Why is it bad practice to call System.gc()?

Question

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?

Fari

I think a good time to call System.gc() is when you're doing something that is a long loading process already. For example I'm working on a game and plan to call System.gc() when the game loads a new level, at the end of the loading process. The user is already waiting a bit, and the extra performance increase may be worth it; but I will also place an option in the configuration screen to disable this behavior. – Sarraute

Another case has been explained in java-monitor.com/forum/showthread.php?t=188 Where it explains how it can be bad practice to call the System.gc() – Lilianaliliane

just my two cents after reading some of the answers (and it should be noted that I am fairly new to Java, so please take my opinion with a grain of salt), but... I think the better question might be "why do you have such a big HashMap in the first place?" – Postcard

Answer 1

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"

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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.

Beauvoir

You may be able to construct some weird Rube Goldberg-esque setup where the method in which the GC is run affects the correctness of your code. Perhaps it's masking some strange threading interaction, or maybe a finalizer has a significant effect on the running of the program. I'm not entirely sure it's possible but it may be, so I figured I'd mention it. – Beauvoir

@Fari you might for example have put critical code in finalizers (which is fundamentally broken code) – Bocage

As for my direct answer to number 2, I'd say that the rule is that it's generally not worth your time. There are exceptions to every rule, though. I'm simply trying to explain the thought process. On a recent Sun JVM it probably won't cause much harm, but when you start moving to other JVMs it may not do what you expect anymore. That's why I'd in general avoid it. – Beauvoir

& @Martin: I didn't think about critical code in finalizers. It's been so long since I've written one I almost forgot about them. Thanks for reminding. – Fari

I'd like to add that there are a few corner cases where System.gc() is useful and might even be necessary. For example in UI applications on Windows it can greatly speed up the restoring-process of a Window when you call System.gc() before you minimize the Window (especially when it stays minimized for quite some time and parts of the process get swapped to disk). – Muscolo

@Steven Schlansker: It's possible: if you were silly and wrote code that used WeakReferences to hold on to objects you actually needed, and gc() added the pressure that caused them to be collected before they would under normal operation, that could break things. (Effectively adding a dependency on things being in-cache.) And I'm pretty sure you can tweak the JVM tuning parameters to ensure this case could happen. Needless to say, code that does so is broken. – Lunula

My 2cents: Having 2GB extra allocated to a process which are not in use doesn't harm performance in the long run (hibernation and first-time swap-off put aside). In fact, when a process asks for, say 1Gb, it is 'granted' instantly, but the real pages in RAM are provided only on the first attempt to access that particular page. That's called 'deferred memory allocation'. – Hypogynous

@AndrewJanke I'd say code that uses WeakReferences for objects you want to hold on to is incorrect from the start, garbage collection or not. You'd have the same issue in C++ with std::weak_ptr (though you might notice the issue in a C++ version earlier than you would in a Java version as object destruction would not be deferred like finalization usually is). – K2

@JAB: Agreed. By "silly" I basically meant "wrote buggy code", and I meant "broken" to refer to any code that was written using WeakReferences like that. – Lunula

Is it just me, or does that bug report not really make sense? The "actual" behavior described sounds like an optimal behavior.[/dead horse flogging] – Sharenshargel

You say that "Any code that depends on it for correctness is certainly broken", but what about file I/O streams that won't close properly without it, as discussed in this thread? https://mcmap.net/q/18562/-file-delete-returns-false-even-though-file-exists-file-canread-file-canwrite-file-canexecute-all-return-true – Shillelagh

@rebeccah That's a bug, so yes, I would call it 'certainly broken'. The fact that System.gc() fixes it is a workaround, not good coding practice. – Beauvoir

@Steven When you use XX:MaxHeapFreeRatio, and use either the GC algorithm like CMS or G1GC, system.gc() will actually release memory back to the OS in recent java versions 1.7+. Please see this https://mcmap.net/q/18563/-why-isn-39-t-the-allocated-heap-memory-shrinking-when-usage-is-below-maxheapfreeratio – Jehiel

Answer 2

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.

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^{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.}

Rooker

+1 Good explanation. Note however that this reasoning only applies if you care about throughput. If you want to optimize latentcy at specific points, forcing GC may make sense. E.g. (hypothetically speaking) in a game you might want to avoid delays during levels, but you don't care about delays during level load. Then it would make sense to force GC after level load. It does decrease overall throughput, but that's not what you are optimizing. – Solidify

@Solidify - what you say is true. However, running the GC between levels is a band-aid solution ... and doesn't solve the latency problem if the levels take long enough that you need to run the GC during a level anyway. A better approach is to use a concurrent (low pause) garbage collector ... if the platform supports it. – Rooker

Not entirely sure what you mean by "needs the garbage collector to run". The conditions applications need to avoid are; allocation failures that can never be satisfied and high GC overhead. Randomly making calls to System.gc() often results in high GC overhead. – Haemostasis

@Haemostasis - "Randomly making calls to System.gc() often results in high GC overhead.". I know that. So would anyone who had read and understood my Answer. – Rooker

@Haemostasis - "Not entirely sure what you mean ..." - I mean programs where the "correct" behaviour of a program depends on the GC running at a particular time; e.g. in order to execute finalizers, or to break WeakReferences or SoftReferences. It is a really bad idea to do this ... but this is what I'm talking about. See also Steven Schlansker's answer. – Rooker

@Stephen C, ahh, that wasn't clear from the original post... i.c. humm, I'm trying hard time thinking of where I might attempt this! But you've got me curious as I do like to write code that performs badly and breaks in some what sane ways. – Haemostasis

"I'm trying hard time thinking of where I might attempt this!" - Sadly people do attempt such things. – Rooker

Answer 3

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:

1. You have a lot of objects that are unreachable and may not have been gc'ed. and
2. 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.

