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Posted to users@tomcat.apache.org by Bill Clarke-Fields <wf...@dadebehring.com> on 2007/10/08 18:57:14 UTC
Very Long Full GC after Inactivity
Hi,
We are running an application on Tomcat 5.0.28 with Java 1.4.2. The usage
of the application is very cyclical. It is used heavily during the day, and
lightly at night. During peak daytime hours, a full garbage collection
takes less than 2 seconds, which is fine. However, after a long period of
inactivity in the evening, a full garbage collection will be triggered and
take a very long time. Sometimes over a minute!
Is there any way to avoid this long garbage collection? Why does it take so
long? (Maybe because it is filling up with many small objects over time
rather than a few larger objects?)
Thanks for your help.
-Bill
p.s. Upgrading to a newer version of Tomcat or Java is unfortunately not an
option at this point.
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RE: Very Long Full GC after Inactivity
Posted by "Caldarale, Charles R" <Ch...@unisys.com>.
> From: Bill Clarke-Fields [mailto:wfields@dadebehring.com]
> Subject: RE: Very Long Full GC after Inactivity
>
> I'm just throwing ideas out now, but would it help to set a
> lower value for the -Xms parameter?
Probably not. For best throughput, you generally want -Xms and -Xmx to
be the same, and as large as your environment allows.
> Would the JVM shrink the heap size down when usage is
> low? (I'm guessing not.)
It does, but somewhat erratically. You really need to get a profiler
running.
> Or, is there a way to schedule (or force) full garbage collections?
Not guaranteed. You can call System.gc(), but there's no requirement
that the JVM will honor the call (it usually does, though). If you
install Lambda Probe (www.lambdaprobe.org), the System Information tab
has a Force Garbage Collection link you can try. On the same page,
Lambda Probe also has a Memory utilization button, but I don't remember
if it works with your somewhat old versions of Tomcat and JVM.
- Chuck
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Re: Very Long Full GC after Inactivity
Posted by Konstantin Kolinko <kn...@gmail.com>.
Just a hypothesis.
During these off-peak hours some other applications are actively running
(maintenance etc.), and as java is not intensively used, its memory is swapped
to the hard disks.
Thus, when it awakens and does the GC, it takes so much time to wake up
the memory walking through it to collect the garbage.
What is memory consumption at night time? Are there memory-intensive
tasks being performed at that time?
May be restarting the tomcat can be scheduled after all those are
done? (Or a forced GC, or filling memory with a bunch of dummy
objects, up to Runtime.freeMemory()?)
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Re: Very Long Full GC after Inactivity
Posted by Christopher Schultz <ch...@christopherschultz.net>.
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Bill,
Bill Clarke-Fields wrote:
> Or, is there a way to schedule (or force) full garbage collections?
Not really. You can issue a call to Runtime.gc(), but it's not
guaranteed to do anything at all. My experience has been that the JVM
does respect your request for a GC, even if it does take a few seconds
to kick-in.
Do you know what causes these 1-minute-long GCs to kick-off in the first
place?
I'm asking because you might be able to his a URL that would kick-start
the full GC at, say, 5 in the morning just before your real customers
got there. It' not perfect, but it might help.
- -chris
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RE: Very Long Full GC after Inactivity
Posted by Bill Clarke-Fields <wf...@dadebehring.com>.
I'm just throwing ideas out now, but would it help to set a lower value for
the -Xms parameter? Would the JVM shrink the heap size down when usage is
low? (I'm guessing not.)
Or, is there a way to schedule (or force) full garbage collections?
Thanks,
-Bill
Caldarale, Charles R wrote:
>
>> From: Bill Clarke-Fields [mailto:wfields@dadebehring.com]
>> Subject: RE: Very Long Full GC after Inactivity
>>
>> The problem is that off-hours usage is very different, and
>> leads to these very long full GCs.
