Failure scenarios

[Per Liden]

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.

/Per

> 
> cheers,
> Per
> 
>>
>> Thanks!
>>