Why the method call System.gc() doesn't guarantee that the Garbage Collector algorithm will run at that moment? Why it cannot certainly reclaim all unused object's memory whenever it is invoked ?
Forcing object destruction is sign of bad coding so Java might wanted to avoid developers from getting addicted to it.
If Java gives you freedom of forcing of object destruction, improper use of it might have adverse affect of application performance.
This restriction allows you (force you) to put more focus into business logic than memory managemnt
JVM is the best person to decide when memory management is required and how to do it.
You can (should) trust JVM and let it handle things in better way than we can.
Still do you really want to force object destruction? If yes, WHY?
In order to ensure that programs run smoothly within the JVM, the JVM itself manages garbage collection.
Garbage collection has become quite sophisticated. When you ask the system for a GC run, which algorithm are you expecting? A "full GC"? There are multiple heaps, as well; which one is the garbage you are concerned about on? You don't know and this method doesn't indicate.
Suppose calling System.gc() always triggered a full GC. An errant program could easily grind JVM performance to a halt. Defensively, the JVM would want to limit the frequency at which it responded to such calls.
If you are running in a JVM on a non-embedded system (e.g. a server or a desktop computer), there should be no reason for you to concern yourself with any aspect of memory management other than to monitor it and code efficiently.
There are several metrics that are utilized to evaluate garbage collector performance , some of them are:
Throughput—the percentage of total time not spent in garbage collection, considered over long periods of time.
Garbage collection overhead—the inverse of throughput, that is, the percentage of total time spent in
garbage collection.
Pause time—the length of time during which application execution is stopped while garbage collection is occurring.
Frequency of collection—how often collection occurs, relative to application execution.
Footprint—a measure of size, such as heap size.
Promptness—the time between when an object becomes garbage and when the memory becomes
available.
Now If JVM listens to System.gc() like good pet and guarantees to perform action on each System.gc() call, Imagine what would be the performance of application if it is called many times within program.!!??
Throughput will decrease
Garbage Collection overhead will increase.
Application will pause many times as it is busy in recollecting the memory.
If Footprint is large , The garbage Collector would have to scan all memory area for recovering the memory , no matter if there are objects eligible for garbage collection or not.
So , after looking through these points I guess it provides the sufficient reason to JVM to not respond to System.gc on the application choice , but on its own algorithm. And Garbage Collection reclaims all unused object's memory for sure , but its invocation is entirely dependent on JVM own Algorithm rather than on the user's choice.
Source: Memory Management in the Java HotSpot™ Virtual Machine - Sun Microsystems
it cannot certainly reclaim all unused object's memory whenever it is invoked
This assumption of yours is false. In most cases the Garbage Collector could reclaim all unused objects at any point in time. However, if the standard Java library provided a method that would guarantee that, it would put a completely unreasonable burden on the GC subsystem to provide a service that is most of the time useless and could be even damaging.
Is it possible to mark java objects non-collectable from gc perspective to save on gc-sweep time?
Something along the lines of http://wwwasd.web.cern.ch/wwwasd/lhc++/Objectivity/V5.2/Java/guide/jgdStorage.fm.html and specifically non-garbage-collectible containers there (non-garbage-collectable?).
The problem is that I have lots of ordinary temporary objects, but I have even bigger (several Gigs) of objects that are stored for Cache purposes. For no reason should the Java GC traverse all those Cache gigabytes trying to find anything to collect, because they contain cached data which have their own timeouts.
This way I could partition my data in a custom way into infinite-lived and normal-lived objects, and hopefully GC would be quite fast because normal objects don't live so long and amount to smaller amounts.
There are some workarounds to this problem, such as Apache DirectMemory and Commercial Terracotta BigMemory(http://terracotta.org/products/bigmemory), but a java-native solution would be nicer (I mean free and probably more reliable?). Also I want to avoid serialization overhead which means it should happen within same jvm. To my understanding DirectMemory and BigMemory operate mainly off heap which means that the objects must be serialized/deserialized to/from memory outside jvm. Simply marking non-gc regions within the jvm would seem a better solution. Using Files for cache is not an option either, it has the same unaffordable serialization/deserialization overhead - use case is a HA server with lots of data used in random (human) order and low latency needed.
