How to find fragmented space in G1 regions

[Thomas Schatzl]

Hi,

On Tue, 2017-02-07 at 19:06 +0530, Amit Balode wrote:
> Any thoughts on how we could find how much space is fragmented in G1
> regions?

  -XX:+G1PrintRegionLivenessInfo prints region information containing
its type after every marking (use gc+liveness=trace for jdk9) twice.
The first "post-marking" output is what you want. The "post-sorting"
one is only interesting for getting details about the collection set
(and this one does not contain free regions).

The first column contains information about the type of the region:

FREE -> free region
SURV -> survivor region
EDEN -> eden region
HUMS -> humongous (start)
HUMC -> humongous (continuation)
OLD -> old region
ARC -> archive regions (jdk9 only I think, not sure, maybe also jdk8)

>From that you can deduce the size and how many contiguous free regions
are available at the moment.

There is also -XX:+PrintHeapAtGC and -XX:+PrintHeapAtGCExtended which
print the region layout at every GC.

Thanks,
  Thomas