RFR: 8207851 JEP Draft: Support ByteBuffer mapped over non-volatile memory

Andrew Dinn adinn at redhat.com
Thu Sep 27 09:23:05 UTC 2018

On 26/09/18 17:00, Alan Bateman wrote:
> The reason that we've mentioned it a few times is because it's a
> significant issue. If you have a byte buffer then you can't have
> different threads accessing different parts of the buffer at the same
> time, at least not with any of the relative get/put methods as they
> depend on the buffer position. Sure you can globally synchronize all
> operations but you'll likely want much finer granularity. This bugbear
> comes up periodically, particularly when using buffers for cases that
> they weren't really designed for. Stuart pointed out the lack of
> absolute bulk get/put operations which is something that I think will
> help some of these cases.

Ok, I see that there is an issue here where only byte puts at absolute
positions can be performed concurrently (assuming threads know how to
avoid overlapping writes) while, by contrast, cursor-based byte[] stores
require synchronization. Is that the problem in full? Or is there still
more that I have missed?

I certainly agree that a retro-fit to ByteBuffer which provided for
byte[] puts at absolute positions would be of benefit for this proposal.
However, such a retro-fix would be equally as useful for volatile memory
buffers. I am not clear why this omission suggests to you that we should
look at a new, alternative model for managing this particular type of
mapped memory rather than just fixing the current one properly for all

>> Also, can you explain what you mean by confinement? (thread
>> confinement?).
> Yes, thread vs. global. I haven't been following Panama close enough to
> say how this is exposed in the API.

Well, my vague stab was obviously in the right ballpark but I'm afraid I
still don't know what baseball is. Could you explain what you mean by

>> Also, I don't think I would label this API an attempt to develop a file
>> system. I think that's rather and overblown characterisation of what it
>> does.
> I think you may have mis-read my mail as was just picking another
> example where MBB would be problematic.

Apologies for my very evident confusion here. I'd be very grateful if
you could talk down a notch or two and/or amplify a bit more to help the
hard of thinking.

>> I'm still not quite sure where this reply leaves the JEP though. Shall I
>> update the Risks and Assumptions section to include mention of
>> JDK-5029431 as suggested to Stuart? Is there anything else I can do to
>> progress things?
> It wouldn't do any harm to have this section mention that an alternative
> that exposes a more memory centric API may be possible in the future.
Ok, I'll certainly add that.


Andrew Dinn
Senior Principal Software Engineer
Red Hat UK Ltd
Registered in England and Wales under Company Registration No. 03798903
Directors: Michael Cunningham, Michael ("Mike") O'Neill, Eric Shander

More information about the core-libs-dev mailing list