With Release 6.0.1_GA_1816.UBUNTU8 UBUNTU8 NETWORK edition.
we run several days just fine, and then
the processor time charged to the zimbra java process jumps up.
User experience with the web interface suffers.

in zmmailboxd.out I've noticed the incidence of two particular message
increase: "CMS: abort preclean due to time" and "concurrent mode failure"

615284.919: [CMS-concurrent-abortable-preclean-start]
CMS: abort preclean due to time 615289.931: [CMS-concurrent-abortable-preclean: 0.275/5.012 secs] [Times: user=0.30 sys=0.06, real=5.02 secs]
615289.932: [GC[YG occupancy: 386350 K (554816 K)]615289.933: [Rescan (parallel) , 1.0865790 secs]615291.019: [weak refs processing, 0.0000270 secs] [1 CMS-remark: 1850959K(1851392K)] 2237310K(2406208K), 1.0871770 secs] [Times: user=0.48 sys=0.39, real=1.09 secs]
Total time for which application threads were stopped: 1.0882570 seconds

This has occurred 2100 times in the last almost 11+ hours. Yet only
0-3 times in earlier days after a restart.

and the second:

587640.400: [GC 587640.400: [ParNew: 554815K->554815K(554816K), 0.0000810 secs]587640.401: [CMS587640.401: [CMS-concurrent-abortable-preclean: 0.236/4.157 secs] [Times: user=0.31 sys=0.23, real=4.16 secs]
(concurrent mode failure)[Unloading class sun.reflect.GeneratedSerializationConstructorAcces sor11]
[Unloading class sun.reflect.GeneratedConstructorAccessor190]
[Unloading class sun.reflect.GeneratedMethodAccessor469]
[Unloading class sun.reflect.GeneratedMethodAccessor456]
[Unloading class sun.reflect.GeneratedMethodAccessor468]
[Unloading class sun.reflect.GeneratedConstructorAccessor191]
: 1837879K->1851392K(1851392K), 12.7999000 secs] 2392695K->1899378K(2406208K), [CMS Perm : 48919K->48900K(131072K)], 12.8004090 secs] [Times: user=10.79 sys=0.08, real=12.80 secs]
Total time for which application threads were stopped: 12.8018620 seconds

We have 5 GB memory (tuned up as a vm from 3GB after the 6.0 upgrade)
and are the zimbra java process is running with:
/opt/zimbra/java/bin/java -server -XX:NewRatio=2 -Djava.awt.headless=true -XX:MaxPermSize=128m -XX:SoftRefLRUPolicyMSPerMB=1 -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintGCApplicationStoppedTime -XX:+UseConcMarkSweepGC -XX:PermSize=128m -Xss256k -Xms2409m -Xmx2409m -Xmn602m -Djava.io.tmpdir=/opt/zimbra/mailboxd/work -Djava.library.path=/opt/zimbra/lib -Djava.endorsed.dirs=/opt/zimbra/mailboxd/common/endorsed -Dzimbra.config=/opt/zimbra/conf/localconfig.xml -Djetty.home=/opt/zimbra/mailboxd -DSTART=/opt/zimbra/mailboxd/etc/start.config -jar /opt/zimbra/mailboxd/start.jar /opt/zimbra/mailboxd/etc/jetty.properties /opt/zimbra/mailboxd/etc/jetty-setuid.xml /opt/zimbra/mailboxd/etc/jetty.xml

Can I restart this process with minimal/no effect to the user's web experience?
Can someone share insight into what's going on, and what action
I might take?