dissecting memory sessions

[Radosław Smogura]

Hi Maurizo,

Thank you for sharing this approach. I would have question related to closing arenas, and I was thinking about creating pooling arena which will return allocated segments back to the pool like in case of pooling allocator.

Firstly, minor thing, Arena class is sealed so I can’t create wrapper around it?

If memory session will not have close() method, I can close session only using Arena close() method.

However in case in pooling areana, I would have one “shared” arena – responsible for allocating native segments and it’s session will be long living session associated with pool.

I would have to create number of child arenas. This would mean that PoolingArena has to track dependencies between arenas on their own, and preventing closing Pool when there’s at least single child arena alive.

Am I’m correct?

Kind regards,
Radoslaw Smogura

From: Maurizio Cimadamore<mailto:maurizio.cimadamore at oracle.com>
Sent: Wednesday, November 2, 2022 10:47 PM
To: panama-dev at openjdk.java.net<mailto:panama-dev at openjdk.java.net>
Subject: Re: dissecting memory sessions

In case you are interested, below are some more pointers.

First, the javadoc which contains the proposed FFM API changes:

http://cr.openjdk.java.net/~mcimadamore/panama/session_handle_hidden_javadoc/javadoc/java.base/java/lang/foreign/package-summary.html

For those of you who are more interested at the code, here is the branch
that contains the code changes:

https://github.com/mcimadamore/panama-foreign/tree/session_drop_min

And, finally, here is also a jextract branch that supports the proposed
FFM API changes:

https://github.com/mcimadamore/jextract/tree/panama_session_refresh

Cheers
Maurizio

On 02/11/2022 17:48, Maurizio Cimadamore wrote:
> Hi,
> After the first preview of the FFM API in Java 19, we have identified
> a couple of areas where the API could use some improvements:
>
> * Clarify the relationship between MemorySegment and MemoryAddress
> (this was addressed in [1]); and
> * Polish the MemorySession API, and make segments easier to (safely)
> share with external clients (what this email is about).
>
> While we have explored solutions to better encapsulate memory sessions
> in the past (e.g. by dropping session accessors, as described in [2]),
> nothing seemed to stick. So, for Java 19 we decided to leave session
> accessors on memory segments in place, but give the option to
> libraries to protect against "sneaky" close, by creating non-closeable
> memory session views.
>
> After staring at this problem long enough, it became increasingly
> clear that memory sessions, in their current shape and form, are
> trying to do too much - from allocation, to lifecycle management and
> more. The issue with "sneaky" close is mostly a manifestation of that
> more fundamental problem. In that spirit, we have put together a
> document which teases apart the various "traits" associated with
> memory sessions, and repackages the same traits into an API that
> provides better encapsulation and composition. The document can be
> found here:
>
> http://cr.openjdk.java.net/~mcimadamore/panama/session_arenas.html
>
> The main move described in the document is to make MemorySession a
> "pure" lifetime abstraction, thus dropping SegmentAllocator and
> AutoCloseable capabilities. Instead, these capabilities are provided
> by a *second* abstraction, called Arena. Crucially, an Arena _has_ a
> memory session, which can e.g. be used to allocate segments that have
> the same lifecycle as that of the arena. This subtle twist, gives us
> an API that is easier to reason about (and to build upon), and one
> where memory segments can be shared freely across clients - premature
> calls to MemorySession::close are no longer possible. At the same
> time, the API now makes a much clearer distinction between sessions
> that are closeable (i.e. sessions created through an Arena) and those
> that aren't (i.e. implicit and global sessions).
>
> Here's a list of the main API changes, and how they will impact
> clients of the FFM API:
>
> * MemorySession no longer has a close() method; try-with-resources
> against MemorySession will now need to use Arena instead;
> * Support for non-closeable session views
> (MemorySession::asNonCloseable), and related methods
> (MemorySession::equals/hashCode) has been removed;
> * MemorySession::addCloseAction has been removed; instead, clients can
> specify a cleanup action when creating an unsafe segment (i.e. using
> MemorySegment::ofAddress);
> * Some of the predicates in MemorySession have been made more robust -
> e.g. instead of MemorySession::ownerThread(), there is now a predicate
> MemorySession::isOwnedBy(Thread).
>
> After careful consideration, we believe that the changes described in
> this document are worth pursuing for the upcoming Java 20 integration
> [3]: they make the API more principled (no more "sneaky" close), while
> retaining a similar expressive power.
>
> Any feedback is greatly appreciated.
>
> Cheers
> Maurizio
>
> [1] -
> https://cr.openjdk.java.net/~mcimadamore/panama/segment_address.html
> [2] -
> https://mail.openjdk.org/pipermail/panama-dev/2022-February/016152.html
> [3] -
> https://mail.openjdk.org/pipermail/panama-dev/2022-February/016152.html
>
>

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