Any memory the JVM manages is also garbage-collected by the JVM. And any “live” objects which are directly available to Java methods without deserialization have to live in JVM memory. Therefore in my understanding you cannot have live objects which are immune to garbage collection.
On the other hand, the usage you describe should make the generational approach to garbage collection quite efficient. If your big objects stay around for a while, they will be checked for reclamation less often. So I doubt there is much to be gained from avoiding those checks.
Is it possible to mark java objects non-collectable from gc perspective to save on gc-sweep time?
No it is not possible.
You can prevent objects from being garbage collected by keeping them reachable, but the GC will still need to trace them to check reachability on each full; GC (at least).
Is simply my assumption, that when the jvm is starving it begins scanning all those unnecessary objects too.
Yes. That is correct. However, unless you've got LOTS of objects that you want to be treated this way, the overhead is likely to be insignificant. (And anyway, a better idea is to give the JVM more memory ... if that is possible.)
Quite simply, for you to be able to do this, the garbage collection algorithm would need to be aware of such a flag, and take it into account when doing its work.
I'm not aware of any of the standard GC algorithms having such a flag, so for this to work you would need to write your own GC algorithm (after deciding on some feasible way to communicate this information to it).
In principle, in fact, you've already started down this track - you're deciding how garbage collection should be done rather than being happy to leaving it to the JVM's GC algo. Is the situation you describe a measurable problem for you; something for which the existing garbage collection is insufficient, but your plan would work? Garbage collectors are extremely well-tuned, so I wouldn't be surprised if the "inefficient" default strategy is actually faster than your naively-optimal one.
(Doing manual memory management is tricky and error-prone at the best of times; managing some memory yourself while using a stock garbage collector to handle the rest seems even worse. I expect you'd run into a lot of edge cases where the GC assumes it "knows" what's happening with the whole heap, which would no longer be true. Steer clear if you can...)
The recommended approaches would be to use either a commerical RTSJ implementation to avoid GC, or to use off heap memory. One could also look into soft references for caches as well (they do get collected).
This is not recommended:
If for some reason you do not believe these options are sufficient, you could look into direct memory access which is UNSAFE (part of sun.misc.Unsafe). You can use the 'theUnsafe' field to get the 'Unsafe' instance. Unsafe allows to allocation/deallocate memory via 'allocateMemory' and 'freeMemory'. This is not under GC control nor limited by JVM heap size. The impact on GC/application, once you go down this route, is not guaranteed - which is why using byte buffers might be the way to go (if you're not using a RTSJ like implementation).
Hope this helps.
Living Java objects will always be part of the GC life cycle. Or said another way, marking an object to be non-gc is the same order of overhead than having your object referenced by a root reference (a static final map for instance).
But thinking a bit further, data put in a cache are most likely to be temporary, and would eventually be evicted. At that point you will start again to like the JVM and the GC.
If you have 100's of GBs of permanent data, you may want to rethink the architecture of your application, and try to shard and distribute your data (horizontally scalability).
Last but not least, lots of work has been done around serialization, and the overhead of serialization (I'm not speaking about the poor reputation of ObjectInputStream and ObjectOutputStream) is not that big.
More than that, if your data is mainly composed of primitive types (including bytes array), there is efficient way to readInt() or readBytes() from off heap buffers (for instannce netty.io's ChannelBuffer). This could be a way to go.
I have a strange doubts. I know garbage collector has its own limitation. and if allocation is
bad then it can cause a problem for application to respond in unusual way.
So my question is that is it good programming habit to call forcefully garbage collector (System.gc()) at the end of each activity?
Update
Every one is saying that calling system.gc() not beneficial at all.Then i am wondering why its present here.DVM will decide when to run garbage collector.Then what is need of that method?
Update 2
Thanks community to help me out. But honestly i got knowledge about Garbage collection real Beauvoir from this link Java Performance Optimization
it isn't good programming habit to call forcefully garbage collector (System.gc()) at the end of each activity
Because it is useless,only DVM decide when it should be call although you called it...
System.gc(), which the VM sometimes ignores at whim, is mostly useful in two cases:
you're gobbling up memory like there's no tomorrow (usually with bitmaps).
you suspect a memory leak (such as accidentally holding onto an old Context), and want to put the VM memory in a quiescent state to see if the memory usage is creeping up, for debugging.
