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https://issues.apache.org/jira/browse/HBASE-22127?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16804582#comment-16804582
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Duo Zhang commented on HBASE-22127:
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by design, LRUBlockCache is on heap, so the correct way is to copy it on heap
and then put it into LRUBlockCache.
Of course you can change the behavior, but IIRC, on heap is faster than off
heap in the past, that why we put index in LRUBlockCache. So here we need a
performance test.
> Introduce an switch to allow the LRUBlockCache offheap or not.
> --------------------------------------------------------------
>
> Key: HBASE-22127
> URL: https://issues.apache.org/jira/browse/HBASE-22127
> Project: HBase
> Issue Type: Sub-task
> Reporter: Zheng Hu
> Assignee: Zheng Hu
> Priority: Major
>
> In here [1], [~anoop.hbase] pointed out an crtial problem , I pasted here:
> bq. So if we read from HDFS into a pooled BB and then give to LRU cache for
> caching (ya mostly cache on read might be true) we will cache the block which
> is backed by this pooled DBB? Unless the block is evicted , this BB wont go
> back to pool. I think this is some thing we can not livw with !! For LRU
> cache the sizing itself is based on what % of heap size we can grow. But here
> in effect we are occupying the off heap space for the cached blocks. All the
> sizing assumptions and calc going out of control !
> It's indeed an big problem here. so we can only make the block ref to an heap
> area if we use LRUCache (both LruBlockCache and CombinedBlockCache case). Or
> we can also make the lru cache offheap ?
> I think we can introduce an switch indicate that whether the lru block cache
> offheap or not, if heap, then coping those bytes from ByteBuff to heap.
> https://reviews.apache.org/r/70153/diff/6?file=2133545#file2133545line398
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