RFR: 8287788: reuse intermediate segments allocated during FFM stub invocations [v8]

[Matthias Ernst]

On Wed, 22 Jan 2025 09:57:15 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 remembers and reuses up to two small intermediate buffers per carrier-thread in subsequent calls.
>> 
>> 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   10  3.291 ? 0.031  ns/op
>> CallOverheadByValue.byValue  avgt   10  5.464 ? 0.007  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:
> 
>   Back buffer allocation with a single carrier-local segment.

> just need a single buffer
> Alternatively we can use locking

I think these are really really great suggestions, thank you!
It simplifies things tremendously, I've pushed a version of it.
As you say, the errno / state capture piece can probably just use it, too.

The extra atomics for acquiring/releasing don't seem to cost that much, so this has still excellent performance (and is also alloc-free):

Benchmark                    Mode  Cnt  Score   Error  Units
CallOverheadByValue.byPtr    avgt   30  3.375 ? 0.138  ns/op
CallOverheadByValue.byValue  avgt   30  6.625 ? 0.057  ns/op


I'll leave this here for inspiration, I'll add a few unit tests for the stack, but feel free to just close it in favor of related work.

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PR Comment: https://git.openjdk.org/jdk/pull/23142#issuecomment-2606794554