Under nominal circumstances, one should not use it.
I really think it depends on your situation.
Because the heap is generational, the GC may not get rid of certain large objects or bitmaps on its first pass, and its heuristics may not indicate that additional garbage collection is necessary, but there are definitely scenarios where the heuristic could be wrong, and we as the developers have knowledge of a pattern, or can predict usage that the GC cannot, and therefore calling system.gc() will benefit us.
I have seen this before in specific scenarios such as dealing with map tiling or other graphic intensive behaviors, where the native GC in Android (even on 3.0+ devices), doesn't get it right, resulting in Out of Memory errors. However, by adding a few GC calls, the Out of Memory errors are prevented, and the system continues to process albeit at a slower rate (due to garbage collection). In graphic intensive operations, this usually is that state desired (a little lag) over the application crashing because it cannot load additional resources into memory.
My only explanation for why this happens in certain scenarios appears to be timing. If user operations are slow, then the native Android GC seems to do great. However, if your user is scrolling fast, or zooming quickly, this is where I have seen the Android GC lag behind, and a few well thought out System.gc() have resulted in my applications not crashing.
calling System.gc(), doesn't do any harm. but you cant be sure that it will be of some use. because you ask the DVM to do garbage collection, but can't command it... Its dependent totally on DVM. It calls when memory is running out or may be at any time..
I tried putting System.gc() on the line before the lines where I created my bitmap in my Android app. The garbage collector freed up several megabytes in some cases and put and end to my OutOfMemoryError conditions. It did not interfere with the normal garbage collection one bit but it did make my app run faster.
No; if the system needs memory, it will call GC on its own.
Any memory used by an instance, that isn't referenced anywhere else, will become eligible for GC when the instance goes away.
Memory used by the instance itself, if no longer referenced, is also eligible for GC. You can do a code review or profiling to see if you're holding on to memory unnecessarily, but that's a different issue.
Calling GC manually is a bad coding habit...
The Developer docs on RAM usage state:
...
GC_EXPLICIT
An explicit GC, such as when you call gc() (which you should avoid calling and instead trust the GC to run when needed).
...
I've highlighted the most important and relevant part here in bold.
It is possible to ask the Android JVM to run the garbage collector by calling System.gc(). As the documentation states:
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.
Emphasis added!
Some care is needed in interpreting "best effort" in the final sentence:
The "best effort" might be to ignore the "suggestion" entirely. Some JVMs have a configuration option to totally ignore System.gc() calls.
The "best effort" may or may not amount to a full garbage collection. That is an implementation detail.
But the bottom line is that you cannot force the GC to run.
Calling System.gc() is generally a bad idea. It makes your application inefficient, and it may introduce unwanted and unnecessary GC pauses.
The inefficiency issue comes down to the way that modern garbage collectors behave. A garbage collector's work has two parts1:
Finding the objects that are reachable.
Dealing with the objects that are not reachable.
The first part involves traversing reference chains and and marking the graph of objects starting at the GC roots. This work is proportional to the number of reachable objects.
The second part can be handled in a couple of ways, but it will typically be proportional to the size of the reachable objects.
Thus the overall cost of a GC run (in CPU time) depends mostly in the amount of non-garbage. But the benefit of the work performed is the amount of space that you managed to reclaim.
To maximize efficiency, you need to run the GC when the benefit of running the GC is at its highest; i.e. when the heap is close to full. But the problem is that if you call System.gc() you may be requesting a garbage collection when there is lots of free space.
Every one is saying that calling system.gc() not beneficial at all. Then I am wondering why its present here. DVM will decide when to run garbage collector. Then what is need of that method?
It is there for largely historical reasons. The method was present in the System class in Java 1.0. Removing it now would break a lot of legacy code. As for why gc() was included in the first place, the decision was made a long, long time ago, and we were not "in the room" when it was made. My guess is that the decision makers (in ~1995):
were a bit too optimistic about how GC technology would develop,
didn't anticipate that naive programmers would try to use gc() calls to solve memory leaks and other bugs, and / or
were simply too rushed to think too hard about it.
