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

[charlie hunt]

Hi Vitaly,

Just to clarify things in case there might be some confusion … one of the terms in G1 can be a little confusing with a term used in Parallel GC, Serial GC and CMS GC, and that is “to-space”.  In the latter case, “to-space” is a survivor space. In G1, “to-space” is any space that a G1 is evacuating objects too.  So a “to-space exhausted” means that during an evacuation of live objects from a G1 region (which could be an eden region, survivor region or old region), and there is not an available region to evacuate those live objects, this constitutes a “to-space failure”.

I may be wrong, but my understanding is that once a humongous object is allocated, it is not evacuated. It stays in the same allocated region(s) until it is marked as being unreachable and can be reclaimed.

charlie

> On Oct 7, 2016, at 11:00 AM, Vitaly Davidovich <vitalyd at gmail.com> wrote:
> 
> Hi Jenny,
> 
> On Fri, Oct 7, 2016 at 11:52 AM, yu.zhang at oracle.com <mailto:yu.zhang at oracle.com> <yu.zhang at oracle.com <mailto:yu.zhang at oracle.com>> wrote:
> Prasanna,
> 
> In addition to what Vitaly said, I have some comments about your question:
> 
> 1)      Humongus allocation request for 72 mb failed, from the logs we can also see we have free space of  around 3 GB. Does this means , our application is encountering high  amount of fragmentation ?. 
> 
> It is possible. What it means is g1 can not find 36 consecutive regions for that 72 mb object.
> I agree the ReservePercent=40 is too high, but that should not prevent allocating to the old gen. G1 tries to honor ReservePercent.
> 
> So just to clarify - is the space (i.e. regions) reserved by G1ReservePercent allocatable to humongous object allocations? All docs/webpages I found talk about this space being for holding survivors (i.e. evac failure/to-space exhaustion mitigation).  It sounds like you're saying these reserved regions should also be used to satisfy HO allocs? 
> 
> Thanks
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