(jdk10) RFR(m): 8170933: Cleanup Metaspace Chunk manager: Unify treatment of humongous and non-humongous chunks

[Thomas Stüfe]

Ping... Did I send this to the wrong mailing list?

On Tue, Mar 7, 2017 at 3:01 PM, Thomas Stüfe <thomas.stuefe at gmail.com>
wrote:

> (Resent to hotspot-dev)
>
> On Tue, Mar 7, 2017 at 7:45 AM, Thomas Stüfe <thomas.stuefe at gmail.com>
> wrote:
>
>> Hi all,
>>
>> I would like to get a review for this cleanup change for the
>> metaspace.cpp. This has been a long time brewing in my backlog, but I had
>> to wait until jdk10 is open. The cleanup is actually quite small, the
>> largest part of the change is the added test coding.
>>
>> Issue: https://bugs.openjdk.java.net/browse/JDK-8170933
>> Webrev: http://cr.openjdk.java.net/~stuefe/webrevs/METASPACE
>> -1-8170933-unify-treatment-of-humongous-and-non-humongous-ch
>> unks-on-chunkmanager-return/jdk10-webrev.00/webrev/
>>
>> The change implements a suggestion made by Mikael Gerdin when
>> reviewing JDK-8170520 last year, who suggested that the ChunkManager class
>> should handle returns of both non-humongous and humongous chunks, see
>> original discussion here:
>>
>> http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2
>> 016-November/021949.html
>>
>> "It would be great if you could have a look at unifying the
>> ChunkManager::return_chunks API for standard and humongous chunks so
>> that we wouldn't need the special casing for this in ~SpaceManager
>> That way we could also make humongous_dictionary() private to
>> ChunkManager which would improve encapsulation."
>>
>> The cleanup does this and a bit more, all changes having to do with the
>> fact that the ChunkManager class unnecessarily exposes internals to the
>> other classes in metaspace.cpp and that with a little bit of reorganization
>> this can be avoided. The benefit is simpler coding and way better
>> encapsulation, which will help a lot with future changes (in preparation
>> for https://bugs.openjdk.java.net/browse/JDK-8166690).
>>
>> --------
>>
>> The changes in detail:
>>
>> The main issue was that ChunkManager::return_chunks() did not handle
>> humongous chunks. To return humongous chunks, one had to access the
>> humongous chunk dictionary inside the ChunkManager and add the chunk
>> yourself, including maintaining all counters. This happened in
>> ~SpaceManager and made this destructor very complicated.
>>
>> This is solved by moving the handling of humongous chunk returns to the
>> ChunkManager class itself. For convenience, I split the old
>> "ChunkManager::return_chunks" method into two new ones,
>> "ChunkManager::return_single_chunk()" which returns a single chunk to
>> the ChunkManager without walking the chunk chain, and
>> "ChunkManger::return_chunk_list()" which returns a chain of chunks to
>> the ChunkManager. Both functions are now able to handle both non-humongous
>> and humongous chunks. ~SpaceManager() was reimplemented (actually, quite
>> simplified) and just calls "ChunkManager::return_chunk_list()"  for all
>> its chunk lists.
>>
>> So now we can remove the public access to the humongous chunk dictionary
>> in ChunkManger (ChunkManager::humongous_dictionary()) and make this
>> function private.
>>
>> ----
>>
>> Then there was the fact that the non-humongous chunk lists were also
>> exposed to public via ChunkManager::free_chunks(). The only reason was that
>> when a VirtualSpaceNode is retired, it accessed the ChunkManager free lists
>> to find out the size of the items of the free lists.
>>
>> This was solved by adding a new function, ChunkManager::size_by_index(),
>> which returns the size of the items of a chunk list given by its chunk
>> index.
>>
>> So ChunkManager::free_chunks() could be made private too.
>>
>> ---
>>
>> The rest are minor changes:
>>
>> - made ChunkManager::find_free_chunks_list() private because it was not
>> used from outside the class
>> - Fixed an assert in dec_free_chunks_total()
>> - I added logging (UL, with the tags "gc, metaspace, freelist"). I tried
>> to keep the existing logging intact or add useful logging where possible.
>>
>> ----
>>
>> A large chunk of the changes is the added gtests. There is a new class
>> now, ChunkManagerReturnTest, which stresses the ChunkManager take/return
>> code.
>>
>> Note that I dislike the fact that this test is implemented inside
>> metaspace.cpp itself. For now I kept my new tests like the existing tests
>> inside this file, but I think these tests should be moved to
>> test/native/memory/test_chunkManager.cpp. I suggest doing this in a
>> separate fix though, because it needs a bit of remodeling (the tests need
>> to access internals in metaspace.cpp).
>>
>> ----
>>
>> I ran gtests and the jtreg hotspot tests on Linux x64, Solaris sparc and
>> AIX. No issues popped up which were associated with my change.
>>
>> Thanks for reviewing and Kind Regards, Thomas
>>
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