There are also a couple of scenarios where calling System.gc() is beneficial. One such scenario is when your application is about to start a "phase" where unscheduled GC pauses are going to give a particularly bad user experience. By running System.gc() you can take the "performance hit" at a point in time where it matters less; e.g. during a user initiated pause or while switching levels in a game.
But I don't think the above scenario corresponds to your "at the end of every activity".
The final thing to note is that calling System.gc() manually does not prevent normal OOMEs. A normal OOME is typically thrown then the JVM decides there is not enough free heap space to continue. This decision is made immediately after running a (full) GC. Running System.gc() manually won't make any difference to the decision making.
Furthermore, calling System.gc() will not cure normal2 memory leaks. If your application has a memory leak, you actually have a situation where a bunch of objects are reachable when they shouldn't be. But since they are reachable, the GC won't delete them.
The cure for OOMEs is one or more of the following:
Find the memory leaks and fix them. There are tools to help you do this.
Modify the application to use memory more efficiently; e.g. don't keep so much data in memory, or represent it in a more compact form.
Increase the application's heap size.
1 - This is a simplification, but the full story is way to complicated for this posting. I recommend you buy an read an up-to-date book on Garbage Collection of you want (or need) a deeper understanding.
2 - There are cases involving non-heap memory where manually running the GC might help as a band-aid for certain kinds of OOME. But a better solution is to find a better way to reduce non-heap memory usage and/or free up non-heap resources in a more timely fashion.
After answering a question about how to force-free objects in Java (the guy was clearing a 1.5GB HashMap) with System.gc(), I was told it's bad practice to call System.gc() manually, but the comments were not entirely convincing. In addition, no one seemed to dare to upvote, nor downvote my answer.
I was told there that it's bad practice, but then I was also told that garbage collector runs don't systematically stop the world anymore, and that it could also effectively be used by the JVM only as a hint, so I'm kind of at loss.
I do understand that the JVM usually knows better than you when it needs to reclaim memory. I also understand that worrying about a few kilobytes of data is silly. I also understand that even megabytes of data isn't what it was a few years back. But still, 1.5 gigabytes? And you know there's like 1.5 GB of data hanging around in memory; it's not like it's a shot in the dark. Is System.gc() systematically bad, or is there some point at which it becomes okay?
So the question is actually double:
Why is or isn't it bad practice to call System.gc()? Is it really merely a hint to the JVM under certain implementations, or is it always a full collection cycle? Are there really garbage collector implementations that can do their work without stopping the world? Please shed some light over the various assertions people have made in the comments to my answer.
Where's the threshold? Is it never a good idea to call System.gc(), or are there times when it's acceptable? If so, what are those times?
The reason everyone always says to avoid System.gc() is that it is a pretty good indicator of fundamentally broken code. Any code that depends on it for correctness is certainly broken; any that rely on it for performance are most likely broken.
You don't know what sort of garbage collector you are running under. There are certainly some that do not "stop the world" as you assert, but some JVMs aren't that smart or for various reasons (perhaps they are on a phone?) don't do it. You don't know what it's going to do.
Also, it's not guaranteed to do anything. The JVM may just entirely ignore your request.
The combination of "you don't know what it will do," "you don't know if it will even help," and "you shouldn't need to call it anyway" are why people are so forceful in saying that generally you shouldn't call it. I think it's a case of "if you need to ask whether you should be using this, you shouldn't"
EDIT to address a few concerns from the other thread:
After reading the thread you linked, there's a few more things I'd like to point out.
First, someone suggested that calling gc() may return memory to the system. That's certainly not necessarily true - the Java heap itself grows independently of Java allocations.
As in, the JVM will hold memory (many tens of megabytes) and grow the heap as necessary. It doesn't necessarily return that memory to the system even when you free Java objects; it is perfectly free to hold on to the allocated memory to use for future Java allocations.
To show that it's possible that System.gc() does nothing, view
JDK bug 6668279
and in particular that there's a -XX:DisableExplicitGC VM option:
By default calls to System.gc() are enabled (-XX:-DisableExplicitGC). Use -XX:+DisableExplicitGC to disable calls to System.gc(). Note that the JVM still performs garbage collection when necessary.
It has already been explained that calling system.gc() may do nothing, and that any code that "needs" the garbage collector to run is broken.