>
> GC pause time is largely dependent on the number of live objects
> existing at the time of the GC, so yes, it could be an application
> problem. A profiler will help determine what these objects are. It's
> definitely unfortunate you can't upgrade, since that precludes you from
> running JConsole.
>
> - Chuck
>
>
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RE: Very Long Full GC after Inactivity
Posted by "Caldarale, Charles R" <Ch...@unisys.com>.
> From: Bill Clarke-Fields [mailto:wfields@dadebehring.com]
> Subject: RE: Very Long Full GC after Inactivity
>
> The problem is that off-hours usage is very different, and
> leads to these very long full GCs.
GC pause time is largely dependent on the number of live objects
existing at the time of the GC, so yes, it could be an application
problem. A profiler will help determine what these objects are. It's
definitely unfortunate you can't upgrade, since that precludes you from
running JConsole.
- Chuck
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RE: Very Long Full GC after Inactivity
Posted by Bill Clarke-Fields <wf...@dadebehring.com>.
Charles,
I have tuned the heap size based on what is ideal during peak hours. The
problem is that off-hours usage is very different, and leads to these very
long full GCs.
Does it sound like an application problem? I have not done any analysis to
determine what kind of objects are hanging around. This is a third party
application, so there is really not much we can do on that front anyway.
Again, unfortunately upgrading is not an option for us at this point.
Thanks,
-Bill
Caldarale, Charles R wrote:
>
>> From: David Delbecq [mailto:delbd+jakarta@oma.be]
>> Subject: Re: Very Long Full GC after Inactivity
>>
>> check the jvm runs in "server" mode (check there is a '-server'
>> parameter given to jvm startup). Server and client mode have different
>> GC mecanisms.
>
> That's not correct; the set of GC algorithms available for server and
> client are identical, but the default heap sizes are different.
>
> For initial problem analysis, turn on -verbose:gc if you haven't
> already, and use JConsole to look at the heap usage. You may need a
> profiler to find out just what objects are hanging around and causing
> the long pause.
>
> You might want to try running with a later JVM - a 1.5 or 6 version
> should give you reduced pause times as well as better performance
> overall.
>
> - Chuck
>
>
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RE: Very Long Full GC after Inactivity
Posted by "Caldarale, Charles R" <Ch...@unisys.com>.
> From: David Delbecq [mailto:delbd+jakarta@oma.be]
> Subject: Re: Very Long Full GC after Inactivity
>
> check the jvm runs in "server" mode (check there is a '-server'
> parameter given to jvm startup). Server and client mode have different
> GC mecanisms.
That's not correct; the set of GC algorithms available for server and
client are identical, but the default heap sizes are different.
For initial problem analysis, turn on -verbose:gc if you haven't
already, and use JConsole to look at the heap usage. You may need a
profiler to find out just what objects are hanging around and causing
the long pause.
You might want to try running with a later JVM - a 1.5 or 6 version
should give you reduced pause times as well as better performance
overall.
- Chuck
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RE: Very Long Full GC after Inactivity
Posted by "Caldarale, Charles R" <Ch...@unisys.com>.
> From: Bill Clarke-Fields [mailto:wfields@dadebehring.com]
> Subject: Re: Very Long Full GC after Inactivity
>
> Tomcat is running as a Windows service, and the "jvm" startup
> parameter is set to: "C:\j2sdk1.4.2_11\jre\bin\server\jvm.dll".
> Do I also need to pass a "-server" parameter?
No, there are no such parameters when running as a service. Your
selection of the correct .dll guarantees server mode, which seems
appropriate for your environment.
- Chuck
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Re: Very Long Full GC after Inactivity
Posted by Bill Clarke-Fields <wf...@dadebehring.com>.
David,
Tomcat is running as a Windows service, and the "jvm" startup parameter is
set to: "C:\j2sdk1.4.2_11\jre\bin\server\jvm.dll". Do I also need to pass a
"-server" parameter?
