Memory Mapped Segment with offsets into the underlying file

[Uwe Schindler]

Cool. Will try that out! Would it work the same way with mmapped files?

Do I need to compile on my own or do you have a prebuilt binary?

Where's the VM patch, I'm interested to see how it's done.

Uwe

Am September 10, 2020 8:12:12 PM UTC schrieb Maurizio Cimadamore <maurizio.cimadamore at oracle.com>:
>Sorry for resurrecting this old thread.
>
>We have just pushed a patch which enables safe support for shared
>memory 
>segments.
>
>In order to remove ownership from a segment you have to call the 
>`withOwnerThread(null)` method - e.g.
>
>MemorySegment shared = 
>MemorySegment.allocateNative(100).withOwnerThread(null);
>
>This segment will be accessible from any thread, and also closeable
>from 
>any thread. Some VM magic prevents hard crashes - but accessing threads
>
>which attempt to access the segment in a racy way while it's being 
>closed might fail with an IllegalStateException.
>
>It would be nice if you could give this a try, and let us know how it 
>works for your use case.
>
>Cheers
>Maurizio
>
>On 02/07/2020 00:49, Maurizio Cimadamore wrote:
>>
>> On 01/07/2020 23:44, Johannes Lichtenberger wrote:
>>> The only thing, which is missing right now might be sharing a 
>>> MemorySegment between threads, whereas I'm making sure that only
>ever 
>>> one thread reads or writes (but that's something where I have to
>wait 
>>> for Java 15 or even later I guess). A basic operation SirixDB is 
>>> offering is a timer based auto-commit with the 
>>> ScheduledExecutorService, which commits in another thread than the 
>>> main parent thread.
>>
>> This is at the top of our list of things to work on see [1] for some 
>> background. The trick is to find a way to have shared segment which 
>> work at speed, and retain some form of deterministic deallocation.
>>
>> In the meantime though, with the Java 15 API you can workaround the 
>> limitation by using the MemorySegment::ofNativeRestricted factory - 
>> which allows you to set a "null" owner thread (so it's effectively a 
>> shared segment, but an unsafe one, so if you manipulate it
>incorrectly 
>> you can crash the VM).
>>
>> Maurizio
>>
>> [1] - 
>>
>https://mail.openjdk.java.net/pipermail/panama-dev/2020-May/009004.html
>>