java 1.7.0u4 GarbageCollectionNotificationInfo API

[John Cuthbertson]

Hi Taras,

I'm going to cc the serviceability alias. I think they might be best 
suited to answer some of your questions. I believe they own the API and 
the GC provides the data.

Answer 1: It should be milliseconds, but there was a bug 
(http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7087969) that is now 
fixed in hs24 and you could be running into that.

Answer 2: This sounds like a bug. Do you have a test case you can share?

Answer 3: I'll leave that to the serviceability guys.

Regards,

JohnC

On 1/28/2013 1:11 PM, Taras Tielkes wrote:
>
> Hi,
>
> I'm playing around with the new(ish) GarbageCollectionNotificationInfo 
> API. We're using ParNew+CMS in all our systems, and my first goal is a 
> comparison between -XX:+PrintGCDetails -verbose:gc output and the 
> actual data coming through the notification API. I'm using Java 
> 1.7.0u6 for the experiments.
>
> So far, I have a number of questions:
> 1) duration times
>
> The javadoc for gcInfo.getDuration() describes the returned value as 
> expressed in milliseconds. However, the values differ to the gc logs 
> by  several orders of magnitude. How are they calculated?
>
> On a 1-core Linux x64 VM, the values actually look like microseconds, 
> but on a Win32 machines I still can't figure out any resemblance to gc 
> log timings.
>
> Apart from the unit, what should the value represent? Real time or 
> user time?
>
> 2) CMS events with cause "No GC"
>
> How exactly do the phases of CMS map to the notifications emitted for 
> the CMS collector?
>
> I sometimes get events with cause "No GC". Does this indicate a 
> background CMS cycle being initiated by hitting the occupancy fraction 
> threshold?
>
> 3) Eden/Survivor
>
> It seems that the MemoryUsage API treats Eden and Survivor separately, 
> i.e. survivor is not a subset of eden. This is different from the gc 
> log presentation. Is my understanding correct?
>
> In general, I think it would be useful to have a code sample for the 
> GC notification API that generates output as close as possible to 
> -XX:+PrintGCDetails -verbose:gc, as far as the data required to do so 
> is available.
>
> The API looks quite promising, it seems it could really benefit from a 
> bit of documentation love :)
>
> Thanks,
> -tt
>
>
>
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