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Re: [Public WebGL] Issues with sharing resources across contexts



On Wed, Jul 18, 2012 at 1:41 PM, Ben Vanik <benvanik@google.com> wrote:
> Oh, in case it wasn't clear: the objects are exclusive write - there can be
> no readers while there are writers, and vice versa.

Excellent. I believe we are actually in agreement.

> I'm not sure how you aren't seeing how immutable resources are bad. Almost
> all of the cases I (and many others) want WebGL in workers for require
> mutable resources. Resources being immutable makes them almost useless,
> except to offload what exists as single-threaded rendering today to a worker
> (which just adds additional latency and not much else).

Your read lock that excludes write makes the resource immutable. Once
all readers are released, acquisition of your write lock is explicit
resource recovery. In no cases do you employ sharing and mutability
simultaneously.

> On Wed, Jul 18, 2012 at 1:37 PM, David Sheets <kosmo.zb@gmail.com> wrote:
>>
>> On Wed, Jul 18, 2012 at 1:15 PM, Ben Vanik <benvanik@google.com> wrote:
>> > All of them ;)
>> > Almost every use case of WebGL on workers needs mutable resources.
>> >
>> > The primary ones I've used in the past (and would love to use in WebGL
>> > with
>> > workers) is modifying buffer data in various worker threads. You always
>> > want
>> > to pool the buffers and double (or triple) buffer them when rendering.
>> > They
>> > may be populated on one worker and drawn on another (or the main
>> > thread).
>> > Same with textures in the case of either a render-to-texture done on a
>> > worker or a plain old upload, or the reverse of reading back the
>> > framebuffer
>> > on another worker for JS processing.
>>
>> Except for subrange mutation, I do not see how these use cases benefit
>> from shared mutable resources over immutable resources.
>>
>> > These cases would be possible with ownership transfer, and transferable
>> > arrays has shown that it's an easy concept to wrap ones head around and
>> > works well.
>> >
>> > One thing that would suffer, though, is the case of multiple readers. I
>> > may
>> > want to do some readback of a texture for GPGPU data, and could imagine
>> > sending that texture to multiple workers - each worker would then
>> > readPixels
>> > a separate region of the texture.
>>
>> Immutable resources solves this problem.
>>
>> > The other thing that suffers with single ownership is reusing shared
>> > resources (shaders/buffers/textures) - I can imagine (and want) many
>> > cases
>> > where I'd have multiple workers rendering the same scene in different
>> > ways -
>> > whether it's a visualization tool or a game that does things like GPGPU
>> > occlusion culling in one worker and real rendering in another. Or
>> > megatexture processing using the same geometry buffers in one worker,
>> > readback of the framebuffer for processing in another, texture uploads
>> > in
>> > yet another, and final scene rendering on the main thread. If I had to
>> > duplicate all of my level geometry, rerun my CPU skinning, process my
>> > particle systems, and consume Nx as much GPU memory for each worker I'd
>> > be
>> > disappointed - especially on embedded devices.
>>
>> Immutable but shared resources solves this problem without using Nx GPU
>> memory.
>>
>> > Hmm... I feel like an acquisition model probably makes the most sense
>> > for
>> > these cases, although it's a bit more complex...
>> > - pass webgl resources to other workers like transferrable arrays (as
>> > additional args to postMessage)
>> > - a resource is not usable once received until acquired
>> > - some WebGLRenderingContext (or extension methods) to work with the
>> > objects:
>> >   - acquireResource(obj, mode [READ_ONLY, READ_WRITE]) --> returns
>> > whether
>> > acquired
>> >   - releaseResource(obj)
>> > - if you try to read or write an unacquired object, fail at API level
>> > - if you try to write to a read only object, fail at API level
>> > - multiple readers are allowed, only one writer
>>
>> What happens when I have 4 reader workers and 1 writer worker that
