RFR: 8287788: Implement a better allocator for downcalls [v18]

[Jorn Vernee]

On Thu, 23 Jan 2025 17:36:11 GMT, Matthias Ernst <duke at openjdk.org> wrote:

>> Certain signatures for foreign function calls (e.g. HVA return by value) require allocation of an intermediate buffer to adapt the FFM's to the native stub's calling convention. In the current implementation, this buffer is malloced and freed on every FFM invocation, a non-negligible overhead.
>> 
>> Sample stack trace:
>> 
>>    java.lang.Thread.State: RUNNABLE
>> 	at jdk.internal.misc.Unsafe.allocateMemory0(java.base at 25-ea/Native Method)
>> ...
>> 	at jdk.internal.foreign.abi.SharedUtils.newBoundedArena(java.base at 25-ea/SharedUtils.java:386)
>> 	at jdk.internal.foreign.abi.DowncallStub/0x000001f001084c00.invoke(java.base at 25-ea/Unknown Source)
>> ...
>> 	at java.lang.invoke.Invokers$Holder.invokeExact_MT(java.base at 25-ea/Invokers$Holder)
>> 
>> 
>> To alleviate this, this PR implements a per carrier-thread stacked allocator.
>> 
>> Performance (MBA M3):
>> 
>> 
>> Before:
>> Benchmark                    Mode  Cnt   Score   Error  Units
>> CallOverheadByValue.byPtr    avgt   10   3.333 ? 0.152  ns/op
>> CallOverheadByValue.byValue  avgt   10  33.892 ? 0.034  ns/op
>> 
>> After:
>> Benchmark                    Mode  Cnt  Score   Error  Units
>> CallOverheadByValue.byPtr    avgt   30  3.311 ? 0.034  ns/op
>> CallOverheadByValue.byValue  avgt   30  6.143 ? 0.053  ns/op
>> 
>> 
>> `-prof gc` also shows that the new call path is fully scalar-replaced vs 160 byte/call before.
>
> Matthias Ernst has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix test under VThread factory

As to what's causing the issue, I wonder if the stack buffer is being freed while it is still in use, somehow. We reinterpret the segment from the shared arena, maybe the frame should keep that arena alive until it is closed itself (with a `Reference.reachabilityFence`). Looking at the code, the shared per-thread arena should be strongly reachable though, so I'm not sure if that is a plausible explanation...

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PR Comment: https://git.openjdk.org/jdk/pull/23142#issuecomment-2627576484