G1-GC - Full GC [humongous allocation request failed]

[Thomas Schatzl]

Hi,

On Sat, 2016-10-08 at 14:30 -0400, Vitaly Davidovich wrote:
> 
> 
> On Friday, October 7, 2016, Gopal, Prasanna CWK <prasanna.gopal at black
> rock.com> wrote:
> > Hi All
> >  
> >  
> > Thanks for all your reply. These discussions certainly help to get
> > good insight J.
> >  
> > So just to summarize
> >  
> > 1)     G1ReservePercent will not affect Humongus allocation , so
> > the full GC we are encountering is due to fragmentation
> > 
> It may or may not - let's see what a G1 dev says (as Jenny
> mentioned).  Either way, you don't have enough contiguous regions to
> satisfy the allocation, so it's fragmentation one way or the other.
> 
> You should drop G1ReservePercent for now unless you have good reason
> for setting it to 40 (that's a large value, btw). 

See the other email in this thread.

> > 2)     I will try chaging G1MixedGCLiveThresholdPercent to 85 to
> > see the mixed GC’s can be increased.
> > 
> Yes, that's a good idea.  Do you see any mixed GCs at all now? If so,
> how long are the concurrent marking phases taking (look for
> concurrent-mark-end in the gc log).

Agree. Making G1 more aggressive with reclaiming old gen regions may
help.

With 8u40+, some simple, quite effective heuristics were added that in
many cases decrease fragmentation a lot. With 7, you can only either
give G1 more memory, try to make it more aggressively reclaim regions,
or minimize old gen allocations that cause fragmentation.

> > 3)     Due to some other dependencies , we were unable to move to
> > latest Jdk’s ( Jdk 8).  Our application is currently running with
> > CMS and we are seeing long GC pause , that why we wanted to explore
> > G1.As we can’t move Jdk 8 soon , Is it good idea to migrate to G1
> > with Jdk 7  

Please at least move to latest 7u (I remember you mentioning 7u40).
There were a few very useful patches for G1 in 7u60 iirc.

> Have you tried just setting Xmx and a reasonable pause time goal?
> As a rule of thumb, setting Xmx to 3x your live set works 
> well(the more headroom you give to G1, the better).  Giving it 
> a reasonable pause time goal allows it to adjust young gen 
> dynamically and possibly raising it high enough such that there's 
> either no promotion or very little - young gen collection efficiency 
> is a function of how many survivors you have when the collection 
> kicks in, so the fewer survivors the better (that applies 
> to all generational copying collectors, not just G1 of course). 

Thanks,
  Thomas