Robinson, Eric wrote:
On 17/06/2010 08:59, Robinson, Eric wrote:
If your heap size is right on the edge of your minimum for a Tomcat
instance, you may be doing more GC work than is really needed.
However, if you're satisfied with the response time and CPU
utilization, you should be ok.
Time to hit the vendor around the head with the cluebat. If
the app is happy with less heap space then increasing it
is only going to cause problems - mainly that GC when it
happens will take longer and trigger longer pauses. You
can mitigate this with GC config (later VMs may make the
right choices for you anyway) but all this is just adding
unecessary complexity.
Mark
With 160 instances of tomcat on the server, and most of them happy with
64-96MB of RAM, could you take an educated guess at the negative impact
on the server of raising the RAM to 512MB for each instance? How much
extra CPU utilization do you think I could possibly see from all the
extra GC?
Just a note here : 160 X 512 MB = 81 GB.
If each Tomcat's JVM is allowed to use up to 512 MB of Heap, there might be moments where
a lot of JVM's will be using close to that amount. Unless your system can really support
that amount of real RAM, you may be in for massive swapping.
As Mark writes above (and my interpretation of things) :
- a bigger Heap means that the JVM will be able to accumulate more dead stuff in it,
before needing a full GC to clean it up, than if the Heap was smaller.
- thus, all else being equal, it will take more time elapsed before a full-GC
is needed.
- when it is needed however, it will take much longer, because there is more stuff to
clean up.
So all in all, you will have less GC's, but each one will take longer. During a full GC,
the JVM does nothing else (applications are frozen). So there may be an impact on the user
experience during that time.
But, if a significant number of JVMs start filling up their Heap simultaneously, then the
memory used will be "real", not just "reserved". And if your system has less physical RAM
than the total required at any one time, it starts swapping. That /would/ have a definite
impact on the user experience.
Note also that the 81 GB above count only the Tomcat Heaps, and disregard any other memory
needed by your 160 instances.
---------------------------------------------------------------------
To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org
For additional commands, e-mail: users-h...@tomcat.apache.org
