Perf: wasted region after humongous alloc?

[Aleksey Shipilev]

Aha, this decision is odd, and as the example below shows, it wastes regions.
Please take it into your work queue?

Thanks,
-Aleksey

On 12/13/2016 10:55 AM, Roman Kennke wrote:
> It's probably because we're unconditionally skipping to the next region
> in ShenandoahFreeSet::claim_contiguous(), assuming that normally the
> 'current' region is already allocated into. This might not be the case
> though, especially when commonly allocating region-sized TLABs.
> 
> In any case, it is wasteful. Do you want to look into this?
> 
> Roman
> 
> Am Dienstag, den 13.12.2016, 10:47 +0100 schrieb Aleksey Shipilev:
>> Hi,
>>
>> Been playing with tests, and realized we have an peculiar behavior
>> when
>> allocating humongous objects, e.g. in:
>>
>> public class Alloc {
>>   static final int SIZE = Integer.getInteger("size", 2_000_000);
>>   static Object sink;
>>
>>   public static void main(String... args) throws Exception {
>>     for (int c = 0; c < 1000000; c++) {
>>       sink = new int[SIZE];
>>     }
>>   }
>> }
>>
>> The region logging prints this:
>>
>> ...
>> region 238, used = 4194304, live = 0, flags = <humongous>
>> region 239, used = 4194304, live = 0, flags = <humongous>
>> region 240, used = 0, live = 0, flags = <none>
>> region 241, used = 4194304, live = 0, flags = <humongous>
>> region 242, used = 4194304, live = 0, flags = <humongous>
>> region 243, used = 0, live = 0, flags = <none>
>> region 244, used = 4194304, live = 0, flags = <humongous>
>> region 245, used = 4194304, live = 0, flags = <humongous>
>> region 246, used = 0, live = 0, flags = <none>
>> ...
>>
>> So there seems to be an empty region right after the humongous
>> allocation. Are
>> we wasting it intentionally, or is it a bug? Seems wasteful either
>> way.
>>
>> Thanks,
>> -Aleksey
>>