However, the pragmatic reason that it is bad practice to call System.gc() is that it is inefficient. And in the worst case, it is horribly inefficient! Let me explain.
A typical GC algorithm identifies garbage by traversing all non-garbage objects in the heap, and inferring that any object not visited must be garbage. From this, we can model the total work of a garbage collection consists of one part that is proportional to the amount of live data, and another part that is proportional to the amount of garbage; i.e. work = (live * W1 + garbage * W2).
Now suppose that you do the following in a single-threaded application.
System.gc(); System.gc();
The first call will (we predict) do (live * W1 + garbage * W2) work, and get rid of the outstanding garbage.
The second call will do (live* W1 + 0 * W2) work and reclaim nothing. In other words we have done (live * W1) work and achieved absolutely nothing.
We can model the efficiency of the collector as the amount of work needed to collect a unit of garbage; i.e. efficiency = (live * W1 + garbage * W2) / garbage. So to make the GC as efficient as possible, we need to maximize the value of garbage when we run the GC; i.e. wait until the heap is full. (And also, make the heap as big as possible. But that is a separate topic.)
If the application does not interfere (by calling System.gc()), the GC will wait until the heap is full before running, resulting in efficient collection of garbage1. But if the application forces the GC to run, the chances are that the heap won't be full, and the result will be that garbage is collected inefficiently. And the more often the application forces GC, the more inefficient the GC becomes.
Note: the above explanation glosses over the fact that a typical modern GC partitions the heap into "spaces", the GC may dynamically expand the heap, the application's working set of non-garbage objects may vary and so on. Even so, the same basic principal applies across the board to all true garbage collectors2. It is inefficient to force the GC to run.
1 - This is how the "throughput" collector works. Concurrent collectors such as CMS and G1 use different criteria to decide when to start the garbage collector.
2 - I'm also excluding memory managers that use reference counting exclusively, but no current Java implementation uses that approach ... for good reason.
Lots of people seem to be telling you not to do this. I disagree. If, after a large loading process like loading a level, you believe that:
You have a lot of objects that are unreachable and may not have been gc'ed. and
You think the user could put up with a small slowdown at this point
there is no harm in calling System.gc(). I look at it like the c/c++ inline keyword. It's just a hint to the gc that you, the developer, have decided that time/performance is not as important as it usually is and that some of it could be used reclaiming memory.
Advice to not rely on it doing anything is correct. Don't rely on it working, but giving the hint that now is an acceptable time to collect is perfectly fine. I'd rather waste time at a point in the code where it doesn't matter (loading screen) than when the user is actively interacting with the program (like during a level of a game.)
There is one time when i will force collection: when attempting to find out is a particular object leaks (either native code or large, complex callback interaction. Oh and any UI component that so much as glances at Matlab.) This should never be used in production code.
People have been doing a good job explaining why NOT to use, so I will tell you a couple situations where you should use it:
(The following comments apply to Hotspot running on Linux with the CMS collector, where I feel confident saying that System.gc() does in fact always invoke a full garbage collection).
After the initial work of starting up your application, you may be a terrible state of memory usage. Half your tenured generation could be full of garbage, meaning that you are that much closer to your first CMS. In applications where that matters, it is not a bad idea to call System.gc() to "reset" your heap to the starting state of live data.
Along the same lines as #1, if you monitor your heap usage closely, you want to have an accurate reading of what your baseline memory usage is. If the first 2 minutes of your application's uptime is all initialization, your data is going to be messed up unless you force (ahem... "suggest") the full gc up front.
You may have an application that is designed to never promote anything to the tenured generation while it is running. But maybe you need to initialize some data up-front that is not-so-huge as to automatically get moved to the tenured generation. Unless you call System.gc() after everything is set up, your data could sit in the new generation until the time comes for it to get promoted. All of a sudden your super-duper low-latency, low-GC application gets hit with a HUGE (relatively speaking, of course) latency penalty for promoting those objects during normal operations.
It is sometimes useful to have a System.gc call available in a production application for verifying the existence of a memory leak. If you know that the set of live data at time X should exist in a certain ratio to the set of live data at time Y, then it could be useful to call System.gc() a time X and time Y and compare memory usage.
This is a very bothersome question, and I feel contributes to many being opposed to Java despite how useful of a language it is.
