though it doesn't guarantee that it will actually do it. Nevertheless the best solution you have. As mentioned in other responses jvisualvm utility (present in JDK since JDK 6 update 7), provides a garbage functionality as well.
EDIT:
your question open my appetite for the topic and I came across this resource:
oracle gc resource
The application performs reasonably fast but as time goes it seems to eat lots of memory and slow down.
These are a classic symptoms of a Java memory. It is likely that somewhere in your application there is a data structure that just keeps growing. As the heap gets close to full, the JVM spends an increasing proportion of its time running the GC in a (futile) attempt to claw back some space.
Forcing the GC won't fix this, because the GC can't collect the data structure. In fact forcing the GC to run just makes the application slower.
The cure for the problem is to find what is causing the memory leak, and fix it.
Performance gain/drop depends how often you need garbage collection and how much memory your jvm has and how much your program needs.
There is no certainity(its just a hint to the interpreter) of garbage collection when you call System.gc() but at least has a probability. With enough number of calls, you can achieve some statistically derived performance multiplier for only your system setup.
Below graph shows an example program's executions' consumptions and jvm was given only 1GB(no gc),1GB(gc),3GB(gc),3GB(no gc) heaps respectively to each trials.
At first, when jvm was given only 1GB memory while program needed 3.75GB, it took more than 50 seconds for the producer thread pool to complete their job because having less garbage management lead to poor object creation rate.
Second example is about %40 faster because System.gc() is called between each production of 150MB object data.
At third example, jvm is given 3GB memory space while keeping System.gc() on. More memory has given more performance as expected.
But when I turned System.gc() off at the same 3GB environment, it was faster!
Even if we cannot force it, we can have some percentage gain or drain of performance trying System.g() if we try long enough. At least on my windows-7 64 bit operating system with latest jvm .
Garbage collector runs automatically. You can't force the garbage collector.
I do not suggest that you do that but to force the garbage collector to run from within your java code you can just use all the available memory, this works because the garbage collector will run before the JVM throws OutOfMemoryError...
try {
List<Object> tempList = new ArrayList<Object>();
while (true) {
tempList.add(new byte[Integer.MAX_VALUE]);
}
} catch (OutOfMemoryError OME) {
// OK, Garbage Collector will have run now...
}
My answer is going to be different than the others but it will lead to the same point.
Explain:
YES it is possible to force the garbage collector with two methods used at the same time and in the same order this are:
System.gc ();
System.runFinalization ();
this two methods call will force the garbage collector to execute the finalise() method of any unreachable object and free the memory. however the performance of the software will down considerable this is because garbage runs in his own thread and to that one is not way to controlled and depending of the algorithm used by the garbage collector could lead to a unnecessary over processing, It is better if you check your code because it must be broken to you need use the garbage collector to work in a good manner.
NOTE: just to keep on mind this will works only if in the finalize method is not a reassignment of the object, if this happens the object will keep alive an it will have a resurrection which is technically possible.
In other words:
I need to know if after calling System.gc() object instances (that are not collected) are distributed between generations in the same way as before calling System.gc().
Thanks,
After a Full GC, which a System.gc() may or may not trigger,
the Eden space will be empty, so anything in there will be moved out or cleaned up.
the Survivor spaces will swap objects from the one which has objects to the empty one, and
the Tenured space will have the same retained objects it had before but some may have been cleaned up and some may be new to that generation.
In short, the only time System.gc() won't change objects between generations is when
it doesn't do anything because it is ignored
no objects have been created or discarded since the last Full GC.
if this could be another reason why calling System.gc() is evil
Mostly because it hurts performance and you gain very little in return. Note: the RMI can trigger a full collection periodically to ensure distributed objects are cleaned up but it tries to keep this to a minimum.
First of all, System.gc() is only a hint to the JVM that you want a GC, it might not trigger one (if using -XX:+DisableExplicitGC with Hotspot for example), and you should not rely on it doing anything -- the JVM usually knows better anyway.
If it does trigger a GC, then it's just like any other GC, and objects might get promoted from the young to the old generation if they satisfy the criteria (enough generations spent in the survivor spaces for example).
