[Vm-dev] Fwd: Memory mapped files and Pharo (aka Unreal Engine 4 integration with Pharo)

Dimitris Chloupis kilon.alios at gmail.com
Mon Jan 18 07:33:46 UTC 2016

Than you very much Eliot. I am both interested in this and threaded FFI,
and I am sure I will need your wisdom with mmap in the future, so I will be
around ;)

On Sun, 17 Jan 2016 at 23:54, Eliot Miranda <eliot.miranda at gmail.com> wrote:

> On Sun, Jan 17, 2016 at 1:12 PM, Dimitris Chloupis <kilon.alios at gmail.com>
> wrote:
>> Eliot as long as you integrate this, because my knowledge of VM is close
>> to zero, and make it a standard part of the VM I am perfectly ok with it. I
>> dont care about an optimal solution just a solution that works .
> Right.  It would become a standard facility.
>> All I want is the ability to do shared memory management from inside
>> Pharo. I am very new to all this so I am not aware of the problems and
>> limititations I will face. But I know that you are far better than me to
>> provide a good implementation.
>> I dont kid myself this will be a long process for me , it will take time
>> to get familiar with shared memory management , short out potential bugs
>> and dust off my knowledge about C++. So I am not in any hurry but willing
>> to learn how to do this in practice, which means I will make some testing
>> C++ apps to really get the hang of this.
>> That means I can wait for the threaded VM and a potential implementation
>> for shared memory access.
> Good.  But remember this and be prepared to nag when the threaded FFI
> arrives :-).  And if there are volunteers out there looking for an
> interesting project that's not too big this is a candidate.
>> So yeah your idea sounds great, the ability to GC shared memory sounds
>> awesome though I am a bit worried how I will be able to alert my C++ code
>> about this GC, so maybe a specific primitive would be better ?
> Good point.
>> On Sun, Jan 17, 2016 at 10:18 PM Eliot Miranda <eliot.miranda at gmail.com>
>> wrote:
>>> Hi Dimitris,
>>> On Sun, Jan 17, 2016 at 8:57 AM, Dimitris Chloupis <
>>> kilon.alios at gmail.com> wrote:
>>>> I attach the forwarded post I posted in pharo-users but if you want the
>>>> summary its a collection of questions whether its possible to use memory
>>>> mapped files shared memory with Pharo VM for IPC. I thought to forward also
>>>> here since this may be a VM related too. Any help is appreciated
>>>> ---------- Forwarded message ---------
>>>> From: Dimitris Chloupis <kilon.alios at gmail.com>
>>>> Date: Sun, Jan 17, 2016 at 6:41 PM
>>>> Subject: Memory mapped files and Pharo (aka Unreal Engine 4 integration
>>>> with Pharo)
>>>> To: Any question about pharo is welcome <pharo-users at lists.pharo.org>
>>>> An apology for the long post but this is not a simple issue and hence
>>>> not a simple question.
>>>> So I am looking into ways to integrate Pharo with Unreal to bring some
>>>> nuclear powered graphics to Pharo and make us first class citizents to 2D
>>>> and 3D world.
>>>> For those not aware this is unreal
>>>> https://www.unrealengine.com/what-is-unreal-engine-4
>>>> As suprising it may sound Unreal and Pharo share a lot in common
>>>> 1) Both integrated languages with IDEs
>>>> 2) Both promote live coding and live manipulation of data
>>>> 3) Both promote visual coding
>>>> 4) Both promote ease of use and wizard based development (aka RAD)
>>>> Unreal is indeed a C++ based project made to be used by C++ coders but
>>>> it comes with a very powerful visual coding language that can map visual
>>>> nodes to any C++ function/Method and another common ground with Pharo is
>>>> the very strict OO nature of both projects. But then OOP is very big in
>>>> game development anyway.
>>>> Of course my goal is not to force Pharoers to learn C++, but rather
>>>> make a Pharo API so Pharo can be used to do some simple graphics
>>>> development in the start and then we can continue importing more and more
>>>> functionality. Unreal is generally a heavy engine that requires quite a
>>>> powerful GPU but its rendering is just amazing.
>>>> So how I make Pharo talk to Unreal is the million dollar question.
>>>> The road of compiling Unreal as a set of DLLs to be loaded by Pharo via
>>>> FFI is a road full of thorns because its quite an undertaking cause Unreal
>>>> is HUGE and it will be nightmare to maintain since Unreal moves forward
>>>> very fast.
>>>> So we come to the subject of IPC, or Inter Process Communication. I am
>>>> not new to this as you know I have build a socket bridge between Pharo and
>>>> Python that allows Pharo to use Python libraries and in my implementation I
>>>> focus on Blender Python API.
>>>> But sockets are not exactly blazzing fast, calling functions is fine
>>>> because you can even get responses bellow 1 millisecond but if try to run
>>>> some heavy loops you will be in trouble. There are work arounds of course,
>>>> like sending the loop with the socket and language etc but they
>>>> overcomplicate something that at least in my opinion should remain simple.
>>>> Another IPC method is shared memory, it is what it says , basically
>>>> processes that share a place in memory where they can access the same data.
>>>> Extremely fast but it has its own traps for example how to make sure the
>>>> processes dont access the same data at the same time etc.
