CMS Garbage collection eating up processor power

[Jones, Doug H]

I think you are hitting a known 'feature' which affects some 1.4.2
versions, certainly 1.4.2_12. The very first CMS GC after a server
restart will happen at 50% occupancy, thereafter the Collector is meant
to dynamically adjust the limit when a CMS GC kicks in (Just-in-time
with a safety margin). With the affected levels it always happens at
approx 50% occupancy.
 
So the problem you are probably seeing is that your tenured space is
actually 50% or more full of live objects. So in this situation the
Collector will initiate continuous CMS GC's at 2 second intervals.
 
The fix is easy, you need to also add the flag
XX:+UseCMSInitiatingOccupancyOnly=true, then it should honour the
OccupancyFraction of 60%. Of course if the the tenured area is more than
60% full of live objects the problem will re-occur (so just increase
this value).
 
Doug.


________________________________

	From: hotspot-gc-use-bounces at openjdk.java.net
[mailto:hotspot-gc-use-bounces at openjdk.java.net] On Behalf Of T.K
	Sent: Friday, 28 March 2008 6:14 a.m.
	To: hotspot-gc-use at openjdk.java.net
	Subject: CMS Garbage collection eating up processor power
	
	
	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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