Improving next_buffer() rpc

Daniel van Vugt daniel.van.vugt at
Thu Jul 10 02:17:02 UTC 2014

Triple buffering is relevant to the conversation because it appears to 
be the only way to make full concurrent use of buffers without a round 
trip on every swap. So if we want better performance that is directly 
relevant to the protocol change.

Presently a client swap_buffers is a delay (round trip) as well as a 
minimum of 2 context switches (probably more due to our generous use of 
threads) as it sends the old buffer and waits for a new one. We can get 
that down to a best case of no round-trip and thus zero context switches 
during swap_buffers. So long as the client already has the next "free" 
buffer at its disposal without having to wait at all. It's a goal worth 
aiming for.

I know triple buffering is traditionally laggy but in the context of 
Mir's design I'm now not sure it applies here. Because the server is 
always compositing new frames as soon as it has new frames from clients. 
Even the slightest pause then, and you can be certain that the "ready" 
sub-queue is emptied and thus it's only one frame lag from client to 
server, no matter how long the overall ring of buffers is. This would 
explain why I've struggled to see any visual difference in triple vs 
double buffering, but I will retest...

On 09/07/14 23:39, Kevin Gunn wrote:
> First
> Not sure we're still on topic necessarily wrt changing from id's to fd's
> do we need to conflate that with the double/triple buffering topic ?
> let's answer this first...
> Second
> while we're at it :) triple buffering isn't always a win. In the case of
> small, frequent renders (as an example "8x8 pixel square follow my
> finger") you'll have potentially 2 extra buffers that need their 16ms of
> fame on the screen in the queue, 1 at session server, 1 at system
> server. Which can look a little laggy. I'm willing to say in the same
> breath though, that this may be lunatic fringe. The win for the triple
> buffering case is likely more common, which is spikey render times
> (14+ms) amongst more normal render times (9-12ms)
> +1 on giving empty buffers back to the clients to allow them to have a
> "queue" of empty buffers at their disposal (i'm not sure if RAOF is
> correct or duflu in that its "synchronously waiting for a round trip
> every swap"...can we already have an empty buffer queue on the client
> side ?)
> On Wed, Jul 9, 2014 at 4:35 AM, Daniel van Vugt
> <daniel.van.vugt at <mailto:daniel.van.vugt at>>
> wrote:
>     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:
>                     <>
>                     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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