[LAD] "enhanced event port" LV2 extension proposal
wdev at foltman.com
Tue Nov 27 10:46:26 UTC 2007
Dave Robillard wrote:
> Why not make it satisfy most everyone by being extensible?
It *is* extensible. Note that commands 0x00-0x6F and 0x73-0x7F are
unused, so further extensions are free to define them (perhaps we need a
scheme for binding extension URIs to command numbers, to make it more
LV2-ey). And 0x72 command can be used for pretty much any data larger
than 8+2 octets.
While it's "efficient first" and not "generic first", so to speak, it
should be fine for the intended uses.
> The idea of a generic event port is not a bad one,
I think it's not just "not a bad one". The other possibility (multiple
event ports) is less efficient, and speed is crucial here. It's also
more complex, looking from plugin author's perspective. So I had little
> idea at all (no matter what, someone is going to want to put something
> in there you didn't think of.
Please don't jump to conclusions, and take more time to read and analyse
Of course, it is possible to add new event types with arbitrary length
data, and the limitation of 8 octets per extended block is not that bad,
because you can always fit an interface pointer (32-bit or 64-bit) there.
Just look how binary data extension is implemented.
Notice that I just took the approach of optimizing for most common case
(MIDI events), and tried to maximize functionality while keeping block
size small and constant (to avoid pointer arithmetic that was
complicating Lars' proposal a bit).
> Trying to pre-define things from the top down like this is un-lv2-ey).
Well, sometimes you need to find the right tradeoff between being
efficient (memory- and speed-wise) and generic. I think I've found an
acceptable tradeoff (definitely favouring speed, but not losing
generality and not very memory-hungry).
However, I had to make some assumptions about how it will be used
(mostly implemented by inexperienced people, mostly used for MIDI and
float parameters, seldom used for advanced stuff). Oh well, I'm
repeating myself here :)
I think those are correct assumptions, but you seem to have a different
angle for looking at those things. Well, it took me years (and
failed/inadequate designs) to grow out of the "everything should be as
generic as possible" approach, so I understand why you're doing that,
but I still prefer the priority-based optimization approach that I've used.
I still think my proposal could be improved, and I don't like some
decisions that I made (basically, I made them because the alternatives
looked even more nasty), but stripping off optimizations is not the way
to go, IMO.
> Something more appropriate (IMO) might be like:
> struct LV2_EVENT
> ev_stamp_t time; ///< (ignoring the timestamp type issue)
> ev_type_t type; ///< (again ignoring type issue)
> size_t buf_size; ///< size of buf in bytes
> char* buf; ///< raw event data
You're suggesting a "classic" textbook chunked data approach, which
works, no doubt. However, it has some problems with it, which might not
be considered very major, but seem to make my approach slightly more
- too much data to be accessed in the most common use case (in 32-bit
environment, 16 bytes of header plus event data possibly in distant
memory); we don't need to save every byte of RAM, but when you need to
read and write twice as much RAM as you could, then maybe it's worth
- separation of event header and event data in the most common case; it
would be better not to cause cache thrashing too much
- it encourages memory fragmentation (experienced people will allocate
event data for all events in the same buffer, wonder about inexperienced
ones, one malloc per event data? :) )
- it doesn't deal with large data properly (because the plugin cannot
start "owning" the raw event data instead of copying it from the buffer
provided); imagine copying a video buffer in the process() function of a
I'm not saying that approach is Really Bad - just that it's kind of a
pre-optimization version of my proposal (I made MIDI data very
efficient, float parameter data slightly less efficient, float parameter
data with deltas even less efficient, and binary data are pretty
inefficient :) ).
The fact that event has to be handled is annoying enough on its own :) -
I have to end the inner loop, store state information somewhere etc. - I
don't want some additional, unnecessary memory accesses which may throw
sample data and buffers out of the cache.
> (Obviously just a quick generic knock-off to get the idea across). In
> networkey terms, this is separating transport from contents, which is
> pretty firmly established as a Good Idea.
In network context, yes. However, _optimizing_ for uncommon case is not
a preferable approach to me.
The arbitrary binary data command (0x72) mentioned in my proposal can
give you practically everything you need, and can be used in a
network-transparent way, as long as data in the "binary data" chunks are
self-contained (don't refer to other buffers).
However, my proposal lacks any mechanism to be used for serializing
arguments of future commands defined by extensions.
Still, that problem was solved many times in history, by deriving extra
interfaces for the new commands from an interface that provides
reference counting and marshalling. IPersistStream type stuff, for the
victims of Microsoft APIs.
It is a bit complex (or at least not as trivial as plain MIDI), but it
would be only used in hosts and complex plugins that use extension
events, so I guess it'd be fine.
> I very strongly feel that if 'more than MIDI' events are going to be
> mixed with MIDI events in the same port (s/MIDI/whatever), then the
> event transport mechanism needs to be 100% event type agnostic.
On the other hand, "100% generic" means "almost 100% unoptimized". By
throwing away extra information, you often throw away the chances for
optimization, so to speak.
Instead of thinking in terms of MIDI vs non-MIDI, try thinking of "my"
event types as "short" (8 octets), "medium" (16 octets) and "large"
(arbitrary-sized blobs). The fact that all short events are MIDI events
is, I think, less important. It's also not set in stone.
> It's the same approach LV2 takes with ports, and it works beautifully there.
On the other hand, it deals with a trivial problem, and solves it in a
complex way. That's not an engineer's dream :)
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