When does System gc do something

Question

I know that garbage collection is automated in Java  But I understood that if you call System gc   in your code that the JVM may or may not decide to perform garbage collection at that point  How does this work precisely  On what basis parameters exactly does the JVM decide to do  or not do  a GC when it sees System gc     Are there any examples in which case it s a good idea to put this in your code

User · Answer

You need to be very careful if you call System.gc(). Calling it can add unnecessary performance issues to your application, and it is not guaranteed to actually perform a collection. It is actually possible to disable explicit System.gc() via the java argument -XX:+DisableExplicitGC.

I'd highly recommend reading through the documents available at Java HotSpot Garbage Collection for more in depth details about garbage collection.

User · Answer

If you want to know if your System gc   is called  you can with the new Java 7 update 4 get notification when the JVM performs Garbage Collection   I am not 100  sure that the GarbageCollectorMXBean class was introduces in Java 7 update 4 though  because I couldn t find it in the release notes  but I found the information in the javaperformancetuning com site

User · Answer

The Java Language Specification does not guarantee that the JVM will start a GC when you call System gc    This is the reason of this  may or may not decide to do a GC at that point    Now  if you look at OpenJDK source code  which is the backbone of Oracle JVM  you will see that a call to System gc   does start a GC cycle  If you use another JVM  such as J9  you have to check their documentation to find out the answer  For instance  Azul s JVM has a garbage collector that runs continuously  so a call to System gc   won t do anything  Some other answer mention starting a GC in JConsole or VisualVM  Basically  these tools make a remote call to System gc     Usually  you don t want to start a garbage collection cycle from your code  as it messes up with the semantics of your application  Your application does some business stuff  the JVM takes care of memory management  You should keep those concerns separated  don t make your application do some memory management  focus on business    However  there are few cases where a call to System gc   might be understandable  Consider  for example  microbenchmarks  No-one wants to have a GC cycle to happen in the middle of a microbenchmark  So you may trigger a GC cycle between each measurement to make sure every measurement starts with an empty heap

User · Answer

There is a LOT to be said in taking the time to test out the various garbage collection settings  but as was mentioned above it usually not useful to do so   I am currently working on a project involving a memory-limited environment and a relatively large amounts of data--there are a few large pieces of data that push my environment to its limit  and even though I was able to bring memory usage down so that in theory it should work just fine  I would still get heap space errors---the verbose GC options showed me that it was trying to garbage collect  but to no avail  In the debugger  I could perform System gc   and sure enough there would be  plenty  of memory available   not a lot of extra  but enough   Consequently  The only time my application calls System gc   is when it is about to enter the segment of code where large buffers necessary for processing the data will be allocated  and a test on the free memory available indicates that I m not guaranteed to have it   In particular  I m looking at a 1gb environment where at least 300mb is occupied by static data  with the bulk of the non-static data being execution-related except when the data being processed happens to be at least 100-200 MB at the source   It s all part of an automatic data conversion process  so the data all exists for relatively short periods of time in the long run   Unfortunately  while information about the various options for tuning the garbage collector is available  it seems largely an experimental process and the lower level specifics needed to understand how to handle these specific situations are not easily obtained     All of that being said  even though I am using System gc    I still continued to tune using command line parameters and managed to improve the overall processing time of my application by a relatively significant amount  despite being unable to get over the stumbling block posed by working with the larger blocks of data    That being said  System gc   is a tool    a very unreliable tool  and if you are not careful with how you use it  you will wish that it didn t work more often than not

User · Answer

In practice  it usually decides to do a garbage collection   The answer varies depending on lots of factors  like which JVM you re running on  which mode it s in  and which garbage collection algorithm it s using     I wouldn t depend on it in your code   If the JVM is about to throw an OutOfMemoryError  calling System gc   won t stop it  because the garbage collector will attempt to free as much as it can before it goes to that extreme   The only time I ve seen it used in practice is in IDEs where it s attached to a button that a user can click  but even there it s not terribly useful

User · Answer

The only example I can think of where it makes sense to call System gc   is when profiling an application to search for possible memory leaks  I believe the profilers call this method just before taking a memory snapshot

User · Answer

Garbage Collection is good in Java  if we are executing Software coded in java in Desktop laptop server  You can call System gc   or Runtime getRuntime   gc   in Java   Just note that none of those calls are guaranteed to do anything  They are just a suggestion for the jvm to run the Garbage Collector  It s up the the JVM whether it runs the GC or not  So  short answer  we don t know when it runs  Longer answer  JVM would run gc if it has time for that   I believe  the same applies for Android  However  this might slow down your system

