Rob van der Heij wrote:
On Wed, Jun 23, 2010 at 8:47 PM, Mrohs, Ray <ray.mr...@usdoj.gov> wrote:
Rob,
Heap size is set to 500M/1000M. I've read recommendations to make default/max
the same number, but I would just be guessing at a value. It has grow close to
800M at least once, but it always falls back to around the default size and
stays there most of the time.
Ray
The suggestion to set min and max the same goes well with the folks
who have plenty of memory and just want to avoid garbage collection...
whether it applies to a shared resource environment needs to be seen
(uh, measured..)
I'm always suspicious about recommendations without motivation or
measurements. If it really were the best setting, would we not expect
the defaults to be like that?
Well, bearing in mind both the Sun and IBM JVMs default to memory
settings that both IBM and Sun say are crap for app servers (e.g.
neither using their 1.4 or later GC algorithms by default), I'm not sure
I'd place too much stock in their defaults.
Our experince, both on standalone Solaris boxes using the Sun JVM, and
on our zLinux guests using the IBM JVM, has been that large min/max
separation has produced poor results, as the JVM tends to consistently
try to push memory use back down to the min setting, often invoking
unnecessary GCs to do so.
Using our actual applications with our stress test suites (which tend to
have pretty good predictave power) we tend to see worse application
behaviour (small stalls, higher average response times) and higher CPU
use when giving a big spread for min/max; on an older Sun JVM/Solaris
combo we also saw failure in the application to ramp up quickly enough
to deal with load spikes, with big stalls as the JVM tried to alloc memory.
(That last behaviour was common to connection pooling - if we didn't
open enough connections to Oracle at startup time, the app could have
bad performance or even fail as load ramped up due to an inability to
open connections quickly enough.)
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