RFR: 8332516: Serial: Always sample promoted bytes to avoid getting stuck in Full GCs

[Thomas Schatzl]

On Wed, 29 May 2024 09:34:18 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> src/hotspot/share/gc/serial/tenuredGeneration.cpp line 398:
>> 
>>> 396:     promoted_in_bytes = used_after_gc - _used_at_prologue;
>>> 397:   } else {
>>> 398:     promoted_in_bytes = 0;
>> 
>> Question about this new code vs. the old code:
>> 
>> So the changed code updates promotion statistics in old gen also during full gc now. The amount of "promotion" is the difference of used in old gen after and before.
>> 
>> I am not sure this is a good idea as the full gc also reduces the amount of existing old gen live objects. Using `0` in that case (if the full gc collects more than the implicit collection of the young gen during full collection) also seems dodgy at best.
>> 
>> If my reading of the code is correct and there isn't a way to correctly measure the amount of live objects in young gen during full gc I would prefer to just not update the promotion counter during full gc.
>> 
>> Also consider "extra" full gcs like System.gc() calls and other causes for full gc that can happen at any time - they only seem to add noise to the promoted bytes sampling.
>
> The new logic updates promoted-bytes metric on every gc, keeping it up to date at best effort. Not updating it can result into using outdated info in deciding whether to young-gc or full-gc. The bm in https://bugs.openjdk.org/browse/JDK-8320165 shows that one can struck in full-gc forever due to this outdated metric.

Thanks for the clarification.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19306#discussion_r1624219990