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<p class="MsoNormal">Hello All, <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This previous <a href="https://lists.ozlabs.org/pipermail/openbmc/2023-May/033495.html">
thread</a> captures the motive behind our interest in chasing multi-threaded solution for bmcweb.
<o:p></o:p></p>
<p class="MsoNormal">Thanks to Ed for putting up this initial patch. <a href="https://gerrit.openbmc.org/c/openbmc/bmcweb/+/63710">
https://gerrit.openbmc.org/c/openbmc/bmcweb/+/63710</a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We have been testing this patch in the recent times and I wanted to put a summary of our observations. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<ol style="margin-top:0in" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l2 level1 lfo1">The original patch was not creating any explicit threads and we did not find boost::asio creating them for us.
<o:p></o:p></li></ol>
<p class="MsoListParagraph">So as per this <a href="https://theboostcpplibraries.com/boost.asio-scalability">
article</a> from boost I modified the patch to create a thread pool and share the same IO context among all threads. <o:p></o:p></p>
<p class="MsoListParagraph">When I tested this change, I found two problems. <o:p>
</o:p></p>
<ol style="margin-top:0in" start="1" type="a">
<li class="MsoListParagraph" style="margin-left:.25in;mso-list:l0 level1 lfo2">Sharing same IO context between multiple threads does not work.
<o:p></o:p></li></ol>
<p class="MsoListParagraph" style="margin-left:.75in">I have logged this issue <a href="https://github.com/chriskohlhoff/asio/issues/1353">https://github.com/chriskohlhoff/asio/issues/1353</a> in boost::asio git hub page with sample code to reproduce the
issue. <o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:.75in">It would be great if someone else test this sample code and share the results based on their platform.
<o:p></o:p></p>
<ol style="margin-top:0in" start="2" type="a">
<li class="MsoListParagraph" style="margin-left:.25in;mso-list:l0 level1 lfo2">Sharing dbus connection across threads is not safe:
<o:p></o:p></li></ol>
<p class="MsoNormal" style="margin-left:.75in">when we share same IO context between multiple threads, it’s possible that the async job posted by one thread, can be picked up by some other thread.
<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.75in">If thread1 makes crow::connections::systemBus().async_method_call then the response lambda can get executed in thead2’s context.
<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.75in">When thread2 is trying to read from the dbus connection, thread1 can make a new request on the same bus connection as part of handling another URI request.
<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.75in">Sdbus is not thread safe when connection object is shared between multiple threads which can perform read/write operations. <o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:.75in"><o:p> </o:p></p>
<ol style="margin-top:0in" start="2" type="1">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l2 level1 lfo1">IO Context per thread.
<o:p></o:p></li></ol>
<p class="MsoListParagraph">Since sharing IO context was not working I took the second approach mentioned in this
<a href="https://theboostcpplibraries.com/boost.asio-scalability">article</a> which is to dedicate IO context per threads.
<o:p></o:p></p>
<p class="MsoListParagraph">Major design challenge with this approach is to decide which jobs must be executed in which IO context.<o:p></o:p></p>
<p class="MsoListParagraph">I started with dedicating one thread/IO context to manage all the incoming requests and handling responses back to the clients.
<o:p></o:p></p>
<p class="MsoListParagraph">I dedicated another thread/IO context to only manage aggregate URIs which have 1K+ sensors response (MRDs) to populate and does not have tighter latency requirements.
<o:p></o:p></p>
<p class="MsoListParagraph">Our goal is to have faster response on the power/thermal URIs which is served by the main thread and is not blocked by huge response handling required by aggregate URIs which is managed by the secondary thread.<o:p></o:p></p>
<p class="MsoListParagraph">From our previous performance experiments, we had found that JSON response preparation for 5K+ sensors was taking around 250 to 300ms in bmcweb during which power/thermals URIs were blocked. <o:p></o:p></p>
<p class="MsoListParagraph"><o:p> </o:p></p>
<p class="MsoListParagraph"> <o:p></o:p></p>
<p class="MsoListParagraph"> MainThread MRD_Handler_Thread<o:p></o:p></p>
<p class="MsoListParagraph"> <span style="font-family:"Arial",sans-serif"></span>
<span style="font-family:"Arial",sans-serif"></span>
<o:p></o:p></p>
<p class="MsoListParagraph"> asio::post(request)
<o:p></o:p></p>
<p class="MsoListParagraph"> > <o:p>
</o:p></p>
<p class="MsoListParagraph">
<o:p></o:p></p>
<p class="MsoListParagraph"> asio::post(response)
<o:p></o:p></p>
<p class="MsoListParagraph"> < <o:p></o:p></p>
<p class="MsoListParagraph"> <span style="font-family:"Arial",sans-serif"></span> <span style="font-family:"Arial",sans-serif"></span><o:p></o:p></p>
<p class="MsoListParagraph"> MainThread MRD_Handler_Thread<o:p></o:p></p>
<p class="MsoListParagraph">
<o:p>
</o:p></p>
<p class="MsoListParagraph"> <o:p></o:p></p>
<p class="MsoListParagraph">Based on the URI main thread decides to continue to process the request or offload it to the MRD handler thread.
<o:p></o:p></p>
<p class="MsoListParagraph">The response received from the MRD thread is returned to the client by the main thread.
<o:p></o:p></p>
<p class="MsoNormal"> The performance results with the solution are great. We see almost 50% improvement in the performance of power/thermal URIs.
<o:p></o:p></p>
<p class="MsoNormal"> Here is performance is measured based on worst case latency seen on power thermal URIs when there are concurrent clients accessing power/thermal + MRD URIs.
<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> However, this solution seems to have some stability issues in the overnight long run tests.
<o:p></o:p></p>
<p class="MsoNormal" style="text-indent:.5in">The crash is seen around boost:post APIs in multi-threading context. I have logged a different bug in boost::asio to demonstrate this problem.
<a href="https://github.com/chriskohlhoff/asio/issues/1352">https://github.com/chriskohlhoff/asio/issues/1352</a><o:p></o:p></p>
<p class="MsoNormal" style="text-indent:.5in">I will follow up to check if boost can help us with this fix.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">What I am looking for <o:p></o:p></p>
<ol style="margin-top:0in" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:.25in;mso-list:l1 level1 lfo3">Does anyone have any different proposal for sharing IO context between threads which can work our bmc platform?
<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:.25in;mso-list:l1 level1 lfo3">Feedback on handling dbus connection between multiple threads in the context of bmcweb?<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:.25in;mso-list:l1 level1 lfo3">Is this a good model to dedicate threads based on the use case as we are not able to make IO sharing between threads work well?<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:.25in;mso-list:l1 level1 lfo3">Any better way to Post asio jobs across threads and make it stable?<o:p></o:p></li></ol>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks <o:p></o:p></p>
<p class="MsoNormal">Rohit PAI <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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