Thanks,
-Bill
David Delbecq-2 wrote:
>
> Hi,
>
> check the jvm runs in "server" mode (check there is a '-server'
> parameter given to jvm startup). Server and client mode have different
> GC mecanisms.
> Bill Clarke-Fields a écrit :
>> Hi,
>> We are running an application on Tomcat 5.0.28 with Java 1.4.2. The
>> usage
>> of the application is very cyclical. It is used heavily during the day,
>> and
>> lightly at night. During peak daytime hours, a full garbage collection
>> takes less than 2 seconds, which is fine. However, after a long period
>> of
>> inactivity in the evening, a full garbage collection will be triggered
>> and
>> take a very long time. Sometimes over a minute!
>>
>> Is there any way to avoid this long garbage collection? Why does it take
>> so
>> long? (Maybe because it is filling up with many small objects over time
>> rather than a few larger objects?)
>>
>> Thanks for your help.
>> -Bill
>>
>> p.s. Upgrading to a newer version of Tomcat or Java is unfortunately not
>> an
>> option at this point.
>>
>
>
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Re: Very Long Full GC after Inactivity
Posted by David Delbecq <de...@oma.be>.
Hi,
check the jvm runs in "server" mode (check there is a '-server'
parameter given to jvm startup). Server and client mode have different
GC mecanisms.
Bill Clarke-Fields a écrit :
> Hi,
> We are running an application on Tomcat 5.0.28 with Java 1.4.2. The usage
> of the application is very cyclical. It is used heavily during the day, and
> lightly at night. During peak daytime hours, a full garbage collection
> takes less than 2 seconds, which is fine. However, after a long period of
> inactivity in the evening, a full garbage collection will be triggered and
> take a very long time. Sometimes over a minute!
>
> Is there any way to avoid this long garbage collection? Why does it take so
> long? (Maybe because it is filling up with many small objects over time
> rather than a few larger objects?)
>
> Thanks for your help.
> -Bill
>
> p.s. Upgrading to a newer version of Tomcat or Java is unfortunately not an
> option at this point.
>
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RE: Very Long Full GC after Inactivity
Posted by Peter Crowther <Pe...@melandra.com>.
> From: Christopher Schultz [mailto:chris@christopherschultz.net]
> My experience with JNI is that you have to pin objects one at a time.
To be clear: I wasn't talking about pinning Java objects to particular
addresses in memory, instead pinning the entire process address space
into memory using Win32 API calls.
> I'd imagine that most OSs have functions to /actually/ pin data into
> memory (making is non-pagable).
Exactly.
> In order to do that, you'd have to
> Java-pin the object, then OS-pin the memory.
Why Java-pin anything? (assuming you use OS-pin the entire Java heap,
which I accept may be a non-trivial exercise)
> I actually favor your "cheap way" since it's super simple and
> likely to work just as well ;)
Well, me too - it involves the least disruption. KISS says it's the one
to go for if it can be made to work.
- Peter
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RE: Very Long Full GC after Inactivity
Posted by Peter Crowther <Pe...@melandra.com>.
> From: Caldarale, Charles R [mailto:Chuck.Caldarale@unisys.com]
> Peter's not talking about the JNI pin function, which only prevents
> objects from moving around in the JVM heap, not in RAM. He's
> referring
> to nailing the entire heap into RAM, which is an OS call requiring
> privileges; operations of GC within the JVM would be unaffected.
"Nailing" is certainly a good term to use - it's the 6 inch blunt
instrument, as opposed to anything as delicate as a pin :-).
> It
> would be a simple thing to do, but would really foul up the
> workings of
> the OS memory manager. (Been there, done that, strongly
> discourage it.)
I was rather hoping you'd have some experience, Chuck. Thanks for
sharing it - I suspected it Wasn't A Smart Idea, but it's good to be
told so by someone who's had to clear up afterwards!
- Peter
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RE: Very Long Full GC after Inactivity
Posted by "Caldarale, Charles R" <Ch...@unisys.com>.
