Question: ByteBuffer vs MemorySegment for binary (de)serializiation and in-memory buffer pool

[Maurizio Cimadamore]

Hi Gavin,
we have some documents on the foreign memory API here:

https://github.com/openjdk/panama-foreign/blob/foreign-memaccess%2Babi/doc/panama_memaccess.md

They have not been updated against the latest version of the API, but 
should be relatively in good shape (at least good enough to get started).

> The API is a fair bit wordier, but that's the price you pay in 
> exchange for greater expressiveness and flexibility I suppose.
As for wordiness, I don't think the API is significantly more verbose 
than the ByteBuffer API. In fact, at a glance the methods on 
MemorySegment and ByteBuffer are quite similar, but there are two 
differences:

* when you allocate native memory you _have_ to pass an extra argument, 
the temporal bounds of the memory segments (MemorySession), which 
signals when the segment memory is ready to be released
* the access methods, MemorySegment::get/set are parameterized by a 
layout argument, so instead of `buffer.getInt(offset)`, you need 
`segment.get(JAVA_INT, offset)`. This will give more extensibility (e.g. 
add support for more carriers, or value types, at a later point) as well 
as more control (e.g. alignment)

In my experience, and also looking at code written by developers that 
migrated projects from ByteBuffer, the MemorySegment API seems more or 
less on par with ByteBuffer in terms of verbosity. There are var handles 
which you can use (as pointed out elsewhere in this thread), and, 
depending on the use case, they can make your life simpler. But you 
don't _have_ to use them unless you want to.

I hope this helps. From the looks of it, the Foreign Memory API seems 
like a great fit for the project you are working on.

Maurizio


On 20/08/2022 18:35, Gavin Ray wrote:
> Hiya, I hope it's okay that I ask questions here (if not, please 
> direct me elsewhere)
>
> I tried to ask on StackOverflow without much luck, and there aren't 
> many resources
> available at the moment to read about the Foreign Memory API so I'm 
> reaching out here.
>
> I'm implementing a basic database on the JVM and am trying to 
> understand the
> technical implications of using either ByteBuffer or MemorySegment for 
> heap page management and in-memory buffer allocation.
>
> Details are in this thread:
> database - Java: Performance of ByteBuffer versus 
> jdk.incubator.foreign (Panama) Foreign Memory methods 
> (MemoryLayout/Segment) - Stack Overflow 
> <https://stackoverflow.com/questions/73423726/java-performance-of-bytebuffer-versus-jdk-incubator-foreign-panama-foreign-me>
>
> Would appreciate any information folks would be willing to share =)
>
> Thank you,
> Gavin Ray
>
> (By the way, I'm not subscribed to the mailing list, will I be able to 
> reply to responses?)
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