RFR(xs): 8185033: On Metaspace OOM, ChunkManager composition should be logged.

[coleen.phillimore at oracle.com]

Hi Thomas,
Thank you for working on this.

On 7/21/17 2:54 AM, Thomas Stüfe wrote:
> Dear all,
>
> May I please get reviews and a sponsor for the following small enhancement.
>
> Issue: https://bugs.openjdk.java.net/browse/JDK-8185033
> Webrev:
> http://cr.openjdk.java.net/~stuefe/webrevs/8185033-On-Metaspace-OOM-chunkManager-composition-should-be-logged/webrev.00/webrev/
>
> This patch adds a bit of logging if a Metaspace OOM happens. We will now
> get a printout of the composition of the global chunk managers (non-class
> and, if necessary, class).
>
> Before it looked like this:
>
> [272.727s][info][gc,metaspace,freelist] All Metaspace:
> [272.728s][info][gc,metaspace,freelist] data space:   Chunk accounting:
> used in chunks 100752K + unused in chunks 650K  +  capacity in free chunks
> 8293K = 109696K  capacity in allocated chunks 101403K
> [272.728s][info][gc,metaspace,freelist] class space:   Chunk accounting:
> used in chunks 4047K + unused in chunks 97K  +  capacity in free chunks
> 6085K = 10230K  capacity in allocated chunks 4145K
> [272.729s][info][gc,metaspace,freelist] Total fragmentation waste (words)
> doesn't count free space
> [272.729s][info][gc,metaspace,freelist]   data: 183 specialized(s) 960, 169
> small(s) 41, 1507 medium(s) 4480, large count 1
> [272.729s][info][gc,metaspace,freelist]  class: 183 specialized(s) 0, 13
> small(s) 12, 111 medium(s) 8, large count 1
>
> Now:
> [250.548s][info][gc,metaspace,freelist] All Metaspace:
> [250.549s][info][gc,metaspace,freelist] data space:   Chunk accounting:
> used in chunks 100748K + unused in chunks 638K  +  capacity in free chunks
> 10357K = 111744K  capacity in allocated chunks 101387K
> [250.550s][info][gc,metaspace,freelist] class space:   Chunk accounting:
> used in chunks 4047K + unused in chunks 97K  +  capacity in free chunks
> 6085K = 10230K  capacity in allocated chunks 4145K
> [250.550s][info][gc,metaspace,freelist] Total fragmentation waste (words)
> doesn't count free space
> [250.550s][info][gc,metaspace,freelist]   data: 183 specialized(s) 960, 165
> small(s) 27, 1507 medium(s) 4504, large count 1
> [250.550s][info][gc,metaspace,freelist]  class: 183 specialized(s) 0, 13
> small(s) 12, 111 medium(s) 14, large count 1
> [250.550s][info][gc,metaspace,freelist] Chunkmanager (non-class):
> [250.550s][info][gc,metaspace,freelist]   89 specialized (128 bytes)
> chunks, total 91136 bytes
> [250.550s][info][gc,metaspace,freelist]   2567 small (512 bytes) chunks,
> total 10514432 bytes
> [250.550s][info][gc,metaspace,freelist]   0 medium (8192 bytes) chunks,
> total 0 bytes
> [250.550s][info][gc,metaspace,freelist]   0 humongous chunks, total 0 bytes
> [250.550s][info][gc,metaspace,freelist]   total size: 10605568.
> [250.550s][info][gc,metaspace,freelist] Chunkmanager (class):
> [250.550s][info][gc,metaspace,freelist]   87 specialized (128 bytes)
> chunks, total 89088 bytes
> [250.550s][info][gc,metaspace,freelist]   2999 small (256 bytes) chunks,
> total 6141952 bytes
> [250.550s][info][gc,metaspace,freelist]   0 medium (4096 bytes) chunks,
> total 0 bytes
> [250.550s][info][gc,metaspace,freelist]   0 humongous chunks, total 0 bytes
> [250.550s][info][gc,metaspace,freelist]   total size: 6231040.

Yes, this looks like it'll help.   Your numbers are less mystifying than 
the existing ones.
>
> This helps in understanding the underlying cause of the OOM, if that cause
> is related to chunks lingering around in the freelist. See also [1], a Jep
> proposing to improve the metaspace chunk allocation to increase the chance
> of chunk reuse. This patch here is also a preparation for the prototype
> work of [1].
>
> Note that admittedly, metaspace.cpp will again gain a bit of complexity by
> this patch. When examining the coding, I was tempted to refactor but
> refrained from doing so to keep the patch focused. But I think
> metaspace.cpp (especially the coding associated with statistics) could be
> cleaned up a bit. I opened a follow up item [2] to collect cleanup-issues.
> I am willing to do that myself, but would prefer it to be done after
> functional patches - which still have a slight chance of being backported
> to jdk9 - are all being added.

My first impression was that get_statistics() and print_statistics() 
should be member functions of ChunkManagerStatistics because of all the 
stat->lines.   But ChunkManager has to own these also.  I can't think of 
anything better.

Yes, the metaspace code could definitely use refactoring with respect to 
statistics collection.  Please!   Also I added to your RFE to rename 
MetaspaceAux.

I agree with Mikael about adding the code to avoid casting.  You could 
change the type to 'int' with another patch (or this one, I don't mind 
looking at it again).

I could sponsor this when it's ready.

Thanks!
Coleen


>
> Patch was built and tested on Windows x64, Linux x64 and AIX. I also ran
> jtreg hotspot/runtime/Metaspace tests, no issues.
>
> Kind Regards, Thomas
>
> ----
>
> [1] https://bugs.openjdk.java.net/browse/JDK-8166690
> [2] https://bugs.openjdk.java.net/browse/JDK-8185034