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<p class="MsoNormal"><span style="color:black">Good afternoon everyone,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black">I checked all video presentations and slides by Alex Shipilev and Roman Kennke about ShenandoahGC to find the answer for my question with no luck. I am trying to find more details about transitions between modes
in ShenandoahGC</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">I am looking for solution to assess concurrent collector health in real time using different metrics.</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">Here is the schema of transitions, and allocation failure causes degenerated GC cycle, but it does not mention allocation pacing at all:</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"><a href="https://github.com/openjdk/jdk/blob/fba763f82528d2825831a26b4ae4e090c602208f/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp#L361" title="https://github.com/openjdk/jdk/blob/fba763f82528d2825831a26b4ae4e090c602208f/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp#L361"><span style="color:#0563C1">https://github.com/openjdk/jdk/blob/fba763f82528d2825831a26b4ae4e090c602208f/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp#L361</span></a></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">I tried to dig further into this logic, but need your help to put all the pieces together</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">I was not able to effectively trace entry point, but this might work, allocation on heap outside of TLAB:</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"><a href="https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shared/memAllocator.cpp#L258" title="https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shared/memAllocator.cpp#L258"><span style="color:#0563C1">https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shared/memAllocator.cpp#L258</span></a></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">in case of ShenandoahGC I assume we call</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"><a href="https://github.com/openjdk/jdk/blame/739769c8fc4b496f08a92225a12d07414537b6c0/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L901" title="https://github.com/openjdk/jdk/blame/739769c8fc4b496f08a92225a12d07414537b6c0/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L901"><span style="color:#0563C1">https://github.com/openjdk/jdk/blame/739769c8fc4b496f08a92225a12d07414537b6c0/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L901</span></a></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">which then calls</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"><a href="https://github.com/openjdk/jdk/blame/739769c8fc4b496f08a92225a12d07414537b6c0/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L821" title="https://github.com/openjdk/jdk/blame/739769c8fc4b496f08a92225a12d07414537b6c0/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L821"><span style="color:#0563C1">https://github.com/openjdk/jdk/blame/739769c8fc4b496f08a92225a12d07414537b6c0/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L821</span></a></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">if mutator is allocating and pacer enabled (default) we enter Pacer:</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"><a href="https://github.com/openjdk/jdk/blame/739769c8fc4b496f08a92225a12d07414537b6c0/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L828" title="https://github.com/openjdk/jdk/blame/739769c8fc4b496f08a92225a12d07414537b6c0/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L828"><span style="color:#0563C1">https://github.com/openjdk/jdk/blame/739769c8fc4b496f08a92225a12d07414537b6c0/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L828</span></a></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"><a href="https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp#L229" title="https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp#L229"><span style="color:#0563C1">https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp#L229</span></a></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">and I assume try to handle it nicely, if not we start pacing:</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"><a href="https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp#L253" title="https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp#L253"><span style="color:#0563C1">https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp#L253</span></a></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">I have few questions here:</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">- Could you please explain a bit how the system of taxes works? I assume mutators claim budget, while GC replenishes it async, but the details are missing and no comments in the code</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">- To pace we use wait function from Monitor class</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"><a href="https://github.com/openjdk/jdk/blame/master/src/hotspot/share/runtime/mutex.cpp#L232" title="https://github.com/openjdk/jdk/blame/master/src/hotspot/share/runtime/mutex.cpp#L232"><span style="color:#0563C1">https://github.com/openjdk/jdk/blame/master/src/hotspot/share/runtime/mutex.cpp#L232</span></a></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> but the first thing it gets current Java thread. Does that mean that each Java thread goes throw runtime -> heap to allocate, and that's how pacer paces it? So we just pace any allocating thread and threads that allocate more will
just hit this code more often.</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">- Pacer uses ShenandoahPacingMaxDelay (10ms) as max, but pace_for_allocation returns void<span class="apple-converted-space"> </span></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"><a href="https://github.com/openjdk/jdk/blob/fba763f82528d2825831a26b4ae4e090c602208f/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L826" title="https://github.com/openjdk/jdk/blob/fba763f82528d2825831a26b4ae4e090c602208f/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L826"><span style="color:#0563C1">https://github.com/openjdk/jdk/blob/fba763f82528d2825831a26b4ae4e090c602208f/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L826</span></a></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"><a href="https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp#L225" title="https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp#L225"><span style="color:#0563C1">https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp#L225</span></a></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">so I assume if there is no budget available it will pace a thread for up to 10ms, but it does not imply allocation failure.</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">Heap class tries to allocate under lock and if unsuccessful considers this as allocation failure and handles it by calling ShenandoahControlThread. Does it mean that Pacer can’t cause GC to switch to degenerated mode or I am missing
something?</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">- If Pacer doesn't have budget to allocate memory it paces thread, but is there any global budget for pacing time or it is only per thread max (ShenandoahPacingMaxDelay)?<span class="apple-converted-space"> </span></span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">- It would really nice if you can sched some light on these transitions: Concurrent Mode -> Pacing (single thread and total pacing time for all threads) -> most importantly logic of transitioning from pacing to degenerated GC</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">I am trying to build a model which can tell me whether GC is healthy (fully concurrent), a bit unhealthy (pacing), unhealthy (degenerated or full GC) and how close are to the edge of the next state (a bit unhealthy -> unhealthy)</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">No rush and thanks a lot in advance.</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">Regards,</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">Alex Dubrouski</span><span style="font-size:13.5pt;color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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