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

Francesco Nigro nigro.fra at gmail.com
Fri Sep 28 07:38:48 UTC 2018

Hi guys!

I'm one of the mentioned devs (like many others) that are using external
(and unsafe) APIs to concurrent access ByteBuffer's content and a developer
of a messaging broker's journal
that would benefit by this JEP :)
Re concurrent access API, how it looks

I don't know how's considered to appear in these discussions without
presenting myself and I hope to not be OT, but both this JEP and the
comments around are so interesting
that I couldn't resist: I apologize if I'm not respecting some rule on it

Thanks for the hard work,

Il giorno ven 28 set 2018 alle ore 09:21 Peter Levart <
peter.levart at gmail.com> ha scritto:

> Hi Stuart,
> I mostly agree with your assessment about the suitability of the
> ByteBuffer API for nice multithreaded use. What would such API look like? I
> think pretty much like ByteBuffer but without things that mutate
> mark/position/limit/ByteOrder. A stripped-down ByteBuffer API therefore.
> That would be in my opinion the most low-level API possible. If you add
> things to such API that coordinate multithreaded access to the underlying
> memory, you are already creating a concurrent data structure for a
> particular set of use cases, which might not cover all possible use cases
> or be sub-optimal at some of them. So I think this is better layered on top
> of such API not built into it. Low-level multithreaded access to memory is,
> in my opinion, always going to be "unsafe" from the standpoint of
> coordination. It's not only the mark/position/limit/ByteOrder that is not
> multithreaded-friendly about ByteBuffer API, but the underlying memory too.
> It would be nice if mark/position/limit/ByteOrder weren't in the way though.
> Regards, Peter
> On 09/28/2018 07:51 AM, Stuart Marks wrote:
> Hi Andrew,
> Let me first stay that this issue of "ByteBuffer might not be the right
> answer" is something of a digression from the JEP discussion. I think the
> JEP should proceed forward using MBB with the API that you and Alan had
> discussed previously. At most, the discussion of the "right thing" issue
> might affect a side note in the JEP text about possible limitations and
> future directions of this effort. However, it's not a blocker to the JEP
> making progress as far as I'm concerned.
> With that in mind, I'll discuss the issue of multithreaded access to
> ByteBuffers and how this bears on whether buffers are or aren't the "right
> answer." There are actually several issues that figure into the "right
> answer" analysis. In this message, though, I'll just focus on the issue of
> multithreaded access.
> To recap (possibly for the benefit of other readers) the Buffer class doc
> has the following statement:
>     Buffers are not safe for use by multiple concurrent threads. If a
> buffer
>     is to be used by more than one thread then access to the buffer should
> be
>     controlled by appropriate synchronization.
> Buffers are primarily designed for sequential operations such as I/O or
> codeset conversion. Typical buffer operations set the mark, position, and
> limit before initiating the operation. If the operation completes partially
> -- not uncommon with I/O or codeset conversion -- the position is updated
> so that the operation can be resumed easily from where it left off.
> The fact that buffers not only contain the data being operated upon but
> also mutable state information such as mark/position/limit makes it
> difficult to have multiple threads operate on different parts of the same
> buffer. Each thread would have to lock around setting the position and
> limit and performing the operation, preventing any parallelism. The typical
> way to deal with this is to create multiple buffer slices, one per thread.
> Each slice has its own mark/position/limit values but shares the same
> backing data.
> We can avoid the need for this by adding absolute bulk operations, right?
> Let's suppose we were to add something like this (considering ByteBuffer
> only, setting the buffer views aside):
>     get(int srcOff, byte[] dst, int dstOff, int length)
>     put(int dstOff, byte[] src, int srcOff, int length)
> Each thread can perform its operations on a different part of the buffer,
> in parallel, without interference from the others. Presumably these
> operations don't read or write the mark and position. Oh, wait. The
> existing absolute put and get overloads *do* respect the buffer's limit, so
> the absolute bulk operations ought to as well. This means they do depend on
> shared state. (I guess we could make the absolute bulk ops not respect the
> limit, but that seems inconsistent.)
> OK, let's adopt an approach similar to what was described by Peter Levart
> a couple messages upthread, where a) there is an initialization step where
> various things including the limit are set properly; b) the buffer is
> published to the worker threads properly, e.g., using a lock or other
> suitable memory operation; and c) all worker threads agree only to use
> absolute operations and to avoid relative operations.
> Now suppose the threads have completed their work and you want to, say,
> write the buffer's contents to a channel. You have to carefully make sure
> the threads are all finished and properly publish their results back to
> some central thread, have that central thread receive the results, set the
> position and limit, after which the central thread can initiate the I/O
> operation.
> This can certainly be made to work.
> But note what we just did. We now have an API where:
>  - there are different "phases", where in one phase all the methods work,
> but in another phase only certain methods work (otherwise it breaks
> silently);
>  - you have to carefully control all the code to ensure that the wrong
> methods aren't called when the buffer is in the wrong phase (otherwise it
> breaks silently); and
>  - you can't hand off the buffer to a library (3rd party or JDK) without
> carefully orchestrating a transition into the right phase (otherwise it
> breaks silently).
> Frankly, this is pretty crappy. It's certainly possible to work around it.
> People do, and it is painful, and they complain about it up and down all
> day long (and rightfully so).
> Note that this discussion is based primarily on looking at the ByteBuffer
> API. I have not done extensive investigation of the impact of the various
> buffer views (IntBuffer, LongBuffer, etc.), nor have I looked thoroughly at
> the implementations. I have no doubt that we will run into additional
> issues when we do those investigations.
> If we were designing an API to support multi-threaded access to memory
> regions, it would almost certainly look nothing like the buffer API. This
> is what Alan means by "buffers might not be the right answer." As things
> stand, it appears quite difficult to me to fix the multi-threaded access
> problem without turning buffers into something they aren't, or fragmenting
> the API in some complex and uncomfortable way.
> Finally, note that this is not an argument against adding bulk absolute
> operations! I think we should probably go ahead and do that anyway. But
> let's not fool ourselves into thinking that bulk absolute operations solve
> the multi-threaded buffer access problem.
> s'marks

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