On Tue, Jan 31, 2017 at 5:51 AM Thomas Schatzl <thomas.scha...@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 <vita...@gmail.com> > > wrote: > > > On Mon, Jan 30, 2017 at 8:19 AM, Amit Balode <amit.bal...@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
_______________________________________________ hotspot-gc-use mailing list hotspot-gc-use@openjdk.java.net http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
