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From "Wang, Xinglong (Jira)" <j...@apache.org>
Subject [jira] [Created] (HADOOP-17237) Improve NameNode RPC throughput with ReadWriteRpcCallQueue
Date Tue, 01 Sep 2020 11:54:00 GMT
Wang, Xinglong created HADOOP-17237:

             Summary: Improve NameNode RPC throughput with ReadWriteRpcCallQueue 
                 Key: HADOOP-17237
                 URL: https://issues.apache.org/jira/browse/HADOOP-17237
             Project: Hadoop Common
          Issue Type: Improvement
          Components: rpc-server
            Reporter: Wang, Xinglong

 In our production cluster, a typical traffic model is read to write raito is 10:1 and sometimes
the ratios goes to 30:1.
 NameNode is using ReEntrantReadWriteLock under the hood of FSNamesystemLock. Read lock is
shared lock while write lock is exclusive lock.

Read RPC and Write RPC comes randomly to namenode. This makes read and write mixed up. And
then only a small fraction of read can really share their read lock.

Currently we have default callqueue and faircallqueue. And we can refreshCallQueue on the
fly. This opens room to design new call queue.

 If we reorder the rpc call in callqueue to group read rpc together and write rpc together,
we will have sort of control to let a batch of read rpc come to handlers together and possibly
share the same read lock. Thus we can reduce Fragments of read locks.
 This will only improve the chance to share the read lock among the batch of read rpc due
to there are some namenode internal write lock is out of call queue.

Under ReEntrantReadWriteLock, there is a queue to manage threads asking for locks. We can
give an example.
 R: stands for read rpc
 W: stands for write rpc
 In this case, we need 16 lock timeslice.

 In this case, we only need 9 lock timeslice.

 Since the execution order of any 2 concurrent or queued rpc in namenode is not guaranteed.
We can reorder the rpc in callqueue into read group and write group. And then dequeue from
these 2 queues by a designed strategy. let's say dequeue 100 read and then dequeue 5 write
rpc and then dequeue read again and then write again.
 Since FairCallQueue also does rpc call reorder in callqueue, for this part I think they share
the same logic to guarantee rpc result correctness.

 In test environment, we can see a 15% - 20% NameNode RPC throughput improvement comparing
with default callqueue. 
 Test traffic is 30 read:3 write :1 list using NNLoadGeneratorMR

This performance is not a surprise. Due to some write rpc is not managed in callqueue. We
can't do reorder to them by reording calls in callqueue. 
 But still we can do a fully read write reorder if we redesign ReEntrantReadWriteLock to achieve
this. This will be further step after this.

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