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https://issues.apache.org/jira/browse/HBASE-16630?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15504518#comment-15504518
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deepankar commented on HBASE-16630:
-----------------------------------

Ha, nice point, I missed this, but this would still not guarantee, that we 
would freeup the buckets needed for the given BucketSize in a single run, it 
might need several runs free-up a completely free bucket and that would 
transfer into the our hot BucketSize. One other reason it might be slow is that 
bytesToFreeWithExtra will depend on the size of the currently hot BucketSize 
(for which there are no free blocks) and that is usually small and the 
transition might be slow. Another problem is that due to sparsely occupied 
buckets we might encounter keep ending up in the freeSpace method again and 
again. 

> Fragmentation in long running Bucket Cache
> ------------------------------------------
>
>                 Key: HBASE-16630
>                 URL: https://issues.apache.org/jira/browse/HBASE-16630
>             Project: HBase
>          Issue Type: Bug
>          Components: BucketCache
>    Affects Versions: 2.0.0, 1.1.6, 1.3.1, 1.2.3
>            Reporter: deepankar
>            Assignee: deepankar
>         Attachments: HBASE-16630.patch
>
>
> As we are running bucket cache for a long time in our system, we are 
> observing cases where some nodes after some time does not fully utilize the 
> bucket cache, in some cases it is even worse in the sense they get stuck at a 
> value < 0.25 % of the bucket cache (DEFAULT_MEMORY_FACTOR as all our tables 
> are configured in-memory for simplicity sake).
> We took a heap dump and analyzed what is happening and saw that is classic 
> case of fragmentation, current implementation of BucketCache (mainly 
> BucketAllocator) relies on the logic that fullyFreeBuckets are available for 
> switching/adjusting cache usage between different bucketSizes . But once a 
> compaction / bulkload happens and the blocks are evicted from a bucket size , 
> these are usually evicted from random places of the buckets of a bucketSize 
> and thus locking the number of buckets associated with a bucketSize and in 
> the worst case of the fragmentation we have seen some bucketSizes with 
> occupancy ratio of <  10 % But they dont have any completelyFreeBuckets to 
> share with the other bucketSize. 
> Currently the existing eviction logic helps in the cases where cache used is 
> more the MEMORY_FACTOR or MULTI_FACTOR and once those evictions are also 
> done, the eviction (freeSpace function) will not evict anything and the cache 
> utilization will be stuck at that value without any allocations for other 
> required sizes.
> The fix for this we came up with is simple that we do deFragmentation ( 
> compaction) of the bucketSize and thus increasing the occupancy ratio and 
> also freeing up the buckets to be fullyFree, this logic itself is not 
> complicated as the bucketAllocator takes care of packing the blocks in the 
> buckets, we need evict and re-allocate the blocks for all the BucketSizes 
> that dont fit the criteria.
> I am attaching an initial patch just to give an idea of what we are thinking 
> and I'll improve it based on the comments from the community.



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