The fact that you can't trust "System.gc" to do anything is incredibly daunting and can easily invoke "Fear, Uncertainty, Doubt" feel to the language.
In many cases, it is nice to deal with memory spikes that you cause on purpose before an important event occurs, which would cause users to think your program is badly designed/unresponsive.
Having ability to control the garbage collection would be very a great education tool, in turn improving people's understanding how the garbage collection works and how to make programs exploit it's default behavior as well as controlled behavior.
Let me review the arguments of this thread.
It is inefficient:
Often, the program may not be doing anything and you know it's not doing anything because of the way it was designed. For instance, it might be doing some kind of long wait with a large wait message box, and at the end it may as well add a call to collect garbage because the time to run it will take a really small fraction of the time of the long wait but will avoid gc from acting up in the middle of a more important operation.
It is always a bad practice and indicates broken code.
I disagree, it doesn't matter what garbage collector you have. Its' job is to track garbage and clean it.
By calling the gc during times where usage is less critical, you reduce odds of it running when your life relies on the specific code being run but instead it decides to collect garbage.
Sure, it might not behave the way you want or expect, but when you do want to call it, you know nothing is happening, and user is willing to tolerate slowness/downtime. If the System.gc works, great! If it doesn't, at least you tried. There's simply no down side unless the garbage collector has inherent side effects that do something horribly unexpected to how a garbage collector is suppose to behave if invoked manually, and this by itself causes distrust.
It is not a common use case:
It is a use case that cannot be achieved reliably, but could be if the system was designed that way. It's like making a traffic light and making it so that some/all of the traffic lights' buttons don't do anything, it makes you question why the button is there to begin with, javascript doesn't have garbage collection function so we don't scrutinize it as much for it.
The spec says that System.gc() is a hint that GC should run and the VM is free to ignore it.
what is a "hint"? what is "ignore"? a computer cannot simply take hints or ignore something, there are strict behavior paths it takes that may be dynamic that are guided by the intent of the system. A proper answer would include what the garbage collector is actually doing, at implementation level, that causes it to not perform collection when you request it. Is the feature simply a nop? Is there some kind of conditions that must me met? What are these conditions?
As it stands, Java's GC often seems like a monster that you just don't trust. You don't know when it's going to come or go, you don't know what it's going to do, how it's going to do it. I can imagine some experts having better idea of how their Garbage Collection works on per-instruction basis, but vast majority simply hopes it "just works", and having to trust an opaque-seeming algorithm to do work for you is frustrating.
There is a big gap between reading about something or being taught something, and actually seeing the implementation of it, the differences across systems, and being able to play with it without having to look at the source code. This creates confidence and feeling of mastery/understanding/control.
To summarize, there is an inherent problem with the answers "this feature might not do anything, and I won't go into details how to tell when it does do something and when it doesn't and why it won't or will, often implying that it is simply against the philosophy to try to do it, even if the intent behind it is reasonable".
It might be okay for Java GC to behave the way it does, or it might not, but to understand it, it is difficult to truly follow in which direction to go to get a comprehensive overview of what you can trust the GC to do and not to do, so it's too easy simply distrust the language, because the purpose of a language is to have controlled behavior up to philosophical extent(it's easy for a programmer, especially novices to fall into existential crisis from certain system/language behaviors) you are capable of tolerating(and if you can't, you just won't use the language until you have to), and more things you can't control for no known reason why you can't control them is inherently harmful.
Sometimes (not often!) you do truly know more about past, current and future memory usage then the run time does. This does not happen very often, and I would claim never in a web application while normal pages are being served.
Many year ago I work on a report generator, that
Had a single thread
Read the “report request” from a queue
Loaded the data needed for the report from the database
Generated the report and emailed it out.
Repeated forever, sleeping when there were no outstanding requests.
It did not reuse any data between reports and did not do any cashing.
Firstly as it was not real time and the users expected to wait for a report, a delay while the GC run was not an issue, but we needed to produce reports at a rate that was faster than they were requested.
Looking at the above outline of the process, it is clear that.
We know there would be very few live objects just after a report had been emailed out, as the next request had not started being processed yet.
It is well known that the cost of running a garbage collection cycle is depending on the number of live objects, the amount of garbage has little effect on the cost of a GC run.