> From: Christopher Schultz [mailto:chris@christopherschultz.net]
> Subject: Re: Very Long Full GC after Inactivity
>
> My experience with JNI is that you have to pin objects one at a time.
Peter's not talking about the JNI pin function, which only prevents
objects from moving around in the JVM heap, not in RAM. He's referring
to nailing the entire heap into RAM, which is an OS call requiring
privileges; operations of GC within the JVM would be unaffected. It
would be a simple thing to do, but would really foul up the workings of
the OS memory manager. (Been there, done that, strongly discourage it.)
- Chuck
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Re: Very Long Full GC after Inactivity
Posted by Christopher Schultz <ch...@christopherschultz.net>.
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Peter,
Peter Crowther wrote:
> The messy way: Run Tomcat as an admin (ick!) and make a JNI call to
> some native code that pins Tomcat's address space into RAM.
Messy indeed.
> I have not tried this, don't know whether there's a "correct" JVM way
> of doing it, and have *no* idea how it might interact with Sun's
> address space management! It's something to try very, very
> cautiously - this might bite you in unexpected ways.
My experience with JNI is that you have to pin objects one at a time.
Since Tomcat manages millions of objects, I'm not sure how you'd do this
effectively. Also, I believe that pinning an object does more than make
is non-relocatable within the heap (see below); I think that if you
pinned objects indefinitely, Bad Things would probably happen. One Bad
Thing that will probably happen is that objects will never be tenured,
and you'll fill up other parts of your heap with objects that ought to
be promoted to more long-lasting areas of the heap.
Finally, pinning an object has no bearing on its "pagability"... it just
means that a native pointer can be used to access it, and the memory
address where the object can be found won't change until it's unpinned.
The memory can still be swapped to disk, as long as the memory address
for that object does not change. Heck, it can even be relocate in
physical memory of the OS wants to do it (which it probably will do
during paging), as long as the process can always use that same pointer
to access the object.
I'd imagine that most OSs have functions to /actually/ pin data into
memory (making is non-pagable). In order to do that, you'd have to
Java-pin the object, then OS-pin the memory. I think that's going to
cause lots more problems than a long wait for the first request of the
day ;)
I actually favor your "cheap way" since it's super simple and likely to
work just as well ;)
- -chris
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RE: Very Long Full GC after Inactivity
Posted by Peter Crowther <Pe...@melandra.com>.
> From: Bill Clarke-Fields [mailto:wfields@dadebehring.com]
> If it turns out to be so, is there
> anything I can do to force it to not page out the JVM heap?
There's cheap and there's messy :-).
The cheap way: Set up a job that runs overnight that exercises your app.
It should keep the critical files paged in.
The messy way: Run Tomcat as an admin (ick!) and make a JNI call to some
native code that pins Tomcat's address space into RAM. I have not tried
this, don't know whether there's a "correct" JVM way of doing it, and
have *no* idea how it might interact with Sun's address space
management! It's something to try very, very cautiously - this might
bite you in unexpected ways.
The cheap *and* messy way: Use Task Manager to boost Tomcat's priority
to realtime. I *think* that pins the process' address space into RAM,
although check as I'm stating this from memory and my memory is erratic
these days. Things will get very unpleasant if Tomcat manages to take
100% CPU on all your cores at once, as a Realtime process is running at
higher priority than the UI so you'd have no way to interrupt it. On
modern multi-core processors, that's less of an issue - set the Affinity
mask for Tomcat *before* raising the priority so there's at least one
core it's not allowed to run on, and you'll be able to regain control.
As I say, not sure this will work, but it's an interesting tactic to try
:-).
Side note: Does your app also use a database? On which machine? Is
your database server also being paged out, adding to the slowness? If
it's SQL Server, it'll be requested to drop cache pages instead as the
system comes under memory pressure, so the heap will quietly shrink...
- Peter
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RE: Very Long Full GC after Inactivity
Posted by Bill Clarke-Fields <wf...@dadebehring.com>.
