[Public WebGL] WebGL2 and no mapBuffer/mapBufferRange

Jeff Gilbert [email protected]
Tue Mar 17 14:39:05 PDT 2015


Just warn on WRITE without INVALIDATE.

Here's what the costs look like to me:

MapBufferRange(READ) ~= GetBufferSubData:
Both require a synchronous GL command followed by a copy.

MapBufferRange(WRITE|INVALIDATE) ~= BufferSubData
MapBufferRange can create a scratch shmem for writing via ArrayBuffer, send
it across IPC on flush, and do Map+memcpy on the GL process. (1 copy)
BufferSubData is at best from an ArrayBuffer which is already shmem, and is
then a copy-on-write (ideally no-copy), but still needs to call
BufferSubData or Map+memcpy on GL process. (2 copies, but only 1 copy if
you have a heuristic which allocates shmem to ArrayBuffers)

With UNSYNCHRONIZED, MapBufferRange can be 'sharper', but potentially more
performant:
READ|UNSYNC: Still synchronous, but lets the GL process use UNSYNCHRONIZED
to prevent stalls.
WRITE|INVAL|UNSYNC: Still async, but lets the GL process use UNSYNCHRONIZED
to prevent stalls.

FLUSH_EXPLICIT lets out-of-process GL reduce the amount of data it needs to
memcpy while not having to allocate many smaller chunks with BufferSubData.
(Multiple discard+write ranges with the same single shmem scratch buffer)

WRITE without INVAL is probably much slower than WRITE|INVAL even on
in-process-GL implementations.

In light of these investigations, I'm increasingly against omitting
MapBuffer from the WebGL 2 spec.

On Tue, Mar 17, 2015 at 1:58 PM, Zhenyao Mo <[email protected]> wrote:

> On Tue, Mar 17, 2015 at 12:15 PM, Florian Bösch <[email protected]> wrote:
> > On Tue, Mar 17, 2015 at 6:35 PM, Zhenyao Mo <[email protected]> wrote:
> >>
> >> With WRITE bit, without INVALIDATE bit, for out-of-process
> >> implementations, we have to append READ bit internally, and read out
> >> the whole buffer range, send it to the js, so it can be written to
> >> partially.  Otherwise, how can you write back in unmap time? unless
> >> you keep track of which elements in the buffer range have been written
> >> to and which remain untouched.
> >
> > You're thinking of one particular way to implement it. The way you're
> > thinking of, is to synchronize the copy, and then flush the whole copy on
> > unmap. That's why you're talking of readback for write.
> >
> > Readback for write makes no sense. An alternative to this necessarily
> slow
> > and cumbersome idea, would be to transfer the data to be written, and
> write
> > that data to the appropriate underlying mapped range. No readback, no
> huge
> > in/out of process performance differences.
> >
> > The queue of writes to perform doesn't need to be individually
> transferred
> > and performed either, an out of process implementation would be free to
> > delay writes to the underlying till it's queued/collated enough writes
> into
> > a block for IPC transfer.
>
> I am aware of this alternative, but that's not currently supported by
> js.  Basically you return a writable buffer to js by MapBufferRange.
> Now you need to keep track of which part are dirty and which are
> clean.  I am not saying it's impossible, but requires new semantics
> and optimization.
>
> I don't see this can be better than BufferSubData in out-of-process
> implementations.
>
> When we expose something in core WebGL, we expect it to be implemented
> the same/similar (semantics, perf, etc) in various platforms (OSs,
> browser vendors, GPUs). If that's not possible, I think extensions are
> the better way, with the correct expectation that this (not just
> semantics, but the perf implication) may not be supported everywhere.
>
> Otherwise, let's say a developer implemented a game using
> MapBufferRange in one browser, but users from another browser can't
> play it at all even on the same hardware because it's too slow, that's
> not good for WebGL at all.
>
> There are many other examples that we have to hold back features that
> developers would love, for example, certain compressed texture formats
> not supported on iOS, etc. It may seem frustrating but in the long
> run, good for WebGL as a web standard.
>
> >
> >>
> >> For out-of-process implementations, using the invalidate bit makes a
> >> huge per difference.  for Map call, it doesn't have to wait for the
> >> service side to return the buffer range, it can just allocate a buffer
> >> and initialize to 0 and allow js to write to it.  Otherwise, it's a
> >> blocking call until service side responded with the readback buffer
> >> data.
> >
> > This argument is borne again out of this flawed idea how to do things
> which
> > you imply as a given, but it isn't, see above.
>
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