Question about G1CollectedHeap::print_on()

[Stefan Johansson]

Hi Thomas,

Seems like you found a bug here. As you say, for G1 the "commit 
watermark" doesn't really make sense, because we can have regions 
uncommitted in the middle of the heap. I think we print the value to 
look similar to the other heaps/GCs. The bug here is that we don't 
multiply with the region size, but just add it. So we should change the 
row in question to:
   p2i(_hrm.reserved().start() + (_hrm.length() * HeapRegion::GrainWords)),

This would give a better value. It would not be the whole truth, since 
there might still be objects above this address if we have uncommitted 
regions in the heap. I better value might be "highest region committed" 
* "heap region size", but that would lie because not all memory below 
that has to be committed.

I created a bug for this:
https://bugs.openjdk.java.net/browse/JDK-8189169

Cheers,
Stefan


On 2017-10-11 12:48, Thomas Stüfe wrote:
> Hi all,
>
> In a customer error file I found the following output:
>
> Heap:
>  garbage-first heap   total 2592768K, used 1914672K 
> [0x0000000679000000, 0x0000000679104f20, 0x00000007f0000000).
>
> I wonder about the middle address value (0x0000000679104f20). I was 
> assuming this to be the commit watermark. But the value seemed awfully 
> low - just ~1MB beyond the start address of the reserved area. Given 
> that about 4/5th of the heap is in-use, this seemed odd.
>
> Looking at the code:
>
> void G1CollectedHeap::print_on(outputStream* st) const {
> ....
>   st->print(" [" PTR_FORMAT ", " PTR_FORMAT ", " PTR_FORMAT ")",
>             p2i(_hrm.reserved().start()),
>         >>    p2i(_hrm.reserved().start() + _hrm.length() + 
> HeapRegion::GrainWords), <<
>             p2i(_hrm.reserved().end()));
> ...
> }
>
> I am still confused. What is "_hrm.reserved().start() + _hrm.length() 
> + HeapRegion::GrainWords"?  We add the number of comitted heap regions 
> to the start address, plus the size of one heap region in words? 
> Sorry, I am not a GC expert, could someone clarify what this address is?
>
> Another question, while looking further into how g1 commits memory I 
> looked at G1PageBasedVirtualSpace and it seems to me that there is no 
> clear commit water mark because pages do not have to be committed 
> sequentially, is that correct?
>
> Thanks for clarifying!
>
> Kind Regards, Thomas