ParNew - how does it decide if Full GC is needed

[Matt Miller]

Are you sure you didn't run a jmap to get a histogram or a heap dump at 
this time?
A full GC with the tenured space that wide open doesn't make much sense 
otherwise.

When you take a histogram or heap dump, the GC log doesn't tell you. I 
think it should, but others have said that this is the expected 
behavior. I'd much rather it be like a System.gc() which prints (System) 
in the GC Log

-Matt

On 5/7/14, 7:34 PM, Vitaly Davidovich wrote:
> Hi guys,
>
> I'd like to get some clarity on exactly how ParNew GC decides whether 
> to follow-up a young GC with a full GC.  Here's a snippet of a young 
> GC followed up by full GC on jdk 7u51:
>
> 29524.949: [GC 11279905K->4112377K(15728640K), 1.6319030 secs]
> 29526.581: [Full GC 4112377K->4085613K(15728640K), 6.8704770 secs]
>
> The vm args are:
>
> -Xms16384m -Xmx16384m -Xmn16384m -XX:NewSize=12288m 
> -XX:MaxNewSize=12288m -XX:SurvivorRatio=10
>
> So it's expected that most objects in the young gen will die after a 
> young collection, and that's the case here (~7gb collected).  We have 
> about 4gb survivors, which obviously overflows the two survivor spaces 
> (they're 1gb each).  Is the survivor space overflow the reason the 
> full gc is initiated, and obviously doesn't clear much (again, as 
> expected)? It also appears that both survivor spaces are completely 
> empty after this full gc, whereas I'd expect some objects to stay 
> there and only some overflow amount would be promoted to tenured size.
>
> The other possible theory behind why full gc occurs in this case is 
> that ParNew cannot predict how much space a young gc will clear (i.e. 
> it doesn't know that majority will be collected) and given that 
> tenured size is pretty small compared to eden, it initiates a full gc. 
>  Alternatively, since all objects apparently got promoted to tenured 
> after this collection (why, by the way? we don't reduce tenuring 
> threshold and this was only the 2nd GC of the day) and the promoted 
> size is something like 98% of old gen capacity, GC panics and does a 
> full GC in hopes of leaving itself some breathing room in the tenured 
> space.
>
> Or is there something else entirely?
>
> I'd greatly appreciate if someone could explain the above.
>
> Thanks
>
>
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