Fae

+1 for GC while analyzing for mem leaks. Note that the information about heap usage (Runtime.freeMemory() et al.) is really only meaningful after forcing a GC, otherwise it would depend on when the system last bothered to run a GC. – Solidify

there is no harm in calling System.gc() it might require stop the world approach and this is a real harm, if happens – Intaglio

There is harm in calling the garbage collector explicitly. Calling the GC at the wrong time wastes CPU cycles. You (the programmer) do not have enough information to determine when the right time is ... but the JVM does. – Rooker

Jetty makes 2 calls to System.gc() after startup and I've rarely seen it make any difference. – Haemostasis

Well the Jetty developers have a bug. It would make a difference ... even if the difference is difficult to quantify. – Rooker

+1. @StephenC whether it's a "stop the world" gc or concurrent gc or something else, hotspot jvm doesn't have a 100% parallel gc. Therefore how much you tune the gc, stop the world occurs at one time(at best one could defer it). As such I think there are situations where you don't want a gc cycle to occur. Therefor I think it's best if you could do a gc beforehand where the performance doesn't really matter. – Thomey

@HarinduDilshan - You are conflating stop the world and full gc. (And parallel) You are correct that all HotSpot GCs stop the world at some time; e.g. to do the new generation collections. (Even for low pause collectors), But a full GC is different, that typically causes stop the world while the entire heap is collected. That's why people are concerned about it. – Rooker

Now if you know for sure that a lengthy pause really doesn't matter, calling System.gc() is OK. But in the Jetty case (for example) they probably don't know for sure. And they won't be able to quantify the benefits ... in general. The Jetty developers will be assuming that when you start up Jetty (or redeploy a webapp, or something) that the end user is going to be OK with a longer pause. But that is an assumption, and in some (edge) cases it will be a bad assumption; e.g. if fast startup times are critical. – Rooker

Answer 4

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).

1. 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.

2. 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.

3. 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.

4. 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.

Singularize

For most generational garbage collectors, objects in the new generation have to survive a certain (often configurable) number of garbage collections, so calling System.gc() once to force the promotion of objects gains you nothing. And you surely don’t want to call System.gc() eight times in a row and pray that now, the promotion has been done and the saved costs of a later promotion justify the costs of multiple full GCs. Depending on the GC algorithm, promoting a lot of objects may not even bear actual costs, as it will just re-assign the memory to the old generation or copy concurrently… – Arin

Answer 5

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.

1. 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.

2. 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.

3. 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.

4. 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.

Ordonnance

Answer 6

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.

Ladylike

Answer 7

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.

Lysozyme

+1 for mentioning that System.gc() does not prevent OutOfMemoryError. Some people believe this. – Solidify

Actually, it may prevent OutOfMemoryError because of the handling of soft references. The SoftReferences created after the last GC run don't get collected in the implementation I know. But this is implementation detail subject to change anytime and a sort of bug and nothing you should rely upon. – Abreast

Answer 8

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'.

Hipolitohipp

This may be so. But if Eclipse or NetBeans was programmed to call System.gc() periodically, you probably would find the behaviour annoying. – Rooker

Answer 9

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:

1. 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.

2. 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)

3. 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.

4. 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.)

Flatcar

Answer 10

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.

Scooter

<stroke>But also from the Javadoc: _When control returns from the method call, the virtual machine has made its best effort to recycle all discarded objects, which I see as a more imperative form of what you've posted about.</stroke> Screw that, there's a bug report about it being misleading. As of which knows better, what are the harms of hinting the JVM? – Fari

The harm is that doing collection at the wrong time can be a huge slow down. The hint you are giving is probably a bad one. As for "best effort" comment, try it and see in a tool like JConsole. Sometimes clicking the "Perform GC" button does nothing – Scooter

Sorry to disagree but calling System.gc() in OpenJDK and anything that is based on it (HP for example) always results in a garbage collection cycle. In fact that seems true of IBM's J9 implementation also – Haemostasis

@Haemostasis - Incorrect: google, and read about -XX:-DisableExplicitGC. – Rooker

Answer 11

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.

Haemostasis

"The reality is, the VM will never ignore a call to System.gc()." - Incorrect. Read about -XX:-DisableExplicitGC. – Rooker

Answer 12

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.

Solidify

Answer 13

1. 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.

2. In some languages, such as C++, dynamically allocated objects must be manually released by use of a delete operator.

3. Java takes a different approach; it handles deallocation for you automatically.
4. 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++.
5. Garbage collection only occurs sporadically (if at all) during the execution of your program.
6. It will not occur simply because one or more objects exist that are no longer used.
7. 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.

Fideicommissary