Proposal for Hybrid Threading Model and simpler Async IO

[Joel Richard]

Hi David,

Thank you for your response. I have started a new thread here:
http://cs.oswego.edu/pipermail/concurrency-interest/2014-May/012638.html

I am aware that the required changes would be complicated, but I think it
is worth a discussion since concurrency, non-blocking IO and high
scalability will be even more important in the future. This feature would
make it much easier to fulfill such requirements. For example, it would not
be necessary anymore to create a second version of each so far blocking
API. Also all standardized APIs like JDBC or JPA would suddenly all have a
non-blocking implementation. The amount of work which could be saved would
be enormous.

Regards, Joel


On Sun, May 4, 2014 at 2:46 AM, David Holmes <david.holmes at oracle.com>wrote:

> Hi Joel,
>
> The best place to raise threading issues/discussions is on Doug Lea's
> concurrency-interest mailing list.
>
> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest
>
> However I don't think this proposal will get much traction. There is
> already async I/O in the core Java libraries. And making the
> synchronization primitives non-blocking would require a completely
> different implementation within the VM - something which would be far too
> destabilizing to ever consider without some very significant benefit.
>
> Further fork-join tasks already go some way towards this, though with a
> more limited programming model.
>
> David Holmes
>
>
> On 3/05/2014 2:34 AM, Joel Richard wrote:
>
>> Hi,
>>
>> Recently I have researched a little about how different languages
>> implement
>> async IO because I wanted to implement a high scalability web server. When
>> I looked at Google Go, I realized that the common callback approach is
>> actually just a language-level workaround to fix a VM issue.
>>
>> So I wondered how it could be possible to implement something similar to
>> goroutines for the JVM. Here are my thoughts: There should be a class
>> ThreadRoutine which implements the Runnable interface. If you use any (so
>> far) blocking methods in it (like Thread.sleep, network/file system IO,
>> Object.wait and synchronization) it uses however not the blocking native
>> implementation, but an async version of it. While it waits for the method
>> to complete, it can run another ThreadRoutine in the same native thread.
>> This means that all ThreadRoutines can share a small native thread pool.
>> If
>> longer computations have to be done, the developer can either call
>> Thread.yield() from time to time or just moves the computation to a
>> classical thread.
>>
>> With such an approach it would be much easier to write highly scalable
>> server software* and it would make it even possible to run most existing
>> libraries with minimal changes as asynchronous IO libraries. Basically,
>> they would just have to make sure that they call Thread.yield() in longer
>> computationally intensive loops and don't start additional normal threads.
>> Last but not least, the behavior of existing threads wouldn't be changed
>> at
>> all.
>>
>> * The advantage of async IO compared to blocking IO is that it doesn’t
>> require expensive thread context switches and that it needs less memory
>> (because fewer threads allocate less memory for stacks).
>>
>> As mentioned above, this would require implementing an asynchronous
>> version
>> of all blocking native methods. I am not that familiar with JNI nor C, but
>> I think this could by achieved by allowing the developer to define a
>> second
>> C function with the _async suffix which calls a callback instead of
>> returning a value. If the Java native method is called in a normal thread,
>> the old C function is called and otherwise the _async version of it. Since
>> Java already supports async IO with NIO, I think it should be technically
>> possible to implement asynchronous versions for almost all of the
>> currently
>> blocking native functions. If an operating system doesn't support certain
>> non-blocking operations, it could just block a special thread instead (as
>> far as I know Go does it similar).
>>
>> What do you think about this idea? Has something like that been discussed
>> before?
>>
>> Regards, Joel
>>
>> P.S. I wasn't sure which mailing list is the correct for such an idea. If
>> it is completely wrong here, please forward it.
>>
>>