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From "Elliott Clark (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8755) A new write thread model for HLog to improve the overall HBase write throughput
Date Tue, 18 Jun 2013 20:58:22 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8755?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13687182#comment-13687182
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Elliott Clark commented on HBASE-8755:
--------------------------------------

The AsyncSyncer can throw an error on data that actually got written to the log.  If the edit
actually made it to the wal but the syncer errors out, then data will be reported as not written
but in fact it could be written.  I don't think there's anything that can be done about it
but maybe a comment about that possibility around 1167.  It's not something new but I expect
since we'll be syncing larger chunks that it's more possible.

As a follow up jira, I think that we can get even better through put by reducing contention
on update and buffer locks.  If we have the AsyncWriter sort wal edits then we can use a read
write lock rather than two different locks (both of which are needed for append).

As another follow up jira I think that we can do away with txid in FSHLog and just use seqnum.

As I see it, I'm +1 but with such a critical piece it would be nice if there were others.
                
> A new write thread model for HLog to improve the overall HBase write throughput
> -------------------------------------------------------------------------------
>
>                 Key: HBASE-8755
>                 URL: https://issues.apache.org/jira/browse/HBASE-8755
>             Project: HBase
>          Issue Type: Improvement
>          Components: wal
>            Reporter: Feng Honghua
>         Attachments: HBASE-8755-0.94-V0.patch, HBASE-8755-0.94-V1.patch, HBASE-8755-trunk-V0.patch
>
>
> In current write model, each write handler thread (executing put()) will individually
go through a full 'append (hlog local buffer) => HLog writer append (write to hdfs) =>
HLog writer sync (sync hdfs)' cycle for each write, which incurs heavy race condition on updateLock
and flushLock.
> The only optimization where checking if current syncTillHere > txid in expectation
for other thread help write/sync its own txid to hdfs and omitting the write/sync actually
help much less than expectation.
> Three of my colleagues(Ye Hangjun / Wu Zesheng / Zhang Peng) at Xiaomi proposed a new
write thread model for writing hdfs sequence file and the prototype implementation shows a
4X improvement for throughput (from 17000 to 70000+). 
> I apply this new write thread model in HLog and the performance test in our test cluster
shows about 3X throughput improvement (from 12150 to 31520 for 1 RS, from 22000 to 70000 for
5 RS), the 1 RS write throughput (1K row-size) even beats the one of BigTable (Precolator
published in 2011 says Bigtable's write throughput then is 31002). I can provide the detailed
performance test results if anyone is interested.
> The change for new write thread model is as below:
>  1> All put handler threads append the edits to HLog's local pending buffer; (it notifies
AsyncWriter thread that there is new edits in local buffer)
>  2> All put handler threads wait in HLog.syncer() function for underlying threads
to finish the sync that contains its txid;
>  3> An single AsyncWriter thread is responsible for retrieve all the buffered edits
in HLog's local pending buffer and write to the hdfs (hlog.writer.append); (it notifies AsyncFlusher
thread that there is new writes to hdfs that needs a sync)
>  4> An single AsyncFlusher thread is responsible for issuing a sync to hdfs to persist
the writes by AsyncWriter; (it notifies the AsyncNotifier thread that sync watermark increases)
>  5> An single AsyncNotifier thread is responsible for notifying all pending put handler
threads which are waiting in the HLog.syncer() function
>  6> No LogSyncer thread any more (since there is always AsyncWriter/AsyncFlusher threads
do the same job it does)

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