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

[Thomas Stüfe]

(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-
> chunks-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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