RFR: 8288966: Better handle very spiky promotion in G1 [v2]

[Ivan Walulya]

On Mon, 8 Aug 2022 12:26:16 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this change that implements "boosting" the PLAB sizes after every "significant" amount of refills.
>> 
>> This removes pause time spikes during garbage collection if the PLAB sizing algorithm has not picked up a good value yet (at startup) or the load is very spiky (e.g. SPECjbb2015).
>> 
>> This works by defining a "tolerated amount of refills", after every time this threshold is met we double the PLAB. The reason for not doing this doubling every time is waste - on tight(er) heaps this can waste a lot of memory for very small allocations, causing more GCs and effective slowdowns.
>> 
>> So this threshold is based on the number of expected refills (per thread); allowing a little more than these expected refills (and discounting the initial PLAB refill).
>> This boosting is also only active with `ResizePLAB` - it's on by default anyway with G1.
>> 
>> The CR contains a graph showing massive pause time spikes on SPECjbb2015 that are fixed by this, as otherwise during these spikes there would be 500k+ PLAB refills that in particular seem to be very harmful on aarch64 (the machines seem to be very problematic wrt to contention on a single atomic counter with lots of threads).
>> 
>> Overall performance (of SPECjbb2015, on aarch64) is slightly better with that change, and in general also improves startup pauses. x64 isn't really affected either way.
>> 
>> I'm aware that now `G1PLABAllocator` has lots of members that are arrays of the same size (the number of generations). I want to fix that to an array of structs in a separate change.
>> 
>> Thanks,
>>   Thomas
>> 
>> Testing: tier1-5
>
> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
> 
>   iwalulya review

Lgtm!

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Marked as reviewed by iwalulya (Reviewer).

PR: https://git.openjdk.org/jdk/pull/9726