>> have all acquired the resource and then the writer mutates the
>> resource? Does it change underneath the readers?
>>
>> To be clear, I am proposing: (1 read-write context, no sharing) -> (0
>> read-write contexts, arbitrary sharing)
>>
>> > This allows multiple workers to render the same objects (all acquire as
>> > read
>> > only), removes the required implicit flush, and substitutes an explicit
>> > flush (an acquire for reading of a dirty object could flush, if that was
>> > tracked). Performance-wise most apps would likely acquire as read only
>> > once
>> > and never have to worry about it again (no synchronization across
>> > workers
>> > required), It's also not magic - meaning that it may require a few more
>> > calls, but it's very clear what's happening and why, and it's not
>> > possible
>> > to fall into some undiscoverable (or unavoidable) performance trap with
>> > implicit flushes.
>> >
>> >
>> > On Wed, Jul 18, 2012 at 12:52 PM, David Sheets <kosmo.zb@gmail.com>
>> > wrote:
>> >>
>> >> On Mon, Jul 16, 2012 at 6:03 PM, Gregg Tavares (社用) <gman@google.com>
>> >> wrote:
>> >> > I agree that solution would work for 2+ contexts in a single webpage
>> >> > since
>> >> > webpages are single threaded. Is that enough for now? Do we care
>> >> > about
>> >> > WebWorkers where that solution will no longer be enough?
>> >> >
>> >> > Lemme throw out some ideas. They probably suck but maybe they'll jog
>> >> > some
>> >> > others.
>> >> >
>> >> > What if a resource could only be used in 1 context at a time and you
>> >> > had
>> >> > to
>> >> > transfer ownership to use it another context? That might solve this
>> >> > issue as
>> >> > well. Would that be too restrictive? What if an object could be used
>> >> > in
>> >> > any
>> >> > context but state only set in the context that "owns" it? Not sure
>> >> > that
>> >> > would help. In the case above it would mean any context could call
>> >> > UseProgram but only the context that owns it could call AttachShaders
>> >> > and
>> >> > LinkProgram since those change state. Of course that wouldn't solve
>> >> > every
>> >> > issue because UseProgram requires the results of LinkProgram.
>> >> >
>> >> > I'm just throwing that out there. The idea is probably not workable
>> >> > but
>> >> > it
>> >> > would be nice to try to design an WebGL API for WebWorkers that
>> >> > didn't
>> >> > have
>> >> > the ambiguity that OpenGL has as far as execution order. Whatever
>> >> > that
>> >> > solution is might also work for 2 contexts in the same page.
>> >>
>> >> What if a means is provided to make a given WebGL resource immutable?
>> >> Once the resource is 'frozen', no further modification would be
>> >> allowed but the resource could be shared between contexts or workers.
>> >>
>> >> What are the use cases for shared mutable WebGL resources?
>> >>
>> >> David
>> >>
>> >> > On Mon, Jul 16, 2012 at 5:36 PM, Ben Vanik <benvanik@google.com>
>> >> > wrote:
>> >> >>
>> >> >> In previous systems I've worked with I've used implicit flushes when
>> >> >> swapping contexts and it made life much easier. Every call in the
>> >> >> graphics
>> >> >> layer basically had this:
>> >> >> void MyGraphicsAPI::SomeCall() {
>> >> >>   MakeCurrent();
>> >> >>   ...
>> >> >> };
>> >> >> void MyGraphicsAPI::MakeCurrent() {
>> >> >>   if (threadCurrentContext != this) {
>> >> >>     Flush();
>> >> >>   }
>> >> >>   // ... set global threadCurrentContext, etc
>> >> >> };
>> >> >>
>> >> >> Using some macro-fu it was made to be a single variable lookup and
>> >> >> branch
>> >> >> that predicted well per call, and in the common case of
>> >> >> single-context
>> >> >> apps
>> >> >> had little impact on performance. In apps with multiple contexts it
>> >> >> made
>> >> >> life much easier when dealing with the sometimes long and hairy code
>> >> >> sequences that touched both (rare, but often unavoidable). Had the
>> >> >> flush not
>> >> >> been implicit it would have required a significant amount of
>> >> >> bookkeeping
>> >> >> logic that every piece of code that touched a context would have to
>> >> >> interact
>> >> >> with - yuck. It also meant that code could be moved between apps
