MinHeapFreeRatio / MaxHeapFreeRatio

[Damon Hart-Davis]

Hi,

First time on this list, so hello!  I see a few familiar names from days of yore!

I am running my favourite (Tomcat/Web) app in a very constrained memory environment (a SheevaPlug) along with all the usual Internet server junk (sendmail,  ntp, sshd, etc) and I am aiming to squeeze it into the even tighter space of a Raspberry Pi in due course.

As such, I want the JVM to give memory back to the OS whenever possible.  In code I set a target to keep about 25% of the heap free, and so for example I stop cacheing some stuff when below that, and cache more vigorously above that.

Here are some relevant options:

CATALINA_OPTS="-Xmx100m -Xms64m"
# Cap size of non-(main-)heap components.
CATALINA_OPTS="$CATALINA_OPTS -XX:MaxPermSize=48m"
# Trim thread stack size.
CATALINA_OPTS="$CATALINA_OPTS -Xss256k"
# Keep the new generation well within the target 25% free...
CATALINA_OPTS="$CATALINA_OPTS -XX:NewRatio=5"
# Be aggressive about giving memory back to the system above target 25% free.
CATALINA_OPTS="$CATALINA_OPTS -XX:MinHeapFreeRatio=10 -XX:MaxHeapFreeRatio=26"
# Run in incremental GC mode to minimise pauses.
CATALINA_OPTS="$CATALINA_OPTS -Xincgc"

I see some evidence that the free ratios are being observed by when the heap is expanded, but I have never seen any variant of JDK 6 or 7, including the Oracle embedded 6, actually give memory back to the OS with this app.

Am I still doing something wrong?  B^>

Rgds

Damon