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https://issues.apache.org/jira/browse/HBASE-16630?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15489646#comment-15489646
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ramkrishna.s.vasudevan commented on HBASE-16630:
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Nice find. Some simiilar issue was reported with file mode also I believe where
it was said that it is not used efficiently. Need to check that JIRA too. I
don't have that ID now.
On the patch
-> The read lock is tried to be released when you get write lock.
-> relocatedCount++; - better to increment after the task is done.
->
{code}
backingMap.put(key, new BucketEntry(newOffset, len, bucketEntry.accessCounter,
697 bucketEntry.getPriority()));
{code}
Should the key be removed once we get the write lock or is it ok to overwrite
the key with the new value? Am asking in terms of some other request asking for
this key at the same time when the deFragmentation happens.
{code}
public void setTo(long free, long used, long itemSize,
557 float nonZeroOccupancyRatio) {
{code}
When does this exact update happen?
Overall is it better if we improve the way the buckets are allocated - will
that improve things?
Also what is the impact of this deFragmentation in real read load. Because we
iterate thro every key. Is it better if do this in a seperate thread where we
hold on to the highest fragmented bucked in a queue and keep defragmenting
that? But may be it won't work because eviction is random and not in our hands?
> 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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