Forgive me for rambling on but I just had an important realisation...
Our current desire to get back to double buffering is only because the
Mir protocol is synchronously waiting for a round trip every swap, and
somehow I thought that the buffer queue length affected time spent in
the ready_to_composite state. Now I'm not so sure that's true.
If we changed the protocol to implement parallelism, then in theory,
keeping triple buffering with a fancy zero-latency swap buffers should
perform better than the current protocol that has to wait for a round trip.
I cannot remember why I thought the length of the buffer queue affected
the time from client-rendering to server-compositing. Perhaps we really
do need to keep triple-buffering always-on so that the performance gain
of a zero-latency client swap-buffers can be achieved...
In summary, I'm back to thinking any protocol change from next_buffer()
needs to support parallelism and not be so synchronous.
- Daniel
On 09/07/14 16:08, Daniel van Vugt wrote:
Oops. I keep forgetting that the new BufferQueue disallows the
compositor to own less than one buffer, so there would no longer be any
benefit to double buffered clients from a more concurrent protocol :(
Maybe Kevin's suggestion is just fine then. So long as the server is
able to figure out the surface(Id) from the Buffer struct.
On 09/07/14 15:41, Daniel van Vugt wrote:
Note that we're working on making double-buffering the default again and
triple the exception. In that case fixing LP: #1253868 may seem
pointless, but it is surprisingly still relevant. Because a fully
parallelized design would significantly speed up double buffering too...
client swap buffers would no longer have to wait for a round-trip before
returning and would instead be almost instant.
On 09/07/14 10:00, Daniel van Vugt wrote:
Sounds better to just pass buffers around although I'm not keen on any
change that doesn't make progress on the performance bottleneck LP:
#1253868. The bottleneck is the swapping/exchanging approach which
limits the client to holding only one buffer, so I don't think it's a
good idea for new designs to still have that problem.
In order to improve parallelism per LP: #1253868 you'd really have to
receive new buffers as soon as they're free, which means getting them as
MirEvents. Then you only need an RPC function to release them back to
the server:
rpc release_buffer(Buffer) returns (Void);
Keep in mind the inter-process communication is the bottleneck here. If
you allow a context switch between the server and client then that's
half to one millisecond (see mirping) per RPC round trip. More than
double that for nested servers and you see the protocol delay could be a
significant factor. So I think any protocol enhancement should have
parallelism designed in.
I also think we need to be careful about not landing any protocol
changes to RTM candidate series' 0.4-0.5, so the foundation for RTM is
maximally mature (albeit not yet optimal).
- Daniel
On 08/07/14 21:10, Kevin DuBois wrote:
Hello mir team,
In order to get the next buffer for the client, we currently have:
rpc next_buffer(SurfaceId) returns (Buffer);
which is problematic for me in working on [1] because this implicitly
releases the buffer from the client side, whereas in working on that
performance improvement, I have to send a fd back to the server. So I
was thinking of adding an rpc method more like:
rpc exchange_buffer(Buffer) returns (Buffer);
This would be sufficient to pass the fd fence back, and the buffer
id in
the Buffer protocol message would be sufficient for the server to
figure
out which surface has sent back its buffer. (given the global buffer
id's we're using)
This does not address the problem noted in:
https://bugs.launchpad.net/mir/+bug/1253868
but I think that might be better addressed by having an exchange type
rpc call (explicit or implicit) and negotiating/increasing how many
buffers the client owns somehow else.
This seems like something that could have diverse opinions, so I'm
hoping to get some input on the protocol change here first.
Thanks!
Kevin
[1]
https://blueprints.launchpad.net/ubuntu/+spec/client-1410-mir-performance
item:
"[kdub] fencing improvements for clients add the ipc plumbing"
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