That when the queue is empty there is nothing better to do then run the GC.
Therefore clearly it was well worth while doing a GC run whenever the request queue was empty; there was no downside to this.
It may be worth doing a GC run after each report is emailed, as we know this is a good time for a GC run. However if the computer had enough ram, better results would be obtained by delaying the GC run.
This behaviour was configured on a per installation bases, for some customers enabling a forced GC after each report greatly speeded up the production of reports. (I expect this was due to low memory on their server and it running lots of other processes, so hence a well time forced GC reduced paging.)
We never detected an installation that did not benefit from a forced GC run every time the work queue was empty.
But, let be clear, the above is not a common case.
These days I would be more inclined to run each report in a seperate process leaving the operating system to clear up memory rather then the garbage collector and having the custom queue manager service use mulple working processes on large servers.
GC efficiency relies on a number of heuristics. For instance, a common heuristic is that write accesses to objects usually occur on objects which were created not long ago. Another is that many objects are very short-lived (some objects will be used for a long time, but many will be discarded a few microseconds after their creation).
Calling System.gc() is like kicking the GC. It means: "all those carefully tuned parameters, those smart organizations, all the effort you just put into allocating and managing the objects such that things go smoothly, well, just drop the whole lot, and start from scratch". It may improve performance, but most of the time it just degrades performance.
To use System.gc() reliably(*) you need to know how the GC operates in all its fine details. Such details tend to change quite a bit if you use a JVM from another vendor, or the next version from the same vendor, or the same JVM but with slightly different command-line options. So it is rarely a good idea, unless you want to address a specific issue in which you control all those parameters. Hence the notion of "bad practice": that's not forbidden, the method exists, but it rarely pays off.
(*) I am talking about efficiency here. System.gc() will never break a correct Java program. It will neither conjure extra memory that the JVM could not have obtained otherwise: before throwing an OutOfMemoryError, the JVM does the job of System.gc(), even if as a last resort.
Maybe I write crappy code, but I've come to realize that clicking the trash-can icon on eclipse and netbeans IDEs is a 'good practice'.
Some of what I am about to write is simply a summarization of what has already been written in other answers, and some is new.
The question "Why is it bad practice to call System.gc()?" does not compute. It assumes that it is bad practice, while it is not. It greatly depends on what you are trying to accomplish.
The vast majority of programmers out there have no need for System.gc(), and it will never do anything useful to them in the vast majority of use cases. So, for the majority, calling it is bad practice because it will not do whatever it is that they think it will do, it will only add overhead.
However, there are a few rare cases where invoking System.gc() is actually beneficial:
When you are absolutely sure that you have some CPU time to spare now, and you want to improve the throughput of code that will run later. For example, a web server that discovers that there are no pending web requests at the moment can initiate garbage collection now, so as to reduce the chances that garbage collection will be needed during the processing of a barrage of web requests later on. (Of course this can hurt if a web request arrives during collection, but the web server could be smart about it and abandon collection if a request comes in.) Desktop GUIs are another example: on the idle event (or, more broadly, after a period of inactivity,) you can give the JVM a hint that if it has any garbage collection to do, now is better than later.
When you want to detect memory leaks. This is often done in combination with a debug-mode-only finalizer, or with the java.lang.ref.Cleaner class from Java 9 onwards. The idea is that by forcing garbage collection now, and thus discovering memory leaks now as opposed to some random point in time in the future, you can detect the memory leaks as soon as possible after they have happened, and therefore be in a better position to tell precisely which piece of code has leaked memory and why. (Incidentally, this is also one of, or perhaps the only, legitimate use cases for finalizers or the Cleaner. The practice of using finalization for recycling of unmanaged resources is flawed, despite being very widespread and even officially recommended, because it is non-deterministic. For more on this topic, read this: https://blog.michael.gr/2021/01/object-lifetime-awareness.html)
When you are measuring the performance of code, (benchmarking,) in order to reduce/minimize the chances of garbage collection occurring during the benchmark, or in order to guarantee that whatever overhead is suffered due to garbage collection during the benchmark is due to garbage generated by the code under benchmark, and not by unrelated code. A good benchmark always starts with an as thorough as possible garbage collection.