>>>> So after some reading I came across to a shared memory model called
>>>> Memory mapped files, basically what that means is essentially a virtual
>>>> file that resides on memory (it may also reside on the hard disk or other
>>>> permanent physical storage but its not necessary) that different processes
>>>> can access.
>>>> So in our case we can have memory mapped file that Unreal , Pharo and
>>>> even Blender can access where we can store command / messages that each
>>>> diffirent application must execute, let them share data and sky is the
>>>> limit.
>>>> Now I know this will not be a walk in the park specially to someone
>>>> like me a C++ noob but I am looking for the Pharo wisemen wisewomen to
>>>> guide me at least through the obstacles of this.
>>>> So the questions are the following
>>>> 1) Do you think this is possible with the current Pharo ?
>>> You should be able to map a file via the FFI calling mmap, and access it
>>> via an ExternalPointer.  But that's crappy.
>>>> 2) Will I be limited by the fact that the VM currently does not
>>>> multithread or cannot use multithreading libraries ? ( I have no intention
>>>> of using multithreading but some handling of process access to the data may
>>>> be necessary to make sure data is safe from concurent modification)
>>> As you've noticed we intend to provide a threaded FFI that will allow
>>> you to use threaded libraries.  But that's not really the issue.  There
>>> must be some handshaking protocol for the two halves to communicate via
>>> shared memory without conflict.  The threaded FFI should make it possible
>>> to call e.g. pthread_cond_wait to synchronise, but a lower-level
>>> test-and-set or conditional move facility would be nicer.  See below.
>>> 3) Is there anything else I should be aware of as common pitfalls for
>>>> such implementation ?
>>>> 4) Can the current FFI help in this task or would I need to implement
>>>> this as a C DLL and load it from Pharo
>>> Forgive me for diving down a bit before I answer your question, but I
>>> find being concrete helps.
>>> Spur has a segmented memory model.  When one grows the heap the VM
>>> allocates memory via mmap and integrates it with the rest of the heap by
>>> using "bridge" objects at the end of each segment.  A bridge object is two
>>> 64-bit words.  It says "I am a bytes object" so that the GC will never look
>>> inside the object, and it lies about its size, saying "I am large enough to
>>> span to the start of the next segment".  So when adding a new segment, the
>>> bridge in the segment before the new one is "shortened" to point to the
>>> start of the new segment, and a new bridge is added to the end of the
>>> segment so that it points to the start of the next segment in memory.  The
>>> last segment's bridge has a zero length.  Spur also provides pinning, as
>>> simple as a per-object flag that tells the GC not to move an object.  So
>>> bridges are pinned, and hence compaction leaves bridges unmolested, but any
>>> object in old space can be pinned also.
>>> So an elegant way of providing shared memory in Spur would be to add a
>>> "map a file as a byte array" primitive.  There are issues with this.  The
>>> first 16 bytes of the mapped file would have to be used to construct the
>>> header for a ByteArray that would comprise the rest of the segment,
>>> excepting another 16 bytes at the end that would need to be a bridge.  So
>>> if you wanted to share this with another application you'd probably want to
>>> allocate a file that was, say, 2k bytes bigger than needed, and use the
>>> first and last 1k bytes to hide the header and the bridge.
>>> Ah, better still would be to construct three objects in the segment, an
>>> initial ByteArray that stretches to 16 bytes before the second page, a
>>> ByteArray whose contents start at the beginning of the second page and
>>> reach all the way to the penultimate page, and then a ByteArray to reach
>>> from the end of the penultimate page to the bridge at the end of the
>>> mmapped file segment.  All three objects could be pinned and be prevented
>>> from being GC'ed until the entire segment was released.  That would give
>>> you a ByteArray (the middle of the three) whose contents were all but the
>>> first and last pages of the mmapped file, aligned on a file page boundary
>>> and whose length was a multiple of the page size.  C++/C clients could then
>>> map the central portion and use e.g. pointers to access it.
>>> To implement this there would be an mmap-file-as-segment primitive and
>>> perhaps a release-segment primitive, or some magic in the GC to release the
>>> segment when the ByteArrays were no longer accessed.
>>> So then I could imagine test-and-set or conditional-move primitives on
>>> ByteArray that supported manipulating locks in a ByteArray, and by
>>> extension, in the shared file.  I think this kind of approach would give
>>> you the fastest, most direct access to shared memory I can think of.  Does
>>> this appeal?
>>> PS: I have no intention of messing with the Pharo VM and I also want to
>>>> avoid the use of plugins as I want this to work with standard Pharo
>>>> distributions.
>>> _,,,^..^,,,_
>>> best, Eliot
> --
> _,,,^..^,,,_
> best, Eliot
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