Does GC Really Matter (Is This Situation)?

["Robinson, Eric" <er...@psmnv.com>]

This is a similar question to one already being discussed in the list
with the subject "Setting the Right Amount of Memory".

We have 160 instances of tomcat on the same server, with most instances
configured to use 64-96MB of RAM. We carefully watch the logs for OOMEs.
If we see any, we increase the RAM allocation for that instance by 32MB,
which is enough to make the OOMEs go away.

Some people say this approach will lead to increased CPU utilization
from frequent GC; however, our server runs 90% idle all day long so CPU
is evidently not being driven up by much, if any. 

Given the circumstances, is there anything to be gained from increasing
the heap size? Our software vendor wants us to increase each tomcat
instance to 512MB, just as a matter of policy, but I don't see a good
technical reason to do that. 

Am I missing something?

--
Eric Robinson


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RE: Does GC Really Matter (Is This Situation)?

["Caldarale, Charles R" <Ch...@unisys.com>]

> From: Robinson, Eric [mailto:eric.robinson@psmnv.com]
> Subject: Does GC Really Matter (Is This Situation)?
> 
> Some people say this approach will lead to increased CPU 
> utilization from frequent GC

If you're referring to what I said, note the numerous caveats I included.  Only if you happened to be right on the borderline of the minimum heap size would CPU usage be excessive, and all evidence indicates that none of your 160 instances are.

> Given the circumstances, is there anything to be gained 
> from increasing the heap size?

No.  Just continue to monitor the CPU and heap usage (as it looks like you have been doing), especially if you have an increase in overall workload.

 - Chuck


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Re: Does GC Really Matter (Is This Situation)?

[Ronald Klop <ro...@base.nl>]

Op dinsdag, 22 juni 2010 18:33 schreef "Robinson, Eric" <er...@psmnv.com>:
> 
>  
> This is a similar question to one already being discussed in the list
> with the subject "Setting the Right Amount of Memory".
> 
> We have 160 instances of tomcat on the same server, with most instances
> configured to use 64-96MB of RAM. We carefully watch the logs for OOMEs.
> If we see any, we increase the RAM allocation for that instance by 32MB,
> which is enough to make the OOMEs go away.
> 
> Some people say this approach will lead to increased CPU utilization
> from frequent GC; however, our server runs 90% idle all day long so CPU
> is evidently not being driven up by much, if any. 
> 
> Given the circumstances, is there anything to be gained from increasing
> the heap size? Our software vendor wants us to increase each tomcat
> instance to 512MB, just as a matter of policy, but I don't see a good
> technical reason to do that. 
> 
> Am I missing something?
> 
> --
> Eric Robinson
> 
> 
> 
> 

You can monitor the gc with jstat.
jstat -gc <pid> 10s
This wil show you the memory usage of a java instance with the time spent in GC. If it does 0.9 sec. of GC every sec. you are running inefficient. :-)

Ronald.