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The Sky Over Berlin (and Elsewhere)

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Linden Lab

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HTTP Project Recap

Earlier this year we blogged about the HTTP project and how, step-by-step, the project is overcoming various limitations. Viewer release 3.4.3 introduced a new HTTP library that made better use of network resources. Texture fetches were the first operations to take advantage of this library, which improved throughput while using fewer connections. But the viewer was still constrained by a one-request-per-connection model.

Changing that model required back-end modifications. Those shipped early in 2013 in the DRTSIM-203 simulator release. For the first time, texture fetches could re-use existing HTTP connections. And for most users, this doubled the theoretical maximum texture request rate.

But all the world is not textures, and mesh fetches were next to receive attention. Meshes required quite a bit more work. Both back-end and viewer engineering was needed, culminating in viewer release 3.7.2. This release brought mesh fetching into behavioral parity with textures.

req.png

These releases have brought the viewer up to the request rate limits of region 'C'. Here, the limits are dictated by serialization, distance, and the speed of light. We are preparing to move beyond this region with changes to concurrency and locality. HTTP request concurrency will be vastly increased by the introduction of HTTP pipelining. Locality will be changed by the use of a Content Delivery Network (CDN) to move texture and mesh data nearer to most users.

 

Pipelining

Common HTTP communication is a simple back-and-forth exchange of requests and responses. A request is issued, a response is returned, and only then is another request issued. As distance increases between endpoints, the time to perform this ping-pong increases, which lowers the effective request rate. Pipelining attacks this distance-induced loss by issuing multiple requests at once without waiting for responses.

The pipelining viewer will use this more aggressive request model for both texture and mesh fetches. This viewer is currently in QA and is expected to go to RC soon after the 3.7.16 release. It has also had trials outside of North America and the results have been very good. The effective request rate has approached the limits imposed by our servers and has exceeded the download rate of UDP texture fetching.

 

CDN

As for those server limits, the operations team at Linden has been making rapid progress on the CDN project (DRTSIM-258). This project will replicate the Linden services that supply meshes and textures to a CDN's PoPs (Points-of-Presence). With PoPs on multiple continents, request service time will be reduced for most users.

Combined with pipelining, CDN experiments are producing results that have only been dreamed about. How fast?  Well, the engineer's universal response of "It depends" applies. But several 100's of fetches per second have been seen far from the USA. This takes us into never before encountered performance territory.

 

And one more thing

The HTTP Project has focused on textures and meshes. But the inventory system, which maintains item ownership, is often described as... sluggish. So as an exercise in expanding the use of the new HTTP library, the pipelining viewer was modified to use it for inventory fetches. As with textures and meshes before, inventory is now fetching in the 'C' region of its specific performance graph. The difference can be surprising.

For several years, HTTP has figured prominently in Linden's plans for Second Life. "HTTP will give you speed and throughput, consistency and robustness."  A promised land, but never quite realized. These next steps are payment on that promise. HTTP done well can support an amazing experience. And you'll have no reason to look back to the V1 world of UDP textures and inventory.

Nous sommes embarqués,

Monty (Linden)

 

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