Parallel GC and array object layout: way off the base and laid out in reverse?

Aleksey Shipilev aleksey.shipilev at oracle.com
Wed Sep 4 13:55:37 PDT 2013


Thanks Igor!

-Aleksey

On 05.09.2013, at 0:50, Igor Veresov <iggy.veresov at gmail.com> wrote:

> For PS, look in psPromotionManager.cpp, here the kernel you need to trivially tweak:
> 
> template <class T> void PSPromotionManager::process_array_chunk_work(
>                                                  oop obj,
>                                                  int start, int end) {
>   assert(start <= end, "invariant");
>   T* const base      = (T*)objArrayOop(obj)->base();
>   T* p               = base + start;
>   T* const chunk_end = base + end;
>   while (p < chunk_end) {
>     if (PSScavenge::should_scavenge(p)) {
>       claim_or_forward_depth(p);
>     }
>     ++p;
>   }
> }
> 
> Like Tony and Thomas said before, you'll still be seeing "surprises" due to array chunking and work stealing. Those, I guess, you'll just have to live with.
> 
> igor
> 
> On Sep 4, 2013, at 1:34 PM, Aleksey Shipilev <aleksey.shipilev at oracle.com> wrote:
> 
>> Here you have it, thanks Igor. 
>> Any reference to the relevant block of code?
>> I can probably try to fix this in background.
>> 
>> -Aleksey.
>> 
>> On 05.09.2013, at 0:00, Igor Veresov <iggy.veresov at gmail.com> wrote:
>> 
>>> Yup, that's a depth-first array-scanning quirk. The work-stealing is done using stacks, so in order to have the first fields followed first the references need to be put of stack in reverse. That's done for regular objects but for arrays it's not. 
>>> 
>>> igor
>>> 
>>> On Sep 4, 2013, at 12:51 PM, Aleksey Shipilev <aleksey.shipilev at oracle.com> wrote:
>>> 
>>>> Hi Jon,
>>>> 
>>>> On 09/04/2013 10:19 PM, Jon Masamitsu wrote:
>>>>> I haven't followed this thread carefully enough but the ParallelGC 
>>>>> collector uses a depth-first traversal while the other collectors use
>>>>> a breadth-first. Would that explain the difference?
>>>> 
>>>> The referenced objects in the array are the leaves in reachability
>>>> graph. I thought there is no difference in depth- vs. breadth-first in
>>>> this case? It looks more like we record the traversed objects on some
>>>> LIFO structure, which polls the elements in the reverse order.
>>>> 
>>>> -Aleksey.
> 
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