Failure scenarios

Simone Bordet simone.bordet at
Wed Apr 10 15:49:01 UTC 2019


On Wed, Apr 10, 2019 at 5:38 PM Per Liden <per.liden at> wrote:
> Hi,
> On 04/10/2019 04:43 PM, Simone Bordet wrote:
> > Hi,
> >
> > I would like to ask a few questions about how ZGC handles failure scenarios.
> >
> > If there is an allocation failure, but the GC is currently running, my
> > understanding is that the allocating thread will pause until the GC
> > can make space for the request allocation.
> > AFAIK, this is called "Allocation Stall" and it's reported by ZGC logging.
> > Is my understanding correct?
> That's correct. And note that space can become available before the GC
> cycle has completed. For example, during relocation set selection,
> ZPages that have zero live objects are immediately free up. And during
> relocation, as ZPages are emptied they are immediately made available
> for new allocations.
> >
> > Also AFAIK there is no fall back to Full GCs. Is this correct?
> Correct. Since ZGC is compacting (as opposed to copying) there's no need
> for any fall back. Put another way, if the normal GC cycle failed to
> free up memory, then any fallback "full GC" will also fail.
> >
> > If ZGC cannot free space, there is no retry and no more stalling of
> > the allocating thread, but just a OOME. Is that right?
> That's right. A stalled Java thread will give up and throw OOME if one
> complete GC cycle has passed and there's still no memory available. So,
> if a thread stalls when a GC cycle is in progress then it will not throw
> OOME until another cycle has been completed.
> cheers,
> Per
> >
> > Thanks!
> >

Simone Bordet
Finally, no matter how good the architecture and design are,
to deliver bug-free software with optimal performance and reliability,
the implementation technique must be flawless.   Victoria Livschitz

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