Introducing Collage: Barrier

While I personally think that barriers are an anti-pattern, they have exactly one valid use case in my line of work — as swap barriers synchronizing the display of a new frame across multiple segments of a display wall or immersive installation.

In an ideal work, swap synchronization would be done using hardware support. Equalizer supports this for nVidia G-Sync, but I haven’t seen many installations using hardware swap synchronization. First, it’s expensive since you need a professional grade card with a special synchronization board. So lower cost installations such as display walls typically don’t even have the hardware. Installations which need the frame synchronization, such as active stereo setups, oftentimes only use the frame (retrace) synchronization and use a software barrier for swap synchronization. The reason is that getting hardware swap synchronization running is such a mess due to driver issues that most people don’t bother. For these reasons, Equalizer both supports hardware and software swap synchronization. The software synchronization used a co::Barrier.

Back to the Collage barrier: Once it is set up, the only call needed is enter, which will block until the height has been reached.

Barriers are versioned, distributed objects. Any process can set up a barrier, register it and communicate the barrier identifier and version to all users of the barrier. The users map, sync and enter the barrier. Since it’s versioned, the master instance can be committed any time, and enter requests are versioned, that is, a barrier operation for an old version will be finished before the enter requests for the new version are processed.

The protocol right now is very simple, a master instance simply tracks enter requests until the height is reached and then unlocks all users. While there are other algorithms which use optimized tree structures or broadcast to reduce the latency, we haven’t seen the need to implement a more complex algorithm.