> -----Original Message-----
> From: George I. Develekos [mailto:gdevele...@omilia.com]
> Sent: Tuesday, November 15, 2016 3:00 PM
> To: 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>
> > 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
