One thing I didn't respond to:
The VM host is unlikely to have something to do here. We have a
coincidence (in time) of three events, consistently.
1. Full GC
2. High CPU
3. App freezes
So it seems really unlikely the host is responsible.
The odd thing is that, the same app, on the same OS, in other
installations, where of course we also have Full-GC periodically, never
shows any freeze.
Regards,
George I. Develekos | SeniorSW Engineer |t: _+30.210.6930664_| e:
gdevele...@omilia.com <mailto:gdevele...@omilia.com>
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On 11/16/2016 6:11 PM, john.e.gr...@wellsfargo.com wrote:
Sorry for top posting. The format got weird.
Those numbers aren’t bad. The most important number for me is the
throughput on the summary tab. Yours is 99.38%. That means the JVM
was doing real work 99.38% of the time and garbage collecting the
other .62%. You could improve that if you worked hard enough, but
going from 99.4 to 99.5 or 99.6 probably isn’t worth the effort. If
the number was 90% or something, then you’d have more room for
improvement.
Look at the GC performance numbers on the summary tab. Obviously
minor GCs are much faster in this regard than major GCs. You can
reduce your total GC time by increasing the size of your young
generation. You will get more or slower young collections but fewer
and faster old collections. Overall the total time will be less than
it is now and the longest pauses will be shorter.
As others have said, though, something doesn’t add up. CMS is only
stop-the-world during certain phases. (Not the ones with “concurrent”
in the name.) If you feel these GC events coincide with pauses in
your app, you can try a thread dump or three (kill -3 <pid>) during
the pause. Use a tool like Samurai to parse the output. This might
only be practical for longer pauses, though.
Also, is it possible the VM itself is having a problem? Maybe you
should talk to your virtualization team to see how stressed the
hardware is. VMWare has an informative java best practices doc:
http://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/techpaper/enterprise-java-applications-on-vmware-best-practices-guide.pdf
John
*From:*George I. Develekos [mailto:gdevele...@omilia.com]
*Sent:* Wednesday, November 16, 2016 5:21 AM
*To:* users@tomcat.apache.org
*Subject:* Re: Please help with Tomcat Garbage Collection
I'm attaching three screenshots of the GCViewer app as it processed
the complete *gc.log* file (about 19 hours).
Please have a look and advise on what I can do to limit Full-GC times.
As of now I have a recommendation to increase the Young Gen..
The setup in summary:
We are using Java 6 (stuck with CentOS 5.8 at this time) and Tomcat
7.0.64.
Xmx is 5G, Xms is 2G, and GC options are -XX:+UseConcMarkSweepGC
-XX:+CMSIncrementalMode
On 11/15/2016 11:45 PM, john.e.gr...@wellsfargo.com
<mailto:john.e.gr...@wellsfargo.com> wrote:
-----Original Message-----
From: George I. Develekos [mailto:gdevele...@omilia.com]
Sent: Tuesday, November 15, 2016 3:00 PM
To:users@tomcat.apache.org <mailto:users@tomcat.apache.org>
Subject: Re: Please help with Tomcat Garbage Collection
The system does very little swapping, both when it's GC'ing and when
it's not.
Less than 100MB worth of swap is taken.
Giving Tomcat its own HW is not an option at this time, especially as
there's no
guarantee it'll solve the problem. Besides it would be a VM anyway, not
physical
dedicated HW. The current server is also a VM.
On 15-Nov-16 10:55 PM, Zdeněk Henek wrote:
I would start with moving this tomcat to its own hw.
Did you check swap? This long pauses could be because part of your
heap is swapped to hdd
Regards,
Zdenek Henek
On Tue, Nov 15, 2016, 21:37 George I. Develekos
<gdevele...@omilia.com> <mailto:gdevele...@omilia.com>
wrote:
On 15-Nov-16 10:22 PM, Christopher Schultz wrote:
George,
On 11/15/16 10:46 AM, George I. Develekos wrote:
Hello guys,
We are having problems on a production system with very long
"full
GC" times, as long as1200sec real time (!!!).
We are using Java 6 (stuck with CentOS 5.8 at this
time) and Tomcat
7.0.64.
Xmx is 5G, Xms is 2G, and GC options are
-XX:+UseConcMarkSweepGC
-XX:+CMSIncrementalMode
No other custom memory-related settings are in place.
Looking at the GC log, the last few Full-GC entries are:
1367.020: [Full GC 1367.020: [CMS:
1178831K->527456K(1926784K),
2.1117220 secs] 1250378K->527456K(2080128K), [CMS Perm :
169762K->56187K(169984K)] icms_dc=0 , 2.1118160 secs]
[Times:
user=1.96 sys=0.13, real=2.11 secs]
2579.317: [Full GC 2579.317: [CMS2581.876:
[CMS-concurrent-mark:
2.558/1212.733 secs] [*Times: user=113.05 sys=28.01,
real=**1212.49 **secs] ** * 3539.969: [Full GC 3539.969:
[CMS3540.056: [CMS-concurrent-sweep: 1.571/23.223 secs]
[Times:
user=6.12 sys=1.36, real=*23.21 secs*]
4070.456: [Full GC 4070.457: [CMS:
1252569K->591200K(1926784K),
2.3447040 secs] 1270617K->591200K(2080128K), [CMS Perm :
169983K->56598K(169984K)] icms_dc=0 , 2.3448140 secs]
[Times:
user=2.18 sys=0.14, real=2.34 secs]
What can we do?
1367.020 Full GC duration=2.11s
2579.317 Full GC duration=1212.49s
So your full GC immediately started another full GC that
took 20
minutes ?
Are you only showing certain FULL GC activity from your
log, or is
that everything?
CMS should have a mark and then a sweep each time, but your
times
don't seem to add up.
also note that the whole point of CMS is that there isn't
any
stop-the-world during the mark portion of the process.
Are you actually experiencing a problem, or are you just
suffering
from instrumentor's remorse?
- -chris
Chris,
What I listed is the result of the command:
grep "Full GC" gc.log
So (obviously) I have skipped other GC activity, i.e. whatever
GC
activity didn't include the "Full GC" string.
Yes we are having app trouble due to the GC delays so this is a
real
problem. Our application has real-time constraints so the GC
delays
cannot be tolerated. I selected those GC options _in order to
avoid
_long GC times.
Additionally, these periods coincide with high CPU for that JVM
process. From 5-20% CPU where it is normally, it jumps to 60%
ore more.
Once GC is done, our app rushes to catch up with tasks that had
to
wait for GC to finish.
Answering another question from a member who has kindly
responded,
yes the server is running other stuff. Basically it runs three
tomcats, the main one being this one. It also runs a DB2
database
that has close-to-zero activity.
George
It might be helpful if you could post a larger chunk of your GC log, at
least long enough to cover the start and end of the CMS phases and maybe even
more. Additionally, try using a tool like GCViewer to analyze the log.
How many CPUs do you have? 60% CPU usage isn't usually a big deal.
Like Chris already said, this is not a stop-the-world phase, so your
application should continue working in parallel with the garbage collector.
Looks like your young generation is only 150MB (2080128k - 1926784k.)
That's very small for a 2-5GB heap. Are you explicitly setting it somewhere or
is the JVM choosing that for you? It's so small that your old generation might
be filling up faster that it should, leading to more frequent full collections.
You could try setting the young generation to something like 25-50% of the
total heap. You'd get a lot of small pauses as the young gen is collected but
fewer long ones.
John
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