I am building a Java web app, using the Play! Framework. I'm hosting it on playapps.net. I have been puzzling for a while over the provided graphs of memory consumption. Here is a sample:
The graph comes from a period of consistent but nominal activity. I did nothing to trigger the falloff in memory, so I presume this occurred because the garbage collector ran as it has almost reached its allowable memory consumption.
My questions:
Is it fair for me to assume that my application does not have a memory leak, as it appears that all the memory is correctly reclaimed by the garbage collector when it does run?
(from the title) Why is java waiting until the last possible second to run the garbage collector? I am seeing significant performance degradation as the memory consumption grows to the top fourth of the graph.
If my assertions above are correct, then how can I go about fixing this issue? The other posts I have read on SO seem opposed to calls to System.gc(), ranging from neutral ("it's only a request to run GC, so the JVM may just ignore you") to outright opposed ("code that relies on System.gc() is fundamentally broken"). Or am I off base here, and I should be looking for defects in my own code that is causing this behavior and intermittent performance loss?
UPDATE
I have opened a discussion on PlayApps.net pointing to this question and mentioning some of the points here; specifically #Affe's comment regarding the settings for a full GC being set very conservatively, and #G_H's comment about settings for the initial and max heap size.
Here's a link to the discussion, though you unfortunately need a playapps account to view it.
I will report the feedback here when I get it; thanks so much everyone for your answers, I've already learned a great deal from them!
Resolution
Playapps support, which is still great, didn't have many suggestions for me, their only thought being that if I was using the cache extensively this may be keeping objects alive longer than need be, but that isn't the case. I still learned a ton (woo hoo!), and I gave #Ryan Amos the green check as I took his suggestion of calling System.gc() every half day, which for now is working fine.
Any detailed answer is going to depend on which garbage collector you're using, but there are some things that are basically the same across all (modern, sun/oracle) GCs.
Every time you see the usage in the graph go down, that is a garbage collection. The only way heap gets freed is through garbage collection. The thing is there are two types of garbage collections, minor and full. The heap gets divided into two basic "areas." Young and tenured. (There are lots more subgroups in reality.) Anything that is taking up space in Young and is still in use when the minor GC comes along to free up some memory, is going to get 'promoted' into tenured. Once something makes the leap into tenured, it sits around indefinitely until the heap has no free space and a full garbage collection is necessary.
So one interpretation of that graph is that your young generation is fairly small (by default it can be a fairly small % of total heap on some JVMs) and you're keeping objects "alive" for comparatively very long times. (perhaps you're holding references to them in the web session?) So your objects are 'surviving' garbage collections until they get promoted into tenured space, where they stick around indefinitely until the JVM is well and good truly out of memory.
Again, that's just one common situation that fits with the data you have. Would need full details about the JVM configuration and the GC logs to really tell for sure what's going on.
Java won't run the garbage cleaner until it has to, because the garbage cleaner slows things down quite a bit and shouldn't be run that frequently. I think you would be OK to schedule a cleaning more frequently, such as every 3 hours. If an application never consumes full memory, there should be no reason to ever run the garbage cleaner, which is why Java only runs it when the memory is very high.
So basically, don't worry about what others say: do what works best. If you find performance improvements from running the garbage cleaner at 66% memory, do it.
I am noticing that the graph isn't sloping strictly upward until the drop, but has smaller local variations. Although I'm not certain, I don't think memory use would show these small drops if there was no garbage collection going on.
There are minor and major collections in Java. Minor collections occur frequently, whereas major collections are rarer and diminish performance more. Minor collections probably tend to sweep up stuff like short-lived object instances created within methods. A major collection will remove a lot more, which is what probably happened at the end of your graph.
Now, some answers that were posted while I'm typing this give good explanations regarding the differences in garbage collectors, object generations and more. But that still doesn't explain why it would take so absurdly long (nearly 24 hours) before a serious cleaning is done.
Two things of interest that can be set for a JVM at startup are the maximum allowed heap size, and the initial heap size. The maximum is a hard limit, once you reach that, further garbage collection doesn't reduce memory usage and if you need to allocate new space for objects or other data, you'll get an OutOfMemoryError. However, internally there's a soft limit as well: the current heap size. A JVM doesn't immediately gobble up the maximum amount of memory. Instead, it starts at your initial heap size and then increases the heap when it's needed. Think of it a bit as the RAM of your JVM, that can increase dynamically.