User · Answer

In practice  it usually decides to do a garbage collection   The answer varies depending on lots of factors  like which JVM you re running on  which mode it s in  and which garbage collection algorithm it s using     I wouldn t depend on it in your code   If the JVM is about to throw an OutOfMemoryError  calling System gc   won t stop it  because the garbage collector will attempt to free as much as it can before it goes to that extreme   The only time I ve seen it used in practice is in IDEs where it s attached to a button that a user can click  but even there it s not terribly useful

User · Answer

Most JVMs will kick off a GC  depending on the -XX DiableExplicitGC and -XX  ExplicitGCInvokesConcurrent switch   But the specification is just less well defined in order to allow better implementations later on   The spec needs clarification  Bug  6668279   spec  System gc   should indicate that we don t recommend use and don t guarantee behaviour  Internally the gc method is used by RMI and NIO  and they require synchronous execution  which  this is currently in discussion   Bug  5025281  Allow System gc   to trigger concurrent  not stop-the-world  full collections

User · Answer

Accroding to Thinking in Java by Bruce Eckel  one use case for explicit System gc   call is when you want to force finalization  i e  the call to finalize method

User · Answer

I can t think of a specific example when it is good to run explicit GC    In general  running explicit GC can actually cause more harm than good  because an explicit gc will trigger a full collection  which takes significantly longer as it goes through every object   If this explicit gc ends up being called repeatedly it could easily lead to a slow application as a lot of time is spent running full GCs   Alternatively if going over the heap with a heap analyzer and you suspect a library component to be calling explicit GC s you can turn it off adding  gc -XX  DisableExplicitGC  to the JVM parameters

User · Answer

while system gc works it will stop the world all respones are stopped so garbage collector can scan every object to check if it is needed deleted  if the application is a web project  all request are stopped until gc finishes and this will cause your web project can not work in a monent

User · Answer

while system gc works it will stop the world all respones are stopped so garbage collector can scan every object to check if it is needed deleted  if the application is a web project  all request are stopped until gc finishes and this will cause your web project can not work in a monent

User · Answer

You have no control over GC in java -- the VM decides   I ve never run across a case where System gc   is needed   Since a System gc   call simply SUGGESTS that the VM do a garbage collection and it also does a FULL garbage collection  old and new generations in a multi-generational heap   then it can actually cause MORE cpu cycles to be consumed than necessary   In some cases  it may make sense to suggest to the VM that it do a full collection NOW as you may know the application will be sitting idle for the next few minutes before heavy lifting occurs   For example  right after the initialization of a lot of temporary object during application startup  i e   I just cached a TON of info  and I know I won t be getting much activity for a minute or so    Think of an IDE such as eclipse starting up -- it does a lot to initialize  so perhaps immediately after initialization it makes sense to do a full gc at that point

User · Answer

I can t think of a specific example when it is good to run explicit GC    In general  running explicit GC can actually cause more harm than good  because an explicit gc will trigger a full collection  which takes significantly longer as it goes through every object   If this explicit gc ends up being called repeatedly it could easily lead to a slow application as a lot of time is spent running full GCs   Alternatively if going over the heap with a heap analyzer and you suspect a library component to be calling explicit GC s you can turn it off adding  gc -XX  DisableExplicitGC  to the JVM parameters

User · Answer

Normally  the VM would do a garbage collection automatically before throwing an OutOfMemoryException  so adding an explicit call shouldn t help except in that it perhaps moves the performance hit to an earlier moment in time   However  I think I encountered a case where it might be relevant  I m not sure though  as I have yet to test whether it has any effect   When you memory-map a file  I believe the map   call throws an IOException when a large enough block of memory is not available  A garbage collection just before the map   file might help prevent that  I think  What do you think

User · Answer

You need to be very careful if you call System.gc(). Calling it can add unnecessary performance issues to your application, and it is not guaranteed to actually perform a collection. It is actually possible to disable explicit System.gc() via the java argument -XX:+DisableExplicitGC.

I'd highly recommend reading through the documents available at Java HotSpot Garbage Collection for more in depth details about garbage collection.

