[Opensim-dev] Question about the udp receiver algorithm
Diva Canto
diva at metaverseink.com
Sat Apr 26 04:50:32 UTC 2014
That is one very specific and unique case, something that happens in the
beginning, and that is necessary, otherwise clients crash. It's an
"exception" wrt the bulk of processing UDP packets. The bulk of them are
processed as you described in your first message: placed in a queue,
consumed by a consumer thread which either processes them directly or
spawns threads for processing them.
In general, my experience is also that limiting the amount of
concurrency is a Good Thing. A couple of years ago we had way too much
concurrency; we've been taming that down.
As Dahlia said, the packet handling layer of OpenSim is really critical,
and the viewers are sensitive to it, so any drastic changes to it need
to go through extensive testing. The current async reading is not bad,
as it empties the socket queue almost immediately. The threads that are
spawn from the consumer thread, though, could use some rethinking.
On 4/25/2014 9:29 PM, Matt Lehmann wrote:
> One example of what I'm trying to say.
>
> In part of the packet handling there is a condition where the server
> needs to respond to the client, but does not yet know the identity of
> the client. So the server responds to the client and then spawns a
> thread which loops and sleeps until it can identify the client.( I
> don't really understand what's going on here,)
>
> Nevertheless in this case you could do without the new thread if you
> queued a lambda function which would check to see if the client can be
> identified. A second event loop could periodically poll this function
> until it completes.
>
> You could also queue other contexts which would complete the handling
> of other types of packets.
>
> Matt
>
> On Friday, April 25, 2014, Dahlia Trimble <dahliatrimble at gmail.com
> <mailto:dahliatrimble at gmail.com>> wrote:
>
> From my experience there are some things that need to happen as
> soon as possible and others which can be delayed. What needs to
> happen ASAP:
> 1). reading the socket and keeping it emptied.
> 2) acknowledge any received packets which may require such
> 3) process any acknowledgements sent by the viewer
> 4) handle AgentUpdate packets. (these can probably be filtered for
> uniqueness and mostly discarded if not unique).
>
> This list is off the top of my head and may not be complete. Most,
> if not all, other packets could be put into queues and process as
> resources permit without negatively affecting the quality of the
> shared state of the simulation.
>
> Please be aware that viewers running on high-end machines can
> constantly send several hundred packets per second, and that under
> extreme conditions there can be several hundred viewers connected
> to a single simulator. Any improvements in the UDP processing
> portions of the code base should probably take these constraints
> into consideration.
>
>
> On Fri, Apr 25, 2014 at 8:17 PM, Matt Lehmann <mattlehma at gmail.com
> <javascript:_e(%7B%7D,'cvml','mattlehma at gmail.com');>> wrote:
>
> That makes sense to me.
>
> If I recall, the packet handlers will create more threads if
> they expect delays, such as when waiting for a client to
> finish movement into the sim.
>
> Considering that I have 65 threads running on my standalone
> instance, with 4 cores that leaves about 15 threads competing.
> You have to do the work at some point.
>
> Matt
>
> On Friday, April 25, 2014, Dahlia Trimble
> <dahliatrimble at gmail.com
> <javascript:_e(%7B%7D,'cvml','dahliatrimble at gmail.com');>> wrote:
>
> Depends on what you mean by "services the packets".
> Decoding and ACKing could probably work well in a socket
> read loop but dispatching the packet to the proper part of
> the simulation could incur many delays which can cause a
> lot of packet loss in the lower level operating system
> routines as the buffers are only so large and any
> excessive data is discarded. Putting them in a queue for
> another thread to service is a good compromise which tends
> to keep things working under most load conditions.
>
> However, if your design seems to improve things under a
> wide range of operating conditions, feel free to submit a
> patch! But please don't expect it to be accepted as it may
> need a *lot* of testing to show it's benefit over the
> current implementation.
>
>
>
>
> On Fri, Apr 25, 2014 at 7:45 PM, Matt Lehmann
> <mattlehma at gmail.com> wrote:
>
> As of the last release, the algorithm for handling udp
> packets from clients is as so...
>
> Start an async read cycle on the socket, adding
> packets to a blocking queue.
> Also start a smart thread which waits on the queue
> and services the packets.
>
> Wouldn't it be more efficient to use a single thread
> that waits on the socket by looping on a socket.poll
> call, immediately servicing packets?
>
> I have tried this locally and I really think it would
> improve efficiency. If you want I can submit a patch.
>
> Thanks
>
> Matt
>
>
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