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Hi Jaroslav,
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<div class="">Exposing the liveness number that we already have in ZGC gc logs in a JFR event sounds pretty straight forward and not controversial to me. It’s worth mentioning though that generational ZGC is around the corner, so might be good to make sure
it fits in there as well. While liveness information will still be logged in generational ZGC, it is tracked per generation collection, and there is a one-to-many relationship between GC cycles and generation collections.</div>
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<div class="">We have defined the model such that there are minor vs major GCs. A minor collection always collects the young generation only, while a major collection collects the young generation (at least once but sometimes twice) and the old generation.
The young collections will log liveness for the young generation, and the old collections will log liveness for the old generation. So we don’t get liveness of the entire system in that sense. But having said that, exposing in events the liveness info we log
seems not controversial to me.</div>
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<div class="">With that said, I agree this sounds like useful information and don’t mind if you want to tinker with it. You can check out the generational ZGC code here: <a href="https://github.com/openjdk/zgc/tree/zgc_generational" class="">https://github.com/openjdk/zgc/tree/zgc_generational</a></div>
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<div class="">Erik Duveblad is currently tinkering a little bit with the JFR code for generational ZGC, and sent an email to jmc-dev about what we would like to do there.</div>
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<div class="">/Erik</div>
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<div class="">On 12 Jan 2023, at 16:56, Jaroslav Bachorík <<a href="mailto:jaroslav.bachorik@datadoghq.com" class="">jaroslav.bachorik@datadoghq.com</a>> wrote:</div>
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<div dir="ltr" class="">Hello,
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<div class="">I am trying to resurrect my PR (<a href="https://github.com/openjdk/jdk/pull/2579" class="">https://github.com/openjdk/jdk/pull/2579</a>) about adding a JFR event for live set size but in a less ambitious form.</div>
<div class="">Having this information in a JFR recording would be extremely useful for Datadog but I am sure that other perf tools vendors might be interested in this as well. <br class="">
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<div class="">Instead of trying to add this event to all GC implementations I would like to circle back and add this event to a single implementation at a time, given that adding such an event would not be controversial or negatively affect the GC implementation
by doing significantly more work just in order to compute the live set size.</div>
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<div class="">I would like to start with ZGC where the live set size is already known and quite easy to use without any additional computation.</div>
<div class="">From my previous work on this problem it seems like adding this event for Shenandoah, ParallelGC and Epsilon should also not create a lot of controversy.<br class="">
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<div class="">I am looking for feedback before investing more time in this effort - I would really like to avoid the situation I experienced with my previous PR when it was basically shot down after quite a lot of time spent on developing and reviewing it.</div>
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<div class="">Cheers,</div>
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<div class="">-JB-</div>
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