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https://issues.apache.org/jira/browse/HBASE-27730?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17801971#comment-17801971
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Becker Ewing commented on HBASE-27730:
--------------------------------------
Sorry about that! I've pushed up the code to this branch:
[https://github.com/jbewing/hbase/tree/HBASE-27730-prelim]
> Optimize reference counting in off-heap ByteBuff
> ------------------------------------------------
>
> Key: HBASE-27730
> URL: https://issues.apache.org/jira/browse/HBASE-27730
> Project: HBase
> Issue Type: Improvement
> Reporter: Bryan Beaudreault
> Assignee: Becker Ewing
> Priority: Major
> Attachments: HBASE-27730-prelim.patch
>
>
> In HBASE-27710 we uncovered a performance regression in reference counting of
> ByteBuff. This was especially prominent in on-heap buffers when doing a
> simple HFile.Reader iteration of a file. For that case, we saw a 4x
> regression when reference counting was in play.
> It stands to reason that this same regression exists in off-heap buffers, and
> I've run a microbenchmark which indeed shows the same issue. With existing
> reference counting, scanning a 20gb hfile takes 40s. With an optimized
> version, scanning the same file takes 20s. We don't typically see this in
> profiling live regionservers where so much else goes on, but optimizing this
> would eliminate some cpu cycles.
> It's worth noting that netty saw this same regression a few years ago:
> [https://github.com/netty/netty/pull/8895]. Hat tip to [~zhangduo] for
> pointing this out.
> One of the fixes there was to copy some internal code from deeper in the ref
> counting, so that the call stack was smaller and inlining was possible. We
> can't really do that.
> Another thing they did was add a boolean field in their CompositeByteBuffer,
> which gets set to true when the buffer is recycled. So they don't need to do
> reference counting on every operation, instead they can just check a boolean.
> I tried adding a boolean to our RefCnt.java, and it indeed fixes the
> regression. The problem is, due to class alignment issues in java, adding
> this boolean field increases the heap size of RefCnt from 24 to 32 bytes.
> This seems non-trivial given it's used in bucket cache where there could be
> many millions of them.
> I think we can get around this by simply nulling out the recycler in RefCnt
> after it has been called. Then, instead of doing a boolean check we can do a
> null check. This performs similarly to the boolean, but without any extra
> memory.
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