User · Answer

You have no control over GC in java -- the VM decides   I ve never run across a case where System gc   is needed   Since a System gc   call simply SUGGESTS that the VM do a garbage collection and it also does a FULL garbage collection  old and new generations in a multi-generational heap   then it can actually cause MORE cpu cycles to be consumed than necessary   In some cases  it may make sense to suggest to the VM that it do a full collection NOW as you may know the application will be sitting idle for the next few minutes before heavy lifting occurs   For example  right after the initialization of a lot of temporary object during application startup  i e   I just cached a TON of info  and I know I won t be getting much activity for a minute or so    Think of an IDE such as eclipse starting up -- it does a lot to initialize  so perhaps immediately after initialization it makes sense to do a full gc at that point

User · Answer

Most JVMs will kick off a GC  depending on the -XX DiableExplicitGC and -XX  ExplicitGCInvokesConcurrent switch   But the specification is just less well defined in order to allow better implementations later on   The spec needs clarification  Bug  6668279   spec  System gc   should indicate that we don t recommend use and don t guarantee behaviour  Internally the gc method is used by RMI and NIO  and they require synchronous execution  which  this is currently in discussion   Bug  5025281  Allow System gc   to trigger concurrent  not stop-the-world  full collections

User · Answer

Garbage Collection is good in Java  if we are executing Software coded in java in Desktop laptop server  You can call System gc   or Runtime getRuntime   gc   in Java   Just note that none of those calls are guaranteed to do anything  They are just a suggestion for the jvm to run the Garbage Collector  It s up the the JVM whether it runs the GC or not  So  short answer  we don t know when it runs  Longer answer  JVM would run gc if it has time for that   I believe  the same applies for Android  However  this might slow down your system

User · Answer

The only example I can think of where it makes sense to call System gc   is when profiling an application to search for possible memory leaks  I believe the profilers call this method just before taking a memory snapshot

User · Answer

I can t think of a specific example when it is good to run explicit GC    In general  running explicit GC can actually cause more harm than good  because an explicit gc will trigger a full collection  which takes significantly longer as it goes through every object   If this explicit gc ends up being called repeatedly it could easily lead to a slow application as a lot of time is spent running full GCs   Alternatively if going over the heap with a heap analyzer and you suspect a library component to be calling explicit GC s you can turn it off adding  gc -XX  DisableExplicitGC  to the JVM parameters

User · Answer

If you use direct memory buffers  the JVM doesn t run the GC for you even if you are running low on direct memory   If you call ByteBuffer allocateDirect   and you get an OutOfMemoryError you can find this call is fine after triggering a GC manually

User · Answer

System gc   is implemented by the VM  and what it does is implementation specific  The implementer could simply return and do nothing  for instance    As for when to issue a manual collect  the only time when you may want to do this is when you abandon a large collection containing loads of smaller collections--a  Map lt String  lt LinkedList gt  gt    for instance--and you want to try and take the perf hit then and there  but for the most part  you shouldn t worry about it  The GC knows better than you--sadly--most of the time

User · Answer

we can never force garbage collection  System gc is only suggesting vm for garbage collection  however  really what time the mechanism runs  nobody knows  this is as stated by JSR specifications

User · Answer

You have no control over GC in java -- the VM decides   I ve never run across a case where System gc   is needed   Since a System gc   call simply SUGGESTS that the VM do a garbage collection and it also does a FULL garbage collection  old and new generations in a multi-generational heap   then it can actually cause MORE cpu cycles to be consumed than necessary   In some cases  it may make sense to suggest to the VM that it do a full collection NOW as you may know the application will be sitting idle for the next few minutes before heavy lifting occurs   For example  right after the initialization of a lot of temporary object during application startup  i e   I just cached a TON of info  and I know I won t be getting much activity for a minute or so    Think of an IDE such as eclipse starting up -- it does a lot to initialize  so perhaps immediately after initialization it makes sense to do a full gc at that point

User · Answer

Normally  the VM would do a garbage collection automatically before throwing an OutOfMemoryException  so adding an explicit call shouldn t help except in that it perhaps moves the performance hit to an earlier moment in time   However  I think I encountered a case where it might be relevant  I m not sure though  as I have yet to test whether it has any effect   When you memory-map a file  I believe the map   call throws an IOException when a large enough block of memory is not available  A garbage collection just before the map   file might help prevent that  I think  What do you think

