Improving next_buffer() rpc

Kevin Gunn kevin.gunn at
Wed Jul 9 15:39:29 UTC 2014

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...

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> 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]
>>>>> mir-performance
>>>>> item:
>>>>> "[kdub] fencing improvements for clients add the ipc plumbing"
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