If the actual memory use of your application starts to reach the current heap size, a garbage collection will typically be instigated. This might reduce the memory use, so an increase in heap size isn't needed. But it's also possible that the application currently does need all that memory and would exceed the heap size. In that case, it is increased provided that it hasn't already reached the maximum set limit.
Now, what might be your case is that the initial heap size is set to the same value as the maximum. Suppose that would be so, then the JVM will immediately seize all that memory. It will take a very long time before the application has accumulated enough garbage to reach the heap size in memory usage. But at that moment you'll see a large collection. Starting with a small enough heap and allowing it to grow keeps the memory use limited to what's needed.
This is assuming that your graph shows heap use and not allocated heap size. If that's not the case and you are actually seeing the heap itself grow like this, something else is going on. I'll admit I'm not savvy enough regarding the internals of garbage collection and its scheduling to be absolutely certain of what's happening here, most of this is from observation of leaking applications in profilers. So if I've provided faulty info, I'll take this answer down.
As you might have noticed, this does not affect you. The garbage collection only kicks in if the JVM feels there is a need for it to run and this happens for the sake of optimization, there's no use of doing many small collections if you can make a single full collection and do a full cleanup.
The current JVM contains some really interesting algorithms and the garbage collection itself id divided into 3 different regions, you can find a lot more about this here, here's a sample:
Three types of collection algorithms
The HotSpot JVM provides three GC algorithms, each tuned for a specific type of collection within a specific generation. The copy (also known as scavenge) collection quickly cleans up short-lived objects in the new generation heap. The mark-compact algorithm employs a slower, more robust technique to collect longer-lived objects in the old generation heap. The incremental algorithm attempts to improve old generation collection by performing robust GC while minimizing pauses.
Copy/scavenge collection
Using the copy algorithm, the JVM reclaims most objects in the new generation object space (also known as eden) simply by making small scavenges -- a Java term for collecting and removing refuse. Longer-lived objects are ultimately copied, or tenured, into the old object space.
Mark-compact collection
As more objects become tenured, the old object space begins to reach maximum occupancy. The mark-compact algorithm, used to collect objects in the old object space, has different requirements than the copy collection algorithm used in the new object space.
The mark-compact algorithm first scans all objects, marking all reachable objects. It then compacts all remaining gaps of dead objects. The mark-compact algorithm occupies more time than the copy collection algorithm; however, it requires less memory and eliminates memory fragmentation.
Incremental (train) collection
The new generation copy/scavenge and the old generation mark-compact algorithms can't eliminate all JVM pauses. Such pauses are proportional to the number of live objects. To address the need for pauseless GC, the HotSpot JVM also offers incremental, or train, collection.
Incremental collection breaks up old object collection pauses into many tiny pauses even with large object areas. Instead of just a new and an old generation, this algorithm has a middle generation comprising many small spaces. There is some overhead associated with incremental collection; you might see as much as a 10-percent speed degradation.
The -Xincgc and -Xnoincgc parameters control how you use incremental collection. The next release of HotSpot JVM, version 1.4, will attempt continuous, pauseless GC that will probably be a variation of the incremental algorithm. I won't discuss incremental collection since it will soon change.
This generational garbage collector is one of the most efficient solutions we have for the problem nowadays.
I had an app that produced a graph like that and acted as you describe. I was using the CMS collector (-XX:+UseConcMarkSweepGC). Here is what was going on in my case.
I did not have enough memory configured for the application, so over time I was running into fragmentation problems in the heap. This caused GCs with greater and greater frequency, but it did not actually throw an OOME or fail out of CMS to the serial collector (which it is supposed to do in that case) because the stats it keeps only count application paused time (GC blocks the world), application concurrent time (GC runs with application threads) is ignored for those calculations. I tuned some parameters, mainly gave it a whole crap load more heap (with a very large new space), set -XX:CMSFullGCsBeforeCompaction=1, and the problem stopped occurring.
Probably you do have memory leaks that's cleared every 24 hours.