Yu Li commented on HBASE-16698:

Current process of {{doMiniBatchMutation}} for branch-1 is like:
1. acquire locks
2. update timestamps
3. build WAL edit
4. append edit to WAL
5. write to memstore
6. sync WAL or defer

W/o patch mvcc number is attained between step #4 and #5, which makes step #4 
and #5 serializing even though appending edit WAL is asynchronous through 

W/ patch mvcc number is preassigned and step#4 and #5 could run in parallel

Since we will update the global {{highestSyncedSequence}} in {{SyncRunner}}, 
the sooner it arrives at {{syncOrDefer}}, the more chance it will release other 
sync task/be released without waiting on real sync operation. To quote and 
emphasize, patch here limit the contention to region level instead of 
regionserver level, and parallel writes on different regions will benefit. 

What's more, all the back and forth discussions here are around SYNC_WAL, but 
don't forget the ASYNC_WAL writes, the improvement on ASYNC_WAL is way more 

> Performance issue: handlers stuck waiting for CountDownLatch inside 
> WALKey#getWriteEntry under high writing workload
> --------------------------------------------------------------------------------------------------------------------
>                 Key: HBASE-16698
>                 URL: https://issues.apache.org/jira/browse/HBASE-16698
>             Project: HBase
>          Issue Type: Improvement
>          Components: Performance
>    Affects Versions: 1.2.3
>            Reporter: Yu Li
>            Assignee: Yu Li
>             Fix For: 2.0.0
>         Attachments: HBASE-16698.branch-1.patch, 
> HBASE-16698.branch-1.v2.patch, HBASE-16698.patch, HBASE-16698.v2.patch, 
> hadoop0495.et2.jstack
> As titled, on our production environment we observed 98 out of 128 handlers 
> get stuck waiting for the CountDownLatch {{seqNumAssignedLatch}} inside 
> {{WALKey#getWriteEntry}} under a high writing workload.
> After digging into the problem, we found that the problem is mainly caused by 
> advancing mvcc in the append logic. Below is some detailed analysis:
> Under current branch-1 code logic, all batch puts will call 
> {{WALKey#getWriteEntry}} after appending edit to WAL, and 
> {{seqNumAssignedLatch}} is only released when the relative append call is 
> handled by RingBufferEventHandler (see {{FSWALEntry#stampRegionSequenceId}}). 
> Because currently we're using a single event handler for the ringbuffer, the 
> append calls are handled one by one (actually lot's of our current logic 
> depending on this sequential dealing logic), and this becomes a bottleneck 
> under high writing workload.
> The worst part is that by default we only use one WAL per RS, so appends on 
> all regions are dealt with in sequential, which causes contention among 
> different regions...
> To fix this, we could also take use of the "sequential appends" mechanism, 
> that we could grab the WriteEntry before publishing append onto ringbuffer 
> and use it as sequence id, only that we need to add a lock to make "grab 
> WriteEntry" and "append edit" a transaction. This will still cause contention 
> inside a region but could avoid contention between different regions. This 
> solution is already verified in our online environment and proved to be 
> effective.
> Notice that for master (2.0) branch since we already change the write 
> pipeline to sync before writing memstore (HBASE-15158), this issue only 
> exists for the ASYNC_WAL writes scenario.

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