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https://issues.apache.org/jira/browse/HDFS-17531?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17849447#comment-17849447
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Jian Zhang commented on HDFS-17531:
-----------------------------------
Hello everyone, I conducted a performance comparison test between sync and
asynchronous router, and the test results showed that in single ns or multi ns
scenarios, Asynchronous router in terms of throughput The utilization of CPU
and thread, as well as the average processing time of client requests, are
better than those of sync router, especially when downstream ns have
performance bottlenecks, The performance of the async router is far greater
than that of the sync router; And in terms of isolation, Asynchronous router is
also better than sync router.
Detailed testing PDF: Comparison of Async router & sync router performance.pdf
> RBF: Asynchronous router RPC
> ----------------------------
>
> Key: HDFS-17531
> URL: https://issues.apache.org/jira/browse/HDFS-17531
> Project: Hadoop HDFS
> Issue Type: New Feature
> Reporter: Jian Zhang
> Assignee: Jian Zhang
> Priority: Major
> Labels: pull-request-available
> Attachments: Async router single ns performance test.pdf, Aynchronous
> router.pdf, Comparison of Async router & sync router performance.pdf,
> HDFS-17531.001.patch, image-2024-05-19-18-07-51-282.png
>
>
> *Description*
> Currently, the main function of the Router service is to accept client
> requests, forward the requests to the corresponding downstream ns, and then
> return the results of the downstream ns to the client. The link is as follows:
> *!image-2024-05-19-18-07-51-282.png|width=900,height=300!*
> The main threads involved in the rpc link are:
> {*}Read{*}: Get the client request and put it into the call queue *(1)*
> {*}Handler{*}:
> Extract call *(2)* from the call queue, process the call, generate a new
> call, place it in the call of the connection thread, and wait for the call
> processing to complete *(3)*
> After being awakened by the connection thread, process the response and put
> it into the response queue *(5)*
> *Connection:*
> Hold the link with downstream ns, send the call from the call to the
> downstream ns (via {*}rpcRequestThread{*}), and obtain a response from ns.
> Based on the call in the response, notify the call to complete processing
> *(4)*
> *Responder:*
> Retrieve the response queue from the queue *(6)* and return it to the client
>
> *Shortcoming*
> Even if the *connection* thread can send more requests to downstream
> nameservices, since *(3)* and *(4)* are synchronous, when the *handler*
> thread adds the call to connection.calls, it needs to wait until the
> *connection* notifies the call to complete, and then Only after the response
> is put into the response queue can a new call be obtained from the call queue
> and processed. Therefore, the concurrency performance of the router is
> limited by the number of handlers; a simple example is as follows: If the
> number of handlers is 1 and the maximum number of calls in the connection
> thread is 10, then even if the connection thread can send 10 requests to the
> downstream ns, since the number of handlers is 1, the router can only process
> one request after another.
>
> Since the performance of router rpc is mainly limited by the number of
> handlers, the most effective way to improve rpc performance currently is to
> increase the number of handlers. Letting the router create a large number of
> handler threads will also increase the number of thread switches and cannot
> maximize the use of machine performance.
>
> There are usually multiple ns downstream of the router. If the handler
> forwards the request to an ns with poor performance, it will cause the
> handler to wait for a long time. Due to the reduction of available handlers,
> the router's ability to handle ns requests with normal performance will be
> reduced. From the perspective of the client, the performance of the
> downstream ns of the router has deteriorated at this time. We often find that
> the call queue of the downstream ns is not high, but the call queue of the
> router is very high.
>
> Therefore, although the main function of the router is to federate and handle
> requests from multiple NSs, the current synchronous RPC performance cannot
> satisfy the scenario where there are many NSs downstream of the router. Even
> if the concurrent performance of the router can be improved by increasing the
> number of handlers, it is still relatively slow. More threads will increase
> the CPU context switching time, and in fact many of the handler threads are
> in a blocked state, which is undoubtedly a waste of thread resources. When a
> request enters the router, there is no guarantee that there will be a running
> handler at this time.
>
> Therefore, I consider asynchronous router rpc. Please view the *pdf* for the
> complete solution.
>
> Welcome everyone to exchange and discuss!
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