CMS Garbage collection eating up processor power
T.K
thankhar at gmail.com
Fri Mar 28 14:38:48 UTC 2008
Thanks, Jon. I've guess we need to figure out the memory usage on the
application itself, and also limit the number users on one instance as well.
Btw, I found your blog just a few days ago, and I ended up reading most of
them. Really good stuffs in there. :)
TK
On 3/28/08, Jon Masamitsu <Jon.Masamitsu at sun.com> wrote:
>
> T.K wrote On 03/27/08 20:19,:
>
> > I've attached an old gclog where it has similar behavior on the heap.
> > But the problem with that is it takes quite a long time, like 20 - 70
> > seconds. The server basically went down with customers complaining
> > during that time, so we had to try CMS, but that doesn't work out well
> > either.
>
>
> A significant fraction of your heap is filled with live data. The GC is
> running
> almost constantly because it is trying to keep the application going by
> recovering
> whatever space it can. If the occupancy of the heap does not go down when
> the number of users drops, there's some code in the application that is
> keeping objects alive that perhaps are never going to be used again. If
> the
> application really needs all that data, then you need a larger heap to
> keep
> GC from running so frequently. If increasing the size of the heap just
> delays the onset of the frequent GC's and you do think that all that data
> should be live, you may have to limit the number of users in some way so
> as to not fill up the heap.
>
> >
> > We are having Sun Portal Server 6.2 on those web servers. Do you know
> > is it a normal behavior for Portal Server with about 400-500 users per
> > instance?
>
>
> Don't know about this.
>
> > Thanks,
> > TK
> >
> > On Thu, Mar 27, 2008 at 5:10 PM, Jon Masamitsu <Jon.Masamitsu at sun.com
> > <mailto:Jon.Masamitsu at sun.com>> wrote:
> >
> > Late in this log I see
> >
> > 162417.510: [GC 162417.510: [ParNew: 370176K->11520K(381696K),
> > 0.4232456
> > secs] 3055096K->2715938K(3134208K), 0.4240171 secs]
> >
> > At that point the heap is about 85% full (2715938K/3134208K). The
> > tenured generation is almost completely full.
> >
> > Do you have similar logs when using the default GC? We could use
> them
> > to verify the amount of live data.
> >
> >
> > T.K wrote:
> > >
> > > Hi Jon,
> > >
> > > Here's the attached gclog. It starts out fine when we put in
> > the change
> > > in the evening. Eventually, the next morning (around 60000
> > seconds on
> > > gc time) when the load starting to get in, CMS starts to run
> > > consectively. The first CMS fails occur when we bump up the
> > users to
> > > 700+, and almost kill the server. Ever since then, I don't see
> > the CMS
> > > ever stops, even when the load goes down to 50 users.
> > >
> > > I cut off the logs in between into 3 portion so that I can
> > attach it. :D
> > >
> > >
> > > Thanks,
> > > TK
> > >
> > >
> > > On 3/27/08, *Jon Masamitsu* <Jon.Masamitsu at sun.com
> > <mailto:Jon.Masamitsu at sun.com>
> > > <mailto:Jon.Masamitsu at sun.com <mailto:Jon.Masamitsu at sun.com>>>
> > wrote:
> > >
> > > Using CMS sometimes needs some tuning (especially
> > > with the 1.4.2 jdk). Do you have any gc logs
> > > (-XX:+PrintGCDetails) so we can see what's happening?
> > >
> > >
> > > T.K wrote:
> > > > Hi All,
> > > > We got 5 Sun Web Servers running on Java 1.4.2, and used
> > to use the
> > > > default GC for Tenured space. The problem with that is
> > that it takes
> > > > 60-80 seconds everytime the GC happens, and the latency
> > on the
> > > site goes
> > > > crazy. So we decided to change it to use the Concurrent
> > Mark Sweep
> > > > Collector on one server to test it out. Here's the setting:
> > > >
> > > > -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -Xms3G -Xmx3G
> > > -XX:NewSize=384M
> > > > -XX:MaxNewSize=384M -XX:PermSize=64M -XX:MaxPermSize=64M
> > > > -XX:CMSInitiatingOccupancyFraction=60
> > > >
> > > > With that setting, the server runs great. But eventually,
> > when the
> > > > server reach a medium load (around 100-200 users), the
> > tenured
> > > space is
> > > > always around half full, and the CMS collector starts to
> run
> > > > continuously one after another. It doesn't hurt the
> > application
> > > for now,
> > > > but it's taking 25% of processing time (we got 4 cpu, so
> > one web
> > > server
> > > > always keep 1 cpu power). I don't see that much cpu
> > utilization
> > > on other
> > > > web server that don't have CMS, and they have more users
> > than the one
> > > > with CMS. If we got CMS on all 5 web servers, I'm
> > wondering if
> > > that will
> > > > crash the server or not. What should I do to decrease
> > the processor
> > > > utilization caused by GC?
> > > >
> > > > Also, I'm thinking to use i-CMS on the JVM, and maybe
> > that might slow
> > > > down the CMS and reduce the amount of CPU utilization by
> > CMS. Any
> > > thought?
> > > >
> > > > Thanks,
> > > > TK
> > > >
> > > >
> > > >
> > >
> >
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