Two basic questions on -verbosegc and -XX:+PrintTenuringDistribution

[Taras Tielkes]

Hi,

We're using a time-series database to store and aggregate monitoring
data from our systems, including GC behavior.

I'm thinking of adding two metrics:
* total allocation (in K per minute)
* total promotion (in K per minute)

The gc logs are the source for this data, and I'd like to verify that
my understanding of the numbers is correct.

Here's an example verbosegc line of output (we're running ParNew+CMS):
[GC 2136581.585: [ParNew:345951K->40960K(368640K), 0.0676780 secs]
3608692K->3323692K(5201920K), 0.0680220 secs]

a) The delta between the ParNew "before" and "after" is: 345951K-40960K=304991K
My understanding is that the 304991K is the total of (collected in
young gen + promoted to tenured gen)
Since this number of composed of two things, it's not directly useful by itself.

b) The delta between the overall heap "before" and "after" is:
3608692K-3323692K=285000K
I assume that this is effectively the volume that was collected in
this ParNew cycle.
Would it be correct to calculate the total allocation rate of the
running application (in a given period) from summing the total heap
deltas (in a given timespan)?

I do realize that it's a "collected kilobytes" metric, but I think
it's close enough to be used as a "delayed" allocation number,
especially when looking at a timescale of 10 minutes or more.
It has the additional convenience of requiring to parse the current
gc.log line only, and not needing to correlate with the preceding
ParNew event.

c) I take it that the difference between the two deltas (ParNew delta
and total heap delta) is effectively the promotion volume?
In the example above, this would give a promotion volume of
(345951K-40960K)-(3608692K-3323692K)=19991K

d) When looking at -XX:+PrintTenuringDistribution output, I assume the
distribution reflects the situation *after* the enclosing ParNew event
in the log.

Thanks in advance for any corrections,
-tt