CMS large objects vs G1 humongous allocations

[Vitaly Davidovich]

On Tue, Jan 31, 2017 at 5:51 AM Thomas Schatzl <thomas.schatzl at oracle.com>
wrote:

> Hi,
>
>   just commenting a bit on Vitaly's advice:
>
> On Mon, 2017-01-30 at 13:49 +0000, Vitaly Davidovich wrote:
> > Also, you can experiment with relaxing the pause time goal a bit.  G1
> > will use it as a heuristic to determine how many old regions (during
> > mixed GC) to add to a collection.  If you add too few, you're not
> > reclaiming fast enough (potentially) for your allocation rate.
> >
> > Is your entire gc log too big? If not, might be good to attach it or
> > put it somewhere (e.g. pastebin) so we can see the bigger picture.
>
> Agree. It is definitely helpful to see more than just a single GC.
> Often the context determines the appropriate response.
>
> >
> > On Mon, Jan 30, 2017 at 8:38 AM Vitaly Davidovich <vitalyd at gmail.com>
> > wrote:
> > > On Mon, Jan 30, 2017 at 8:19 AM, Amit Balode <amit.balode at gmail.com
> > > > wrote:
> > > > Italy, thank you.
> > > >
> > > > There is no explicit line which says Full GC. I thought line
> > > > printed as "(Heap Sizing) did not expand the heap, reason: heap
> > > > already fully expanded" is an implicit indication that G1 have
> > > > taken all necessary actions which full GC would take and it lead
> > > > to 7.7sec pause.
> > > >
> > > Oh really, there's no Full GC that ensues after those lines?
>
> :) Sometimes trying to just continue opposed to start a full gc right
> away pays off.
>
> > >
> > > The (Heap Sizing) output is G1's ergonomic output.  It's saying
> > > that it would like to expand the heap because it failed to allocate
> > > a new region out of the existing heap.  The reason for expansion is
> > > G1 may start out with a heap smaller than capacity (i.e. max heap
> > > size), and will try to expand the heap (if under capacity/max) as
> > > needed.  Here, it cannot expand because you're already at max heap
> > > size.
> > >
> > > The 7.7s pause is due to to-space exhaustion, which basically means
> > > G1 ran out of space to copy survivors (and you can see that this 7s
> > > is all in the Object Copy phase of the pause).  In my experience,
> > > when you start seeing to-space exhaustion, you pretty soon see a
> > > Full GC.
>
> With JDK9 to-space exhaustion management got significantly faster. It
> should be very close to a regular GC in most cases.
>
> In JDK8, the only way to make this problem disappear is as mentioned by
> Vitaly to avoid them, either by
>   - increasing the heap (to let marking have more time)
>   - decrease the initial heap occupancy (to start marking earlier) via
> -XX:InitialHeapOccupancyPercent
>   - increase the speed of marking by increasing the number of marking
> threads via -XX:ConcGCThreads.
>
> > > > Sorry a bit confused about how can 'G1's concurrent cycles' &
> > > > 'parallel phases of young collections' run concurrently? Does
> > > > that mean ConcGCThreads + ParallelGCThreads cannot be greater
> > > > than cpu cores.?
> > > >
> > > G1 has concurrent GC cycles, such as concurrent marking - this runs
> > > in the background, on the ConcGCThreads, and marks old regions to
> > > gather their liveness information (so that these regions can either
> > > be cleaned up in the background, if fully empty, or they'll be
> > > added to young GCs to make them mixed GCs).  At the same time,
> > > while that concurrent work is ongoing, your Java threads continue
> > > to run and allocate.  You can then hit a young pause, where
> > > ParallelGCThreads will perform some of the parallel work associated
> > > with a young GC.  Young GCs look at eden regions (and do some
> > > processing of them in parallel), while concurrent GC threads can be
> > > processing old regions at the same time.  So yes, you can
> > > theoretically hit a case where you have ConcGCThreads +
> > > ParallelGCThreads threads are runnable, and you may oversubscribe
> > > the machine (assuming each of those threads don't stall themselves
> > > due to internal issues, such as lock contention, etc).
>
> The marking threads will always suspend during GC pauses, so this
> situation can't occur.

Ah, thanks for the correction Thomas.  I thought they continue as they
would be looking at old regions while a young GC is looking at
eden/survivor.

>
>
> Thanks,
>   Thomas
>
-- 
Sent from my phone
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.openjdk.java.net/pipermail/hotspot-gc-use/attachments/20170131/838d28a0/attachment.html>