User · Answer

In practice  it usually decides to do a garbage collection   The answer varies depending on lots of factors  like which JVM you re running on  which mode it s in  and which garbage collection algorithm it s using     I wouldn t depend on it in your code   If the JVM is about to throw an OutOfMemoryError  calling System gc   won t stop it  because the garbage collector will attempt to free as much as it can before it goes to that extreme   The only time I ve seen it used in practice is in IDEs where it s attached to a button that a user can click  but even there it s not terribly useful

User · Answer

System gc   is implemented by the VM  and what it does is implementation specific  The implementer could simply return and do nothing  for instance    As for when to issue a manual collect  the only time when you may want to do this is when you abandon a large collection containing loads of smaller collections--a  Map lt String  lt LinkedList gt  gt    for instance--and you want to try and take the perf hit then and there  but for the most part  you shouldn t worry about it  The GC knows better than you--sadly--most of the time

User · Answer

Most JVMs will kick off a GC  depending on the -XX DiableExplicitGC and -XX  ExplicitGCInvokesConcurrent switch   But the specification is just less well defined in order to allow better implementations later on   The spec needs clarification  Bug  6668279   spec  System gc   should indicate that we don t recommend use and don t guarantee behaviour  Internally the gc method is used by RMI and NIO  and they require synchronous execution  which  this is currently in discussion   Bug  5025281  Allow System gc   to trigger concurrent  not stop-the-world  full collections

User · Answer

There is a LOT to be said in taking the time to test out the various garbage collection settings  but as was mentioned above it usually not useful to do so   I am currently working on a project involving a memory-limited environment and a relatively large amounts of data--there are a few large pieces of data that push my environment to its limit  and even though I was able to bring memory usage down so that in theory it should work just fine  I would still get heap space errors---the verbose GC options showed me that it was trying to garbage collect  but to no avail  In the debugger  I could perform System gc   and sure enough there would be  plenty  of memory available   not a lot of extra  but enough   Consequently  The only time my application calls System gc   is when it is about to enter the segment of code where large buffers necessary for processing the data will be allocated  and a test on the free memory available indicates that I m not guaranteed to have it   In particular  I m looking at a 1gb environment where at least 300mb is occupied by static data  with the bulk of the non-static data being execution-related except when the data being processed happens to be at least 100-200 MB at the source   It s all part of an automatic data conversion process  so the data all exists for relatively short periods of time in the long run   Unfortunately  while information about the various options for tuning the garbage collector is available  it seems largely an experimental process and the lower level specifics needed to understand how to handle these specific situations are not easily obtained     All of that being said  even though I am using System gc    I still continued to tune using command line parameters and managed to improve the overall processing time of my application by a relatively significant amount  despite being unable to get over the stumbling block posed by working with the larger blocks of data    That being said  System gc   is a tool    a very unreliable tool  and if you are not careful with how you use it  you will wish that it didn t work more often than not

User · Answer

Accroding to Thinking in Java by Bruce Eckel  one use case for explicit System gc   call is when you want to force finalization  i e  the call to finalize method

User · Answer

The only example I can think of where it makes sense to call System gc   is when profiling an application to search for possible memory leaks  I believe the profilers call this method just before taking a memory snapshot

User · Answer

If you use direct memory buffers  the JVM doesn t run the GC for you even if you are running low on direct memory   If you call ByteBuffer allocateDirect   and you get an OutOfMemoryError you can find this call is fine after triggering a GC manually

User · Answer

System gc   is implemented by the VM  and what it does is implementation specific  The implementer could simply return and do nothing  for instance    As for when to issue a manual collect  the only time when you may want to do this is when you abandon a large collection containing loads of smaller collections--a  Map lt String  lt LinkedList gt  gt    for instance--and you want to try and take the perf hit then and there  but for the most part  you shouldn t worry about it  The GC knows better than you--sadly--most of the time

User · Answer

You need to be very careful if you call System.gc(). Calling it can add unnecessary performance issues to your application, and it is not guaranteed to actually perform a collection. It is actually possible to disable explicit System.gc() via the java argument -XX:+DisableExplicitGC.

I'd highly recommend reading through the documents available at Java HotSpot Garbage Collection for more in depth details about garbage collection.