>> >> >> that
>> >> >> used
>> >> >> single contexts and multiple contexts without having to change, or
>> >> >> the
>> >> >> app
>> >> >> could even decide at runtime with no ill effect. Explicit flushes
>> >> >> would
>> >> >> have
>> >> >> made that a nightmare.
>> >> >>
>> >> >> The one downside of the implicit flushes is that it's easy to start
>> >> >> flushing all over the place without knowing about it. A simple
>> >> >> counter
>> >> >> of
>> >> >> flushes/frame was enough to help warn that they were happening
>> >> >> though,
>> >> >> as
>> >> >> the target was usually <5 and if you mess things up you would see
>> >> >> dozens.
>> >> >> It's also often much trickier to find missing explicit flushes that
>> >> >> cause
>> >> >> subtle and sometimes very hard to identify behavioral differences.
>> >> >> Just
>> >> >> look
>> >> >> at the number of WebGL pages out there today that emit warnings and
>> >> >> imagine
>> >> >> what it'd be like with this x_x
>> >> >>
>> >> >>
>> >> >> On Mon, Jul 16, 2012 at 5:18 PM, Gregg Tavares (社用)
>> >> >> <gman@google.com>
>> >> >> wrote:
>> >> >>>
>> >> >>> So ........... I've been playing around with sharing resources
>> >> >>> across
>> >> >>> WebGL contexts and I'm running into issues and looking for
>> >> >>> solutions.
>> >> >>>
>> >> >>> The biggest issue is that GL is command buffer driven and so
>> >> >>> calling
>> >> >>> some
>> >> >>> GL function doesn't mean it's actually been executed. You have to
>> >> >>> call
>> >> >>> glFlush. This raises lots of issues of where a program might work
>> >> >>> on
>> >> >>> some
>> >> >>> browser / driver / platform combo but not others if the users
>> >> >>> forgets
>> >> >>> to
>> >> >>> call flush.
>> >> >>>
>> >> >>> For example, assume I have 2 contexts sharing resources. gl1, and
>> >> >>> gl2
>> >> >>>
>> >> >>>
>> >> >>>   var vs = gl1.createShader(gl1.VERTEX_SHADER);
>> >> >>>   var fs = gl1.createShader(gl1.FRAGMENT_SHADER);
>> >> >>>   //...
>> >> >>>   // assume shaders are validly compiled
>> >> >>>   // ...
>> >> >>>   var p = gl1.createProgram();
>> >> >>>   gl1.attachShader(p, vs);
>> >> >>>   gl1.attachShader(p, fs);
>> >> >>>   gl2.linkProgram(p);
>> >> >>>
>> >> >>> I attached on gl1 but linked on gl2. There's is no guarantee in GL
>> >> >>> that
>> >> >>> that link will succeed because the 2 attach commands may have only
>> >> >>> been
>> >> >>> queued and not excuted in which case the linkProgram call will fail
>> >> >>> with
>> >> >>> "missing shaders". The correct code is
>> >> >>>
>> >> >>>   var p = gl1.createProgram();
>> >> >>>   gl1.attachShader(p, vs);
>> >> >>>   gl1.attachShader(p, fs);
>> >> >>>   gl1.flush(); // <--- important
>> >> >>>   gl2.linkProgram(p);
>> >> >>>
>> >> >>> That seems unacceptable.
>> >> >>>
>> >> >>> 2 Approaches off the of my head
>> >> >>>
>> >> >>> 1) Try to make it just work without the flush
>> >> >>>
>> >> >>> One solution might be for the implementation to track which context
>> >> >>> last
>> >> >>> had a call. Any call to a different context causes an automatic
>> >> >>> implicit
>> >> >>> flush
>> >> >>>
>> >> >>> 2) Try to make it fail when not done correctly
>> >> >>>
>> >> >>> This solution would be to try to track of an object is "dirty"
>> >> >>> (state
>> >> >>> has
>> >> >>> been changed) and no flush has been issued since then for that
>> >> >>> object.
>> >> >>> When
>> >> >>> an object is used, if it's dirty then either a flush is issued
>> >> >>> (which
>> >> >>> is
>> >> >>> solution #1) or a error is generated. ("called function on dirty
>> >> >>> object, did
>> >> >>> you forget to call flush?")
>> >> >>>
>> >> >>> Neither solution seems ideal. Worse, whatever solution is chosen
>> >> >>> also
>> >> >>> has
>> >> >>> issues if we ever get WebGL in Workers.
>> >> >>>
>> >> >>> A 3rd solution is just to leave it with the flush required and
>> >> >>> forgetting
>> >> >>> means you get random success / failure. No such a great prospect.
>> >> >>> :-(
>> >> >>>
>> >> >>> Thoughts?
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>
>> >> >
>> >
>> >
>
>

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