Affects Version/s: None
Release Note:Redo of thread model writing edits to the WAL; slower when few clients but as concurrency rises, it makes for better throughput.
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)