User · Answer

If you want to know if your System gc   is called  you can with the new Java 7 update 4 get notification when the JVM performs Garbage Collection   I am not 100  sure that the GarbageCollectorMXBean class was introduces in Java 7 update 4 though  because I couldn t find it in the release notes  but I found the information in the javaperformancetuning com site

User · Answer

Most JVMs will kick off a GC  depending on the -XX DiableExplicitGC and -XX  ExplicitGCInvokesConcurrent switch   But the specification is just less well defined in order to allow better implementations later on   The spec needs clarification  Bug  6668279   spec  System gc   should indicate that we don t recommend use and don t guarantee behaviour  Internally the gc method is used by RMI and NIO  and they require synchronous execution  which  this is currently in discussion   Bug  5025281  Allow System gc   to trigger concurrent  not stop-the-world  full collections

User · Answer

we can never force garbage collection  System gc is only suggesting vm for garbage collection  however  really what time the mechanism runs  nobody knows  this is as stated by JSR specifications

User · Answer

The only example I can think of where it makes sense to call System gc   is when profiling an application to search for possible memory leaks  I believe the profilers call this method just before taking a memory snapshot

User · Answer

I can t think of a specific example when it is good to run explicit GC    In general  running explicit GC can actually cause more harm than good  because an explicit gc will trigger a full collection  which takes significantly longer as it goes through every object   If this explicit gc ends up being called repeatedly it could easily lead to a slow application as a lot of time is spent running full GCs   Alternatively if going over the heap with a heap analyzer and you suspect a library component to be calling explicit GC s you can turn it off adding  gc -XX  DisableExplicitGC  to the JVM parameters

User · Answer

System gc   is implemented by the VM  and what it does is implementation specific  The implementer could simply return and do nothing  for instance    As for when to issue a manual collect  the only time when you may want to do this is when you abandon a large collection containing loads of smaller collections--a  Map lt String  lt LinkedList gt  gt    for instance--and you want to try and take the perf hit then and there  but for the most part  you shouldn t worry about it  The GC knows better than you--sadly--most of the time

User · Answer

You need to be very careful if you call System.gc(). Calling it can add unnecessary performance issues to your application, and it is not guaranteed to actually perform a collection. It is actually possible to disable explicit System.gc() via the java argument -XX:+DisableExplicitGC.

I'd highly recommend reading through the documents available at Java HotSpot Garbage Collection for more in depth details about garbage collection.

User · Answer

In practice  it usually decides to do a garbage collection   The answer varies depending on lots of factors  like which JVM you re running on  which mode it s in  and which garbage collection algorithm it s using     I wouldn t depend on it in your code   If the JVM is about to throw an OutOfMemoryError  calling System gc   won t stop it  because the garbage collector will attempt to free as much as it can before it goes to that extreme   The only time I ve seen it used in practice is in IDEs where it s attached to a button that a user can click  but even there it s not terribly useful

User · Answer

You have no control over GC in java -- the VM decides   I ve never run across a case where System gc   is needed   Since a System gc   call simply SUGGESTS that the VM do a garbage collection and it also does a FULL garbage collection  old and new generations in a multi-generational heap   then it can actually cause MORE cpu cycles to be consumed than necessary   In some cases  it may make sense to suggest to the VM that it do a full collection NOW as you may know the application will be sitting idle for the next few minutes before heavy lifting occurs   For example  right after the initialization of a lot of temporary object during application startup  i e   I just cached a TON of info  and I know I won t be getting much activity for a minute or so    Think of an IDE such as eclipse starting up -- it does a lot to initialize  so perhaps immediately after initialization it makes sense to do a full gc at that point

User · Answer

The Java Language Specification does not guarantee that the JVM will start a GC when you call System gc    This is the reason of this  may or may not decide to do a GC at that point    Now  if you look at OpenJDK source code  which is the backbone of Oracle JVM  you will see that a call to System gc   does start a GC cycle  If you use another JVM  such as J9  you have to check their documentation to find out the answer  For instance  Azul s JVM has a garbage collector that runs continuously  so a call to System gc   won t do anything  Some other answer mention starting a GC in JConsole or VisualVM  Basically  these tools make a remote call to System gc     Usually  you don t want to start a garbage collection cycle from your code  as it messes up with the semantics of your application  Your application does some business stuff  the JVM takes care of memory management  You should keep those concerns separated  don t make your application do some memory management  focus on business    However  there are few cases where a call to System gc   might be understandable  Consider  for example  microbenchmarks  No-one wants to have a GC cycle to happen in the middle of a microbenchmark  So you may trigger a GC cycle between each measurement to make sure every measurement starts with an empty heap

[java] When does System.gc() do something?

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