Peter,
Thanks! This is a very good idea. I know that there are some virus scans
and other processes running at night. I will do some monitoring to
determine if paging is the problem. If it turns out to be so, is there
anything I can do to force it to not page out the JVM heap?
Thanks again,
-Bill
Peter Crowther wrote:
>
>> From: Bill Clarke-Fields [mailto:wfields@dadebehring.com]
>> We are running an application on Tomcat 5.0.28 with Java
>> 1.4.2. The usage
>> of the application is very cyclical. It is used heavily
>> during the day, and
>> lightly at night. During peak daytime hours, a full garbage
>> collection
>> takes less than 2 seconds, which is fine. However, after a
>> long period of
>> inactivity in the evening, a full garbage collection will be
>> triggered and
>> take a very long time. Sometimes over a minute!
>
> I suspect the app is being paged out overnight by other jobs. Overnight
> virus scans will do this on Windows, for example, as the disk cache
> expands to fill available memory.
>
> You could test this by running Performance Monitor (Windows) or your
> preferred logging tool (UNIX) overnight in log mode and looking for high
> page-in and disk I/O values around the time of the full GC. For
> preference, use the Process counters on Windows, as they'll tell you for
> sure whether it's the Java process that's causing the paging traffic.
>
> - Peter
>
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RE: Very Long Full GC after Inactivity
Posted by Peter Crowther <Pe...@melandra.com>.
> From: Bill Clarke-Fields [mailto:wfields@dadebehring.com]
> We are running an application on Tomcat 5.0.28 with Java
> 1.4.2. The usage
> of the application is very cyclical. It is used heavily
> during the day, and
> lightly at night. During peak daytime hours, a full garbage
> collection
> takes less than 2 seconds, which is fine. However, after a
> long period of
> inactivity in the evening, a full garbage collection will be
> triggered and
> take a very long time. Sometimes over a minute!
I suspect the app is being paged out overnight by other jobs. Overnight
virus scans will do this on Windows, for example, as the disk cache
expands to fill available memory.
You could test this by running Performance Monitor (Windows) or your
preferred logging tool (UNIX) overnight in log mode and looking for high
page-in and disk I/O values around the time of the full GC. For
preference, use the Process counters on Windows, as they'll tell you for
sure whether it's the Java process that's causing the paging traffic.
- Peter
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Re: Very Long Full GC after Inactivity
Posted by Filip Hanik - Dev Lists <de...@hanik.com>.
garbage collection is a reactive process, ie, it wont kick in until it
really needs to.
and the time it takes for the GC, depends on your java heap size, -Xmx,
in JDK 1.5 you can specify GC goals like
-XX:MaxGCMinorPauseMillis=<value>
-XX:MaxGCPauseMillis=<value>
to tell the JVM to try to align its GC to meet these goals, however,
with JDK 1.4.2 you could try to make your heap smaller or to put in
manual System.gc() at a periodic interval,
neither of these options is what I like, I would just upgrade the JDK,
even though you said its not an option, it would be your best bet
for a list of all your jvm options at hand, please take a look at
http://blogs.sun.com/watt/resource/jvm-options-list.html
Filip
Bill Clarke-Fields wrote:
> Hi,
> We are running an application on Tomcat 5.0.28 with Java 1.4.2. The usage
> of the application is very cyclical. It is used heavily during the day, and
> lightly at night. During peak daytime hours, a full garbage collection
> takes less than 2 seconds, which is fine. However, after a long period of
> inactivity in the evening, a full garbage collection will be triggered and
> take a very long time. Sometimes over a minute!
>
> Is there any way to avoid this long garbage collection? Why does it take so
> long? (Maybe because it is filling up with many small objects over time
> rather than a few larger objects?)
>
> Thanks for your help.
> -Bill
>
> p.s. Upgrading to a newer version of Tomcat or Java is unfortunately not an
> option at this point.
>
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