memorySegment.asByteBuffer().duplicate() causes multi-MB allocations

[Maurizio Cimadamore]

On 25/10/2022 12:04, Maurizio Cimadamore wrote:
> ```
> public MappedByteBuffer duplicate() {
>         return new DirectByteBuffer(this,
>                                               this.markValue(),
>                                               this.position(),
>                                               this.limit(),
>                                               this.capacity(),
>                                               0,
>
>                                               fileDescriptor(),
>                                               isSync(),
>
>                                               segment);
>     }
> ``` 

Btw, the implementation of `adReadOnlyBuffer` is virtually identical to 
the above - the only difference is that DirectBuffer::asReadOnlyBuffer 
creates a new DIrectBufferR (where R stands for Read Only), not just 
another DirectBuffer. On read-only direct buffers (DirectBufferR), 
asReadOnly buffer just calls duplicate().

So, I think it's very strange that one one of them misbehave, since the 
implementation is so close.

What do you use to monitor the allocations? One theory is that perhaps 
one version of the code "escapes" better than the other, so the numbers 
you see are skewed, and in some cases you see the actual BB allocations 
on the heap, whereas in the other case you don't see anything because 
the JVM scalarizes the BB views. But again, the `duplicate` and 
`adreadOnlyBuffer` are so close to each other that it's hard to imagine 
meaningful differences in terms of escape analysis (even though escape 
analsyis can sometimes act in surprising ways).

Maurizio