Failure scenarios

Per Liden per.liden at
Wed Apr 10 15:49:52 UTC 2019

Just a small clarification below.

On 04/10/2019 05:38 PM, Per Liden 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.

... and there's till no memory available.


> cheers,
> Per
>> Thanks!

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