RFR (M): 8159422: Very high Concurrent Mark mark stack contention

Thomas Schatzl thomas.schatzl at oracle.com
Wed Sep 14 12:54:15 UTC 2016


Hi Kim,

On Tue, 2016-09-13 at 12:19 -0400, Kim Barrett wrote:
> > 
> > On Sep 13, 2016, at 6:15 AM, Thomas Schatzl <thomas.schatzl at oracle.
> > com> wrote:
> > http://cr.openjdk.java.net/~tschatzl/8159422/webrev.2_to_3/ (diff)
> > http://cr.openjdk.java.net/~tschatzl/8159422/webrev.3/ (full)
> -------------------------------------------------------------------
> ----------- 
> src/share/vm/gc/g1/g1ConcurrentMark.cpp
>  242 G1CMMarkStack::OopChunk*
> G1CMMarkStack::remove_chunk_from_chunk_list() {
>  243   MutexLockerEx x(MarkStackChunkList_lock,
> Mutex::_no_safepoint_check_flag);
>  244   _chunks_in_chunk_list--;
>  245   return remove_chunk_from_list(&_chunk_list);
>  246 }
> 
> Decrement of _chunks_in_chunk_list needs to be conditional on result
> of remove_chunk_from_list, only decrementing if got a chunk.
> 

  it did not fail with the latest jprt run (I am basically always doing
jprt runs for all changes I send out) because this method (and all
these new shorthand methods) have not actually been called.

Fixed. Sorry :(

> -------------------------------------------------------------------
> ----------- 
> src/share/vm/gc/g1/g1ConcurrentMark.hpp
> I think some of the padding in G1CMMarkStack isn't useful with the
> lock-based approach.  I'm assuming you've left it because you plan to
> get back to the lock-free approach later.  Is there a CR for that?
> 
> -------------------------------------------------------------------
> -----------
> 

I can only see that the padding for _chunks_in_chunk_list/_chunk_list
might not be useful as they are always accessed together.
I removed it.

The new CR is JDK-8165848 (also referenced by this CR).

Changes at
http://cr.openjdk.java.net/~tschatzl/8159422/webrev.3_to_4/ (diff)
http://cr.openjdk.java.net/~tschatzl/8159422/webrev.4/ (full)

Testing:
jprt

Thanks,
  Thomas



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