When you are measuring the memory consumption of code, in order to determine how much garbage is generated by a piece of code. The idea is to perform a full garbage collection so as to start in a clean state, run the code under measurement, obtain the heap size, then do another full garbage collection, obtain the heap size again, and take the difference. (Incidentally, the ability to temporarily suppress garbage collection while running the code under measurement would be useful here, alas, the JVM does not support that. This is deplorable.)
Note that of the above use cases, only one is in a production scenario; the rest are in testing / diagnostics scenarios.
This means that System.gc() can be quite useful under some circumstances, which in turn means that it being "only a hint" is problematic.
(For as long as the JVM is not offering some deterministic and guaranteed means of controlling garbage collection, the JVM is broken in this respect.)
Here is how you can turn System.gc() into a bit less of a hint:
private static void runGarbageCollection()
{
for( WeakReference<Object> ref = new WeakReference<>( new Object() ); ; )
{
System.gc(); //optional
Runtime.getRuntime().runFinalization(); //optional
if( ref.get() == null )
break;
Thread.yield();
}
}
This still does not guarantee that you will get a full GC, but it gets a lot closer. Specifically, it will give you some amount of garbage collection even if the -XX:DisableExplicitGC VM option has been used. (So, it truly uses System.gc() as a hint; it does not rely on it.)
Yes, calling System.gc() doesn't guarantee that it will run, it's a request to the JVM that may be ignored. From the docs:
Calling the gc method suggests that the Java Virtual Machine expend effort toward recycling unused objects
It's almost always a bad idea to call it because the automatic memory management usually knows better than you when to gc. It will do so when its internal pool of free memory is low, or if the OS requests some memory be handed back.
It might be acceptable to call System.gc() if you know that it helps. By that I mean you've thoroughly tested and measured the behaviour of both scenarios on the deployment platform, and you can show it helps. Be aware though that the gc isn't easily predictable - it may help on one run and hurt on another.
First, there is a difference between spec and reality. The spec says that System.gc() is a hint that GC should run and the VM is free to ignore it. The reality is, the VM will never ignore a call to System.gc().
Calling GC comes with a non-trivial overhead to the call and if you do this at some random point in time it's likely you'll see no reward for your efforts. On the other hand, a naturally triggered collection is very likely to recoup the costs of the call. If you have information that indicates that a GC should be run than you can make the call to System.gc() and you should see benefits. However, it's my experience that this happens only in a few edge cases as it's very unlikely that you'll have enough information to understand if and when System.gc() should be called.
One example listed here, hitting the garbage can in your IDE. If you're off to a meeting why not hit it. The overhead isn't going to affect you and heap might be cleaned up for when you get back. Do this in a production system and frequent calls to collect will bring it to a grinding halt! Even occasional calls such as those made by RMI can be disruptive to performance.
In my experience, using System.gc() is effectively a platform-specific form of optimization (where "platform" is the combination of hardware architecture, OS, JVM version and possible more runtime parameters such as RAM available), because its behaviour, while roughly predictable on a specific platform, can (and will) vary considerably between platforms.
Yes, there are situations where System.gc() will improve (perceived) performance. On example is if delays are tolerable in some parts of your app, but not in others (the game example cited above, where you want GC to happen at the start of a level, not during the level).
However, whether it will help or hurt (or do nothing) is highly dependent on the platform (as defined above).
So I think it is valid as a last-resort platform-specific optimization (i.e. if other performance optimizations are not enough). But you should never call it just because you believe it might help(without specific benchmarks), because chances are it will not.
Since objects are dynamically allocated by using the new operator,
you might be wondering how such objects are destroyed and their
memory released for later reallocation.
In some languages, such as C++, dynamically allocated objects must
be manually released by use of a delete operator.
Java takes a different approach; it handles deallocation for you
automatically.
The technique that accomplishes this is called garbage collection.
It works like this: when no references to an object exist, that object is assumed to be no longer needed, and the memory occupied by the object can be reclaimed. There is no explicit need to destroy objects as in C++.
Garbage collection only occurs sporadically (if at all) during the
execution of your program.
It will not occur simply because one or more objects exist that are
no longer used.
Furthermore, different Java run-time implementations will take
varying approaches to garbage collection, but for the most part, you
